# Glossary

> Extracted from the fifth-edition PDF Markdown source.

Aa A cooler or thicker lava flow that solidifies with a blocky surface texture.
Abrupt climate change Large-scale
change in the global climate system that
takes place over a few decades or less.
Absolute dating Determination of the
geologic age at which time a fossil organism, earth material, or geologic feature
formed; the age is usually expressed in
years; commonly established by the analysis of radioactive isotopes and their decay
products.
Acceptible risk Exposure to danger that
an individual or society is willing to take.
Acid rain Water droplets falling from
clouds that have been made artificially
acidic by pollutants. In volcanoes, this
acidity commonly comes from sulfate
aerosols in eruption clouds, or sulfur dioxide fumes or from hydrogen chloride gas
produced when lava vaporizes seawater.
Rainwater normally has a slight acidity
from dissolved carbon dioxide.
Active fault A fault along which future
movement is thought possible. Two criteria are mappable displacement along a
fracture or fractures in the past 10,000
years or several mappable displacements
in the past 35,000 years.
Adaptive management Structured management with the use of science that promotes sustainable coasts, and allows for
changing conditions during management.
Advance With respect to a glacier, an interval of time during which the area occupied by glacial ice increased.
Aerosol With respect to climate, a suspension of microscopic liquid and solid
particles, such as mineral dust and soot, in
the atmosphere.
Aftershock A subsequent, less intense
earthquake that occurs a few minutes to a
year or so after a main earthquake, and in
the same general area. Usually one of
many quakes of lesser magnitude than the
main shock.
Airburst With respect to collision with extraterrestrial objects, the explosion of a
meteoroid or comet in the atmosphere,
generally at an altitude of 12 to 50 km (~7.5
to 30 mi.).

Air mass thunderstorm An ordinary
storm with lightning and thunder that develops from local convection within an
unstable air mass. Commonly forms in the
afternoon and is generally not severe.
Albedo A measure of the reflectivity of a
material or surface to electromagnetic radiation, especially sunlight. Commonly
expressed as the percentage of radiation
reflected versus absorbed.
Alberta Clipper An extratropical cyclone
that forms east of the Rocky Mountains in
the Canadian province of Alberta and
moves rapidly to the southeast across
Canada and the northern United States.
Commonly associated with snowstorms,
especially blizzards.
Alluvial fan Fan-shaped deposit composed of coarse sediment that is dropped
by a stream as it emerges from a mountain
front onto flatter terrain; typically made
of a variable proportion of stream and
debris-flow deposits. The fan takes the
shape of a segment of a cone.
Alluvium Unconsolidated sediment, such
as sand, gravel, and silt, deposited by a
stream.
Andesite A type of lava that has a silica
content intermediate between basalt and
rhyolite. Commonly, but not exclusively,
associated with composite volcanoes
where it forms by the partial melting of
oceanic crust in a subduction zone below
the volcano.
Angle of repose Maximum angle that
loose material will sustain.
Anthropogenic Produced by humans or
originating in human activity.
Anticipate In respect to natural hazards
and disasters, advance planning for the effects of the natural process.
Aquifer A useable accumulation of subsurface water in earth material that is saturated with water.
Artificial bypass Human action that
moves sand or other sediment around an
obstruction, such as dredging and pumping sand from updrift to downdrift sides
of a jetty or harbor.
Artificial control of natural processes
An action or structure made by humans to

influence phenomena that are not caused
or produced by humans.
Ash fall Fine airborne material that settles onto the land surface. The material is
unconsolidated volcanic debris (ash), less
than 4 mm in diameter, that has been
blown out of a volcano during an eruption; it may travel a considerable distance
from the volcano.
Ash flow See Pyroclastic flow.
Asteroid Rock or metallic particle in
space with a diameter between 10 m (~30
ft.) and 1000 km (~620 mi.).
Asteroid belt Region between orbits of
Mars and Jupiter that contains most of the
large rock or metallic particles in our solar
system.
Asthenosphere Upper zone of Earth’s
mantle, located directly below the lithosphere; a hot, slowly flowing layer of relatively weak rock upon which tectonic
plates move.
Atmosphere Layer of gases surrounding
a planet, such as Earth.
Atmospheric pressure The force per
unit area exerted by the weight of gas
molecules that surround a planet; also
called barometric pressure, because it is
commonly measured with a barometer.
Atmospheric stability A condition of
equilibrium in which a rising or sinking
parcel of air tends to return to its original
position. This equilibrium develops when
a relatively small difference in air temperature exists between the surface and
the air aloft.
Atoll A ring or horseshoe-shaped group
of coral reefs, low islands, and shoals
­enerally surrounding a large island,
g
­submerged island, bank, or underwater
plateau.
Attenuation With respect to earthquakes,
the loss of energy as seismic waves pass
through earth materials. For example, this
process reduces the energy of seismic
waves from deep subduction-zone earthquakes before they reach Earth’s surface.
Avalanche A type of landslide involving
a large mass of snow, ice, or entrained
rock debris that slides, flows, or falls rapidly down a mountainside.

613



---

614   Glossary
Avulsion The process by which all or part
of a river or distributary channel is abandoned in favor of a new channel.
Bankfull discharge Flow condition in
which water completely fills a stream channel. Many stream channels are formed and
maintained by bankfull discharge.
Barometric pressure See Atmospheric
pressure.
Barrier island An elongate, offshore accumulation of sand that is above water
level and is separated from the mainland
by a bay or lagoon; stops large waves from
reaching the shore of the mainland.
Basalt A common volcanic rock ­consisting
of feldspar and other silicate minerals rich
in iron and magnesium; has a r­elatively
low silica content of about 50 percent;
forms shield volcanoes.
Base level The theoretical lowest elevation to which a river may erode, generally
at or about sea level.
Beach Associated with an ocean or lake,
an accumulation of loose material, most
commonly sand, gravel, or bits of shell,
on a shoreline as the result of wave action. As used by river rafters, a wide sand
accumulation along a low bank of the
channel.
Beach budget Inventory of sediment
­influx and loss along a particular stretch of
coastline.
Beach drift Continual movement of sand
grains along the shore by the runup and
backwash of waves.
Beach face The part of the shore that is
normally exposed to swash action, that is,
the runup and backwash of the waves; the
sloping surface between the ocean or lake
and the berm.
Beach nourishment Mechanical addition of sediment, generally sand, to a
shore for recreational and aesthetic purposes, as well as to buffer coastal erosion.
Bedding plane A surface separating layers of sedimentary rocks. In mass wasting, this is commonly a weakness along
which weathering occurs and landslides
develop.
Berm The relatively flat or landward
sloping part of a beach profile produced
by wave-deposited sediment; the part of
the beach where most people sunbathe.
Bermuda-Azores High A persistent
high-pressure region in the North Atlantic during the summer and fall months
that lies approximately between the Island

of Bermuda, east of North Carolina, and
Azores Islands, west of Portugal.
Blind fault A fracture or zone of rupture
along which displacement does not extend to the Earth’s surface.
Blizzard A severe winter storm producing large amounts of falling or blowing
snow that create low visibilities for extended periods of time. In the United
States, for a storm to be classified as a blizzard winds must exceed 56 km (~35 mi.)
per hour with visibilities of less than
0.4 km (~0.25 mi.) for at least 3 hours in
Canada, for a storm to be classified as a
blizzard winds must exceed 40 km (~25
mi.) per hour with visibilities of less than
1 km (~0.6 mi.) for at least 4 hours.
Block With respect to a fragment ejected
from a volcano, an angular particle that is
longer than 64 mm (~2.5 in.).
Bluff As a landform, a high steep bank or
cliff along a lakeshore or river valley commonly made of weakly consolidated earth
material.
Bomb With respect to a volcanic fragment, a smooth-surfaced particle that is
longer than 64 mm (~2.5 in.); commonly
forms from a blob of magma ejected from
a lava fountain; the magma cools and forms
a streamlined shape as it falls to the surface.
Braided With respect to a river or stream,
a channel pattern with numerous islands
and sand and gravel bars that continually
divide and subdivide the flow of water.
This pattern is most evident during low
and moderate stream flow.
Braided river See Braided stream.
Braided stream A flowing channel of
water with numerous islands and sand and
gravel bars that continually divide and
subdivide the flow.
Breaker zone With respect to a shoreline, that part of the beach and nearshore
environment where incoming waves peak
up, become unstable, and collapse toward
the shore.
Breakwater A structure, such as a wall,
that may be attached to a beach or located
offshore, designed to protect a beach or
harbor from the force of waves.
Breccia A rock or zone within a rock
composed of angular fragments; may have
a sedimentary, volcanic, tectonic, or impact origin.
C soil horizon Lowest soil horizon, sometimes known as the zone of partially altered
parent material.

Caldera A semi-circular region of low topography that develops at the top of a volcano or in the continental crust due to
collapse of the surface due to removal of
magma from the subsurface during a volcanic eruption.
Calving The separation of large blocks of
ice from the terminus of a glacier that
flows into a large body of water; calved
blocks may form icebergs.
Carbon-14 Radioactive isotope of carbon with a half-life of approximately 5730
years; used in absolute dating of organic
materials back to approximately 40,000
years.
Carbon dioxide (CO2) A colorless,
­odorless gas that is denser than air and is
produced by plant and fossil fuel combustion, animal and microbe respiration, and
volcanic eruptions. In the atmosphere, it
is the primary cause of global warming
because, as a greenhouse gas, it heats the
air by absorbing infrared radiation.
Carbon monoxide (CO) A colorless, odorless gas that forms in low-oxygen conditions
with incomplete combustion or is released
from volcanoes; in low concentrations it is
highly toxic to humans and animals.
Carbon sequestration The removal and
isolation of carbon compounds, particularly carbon dioxide, from chemical
r­eactions in the carbon biogeochemical
cycle; a term commonly used to describe
the removal of atmospheric carbon dioxide and its isolation underground or in the
ocean.
Carrying capacity Maximum number of
a population of a species that may be
maintained within a particular environment without reducing the ability of that
environment to maintain that population
in the future.
Catastrophe An event or situation causing sufficient damage to people, property,
or society that recovery or rehabilitation
is long and complex; natural processes
most likely to produce a catastrophe include floods, hurricanes, tornadoes, tsunamis, volcanoes, and large fires.
Cave A natural subterranean void commonly consisting of a series of chambers
that are large enough for a person to enter;
formed most often in limestone or marble.
Cave system See Cave.
Cavern A large cave.
Cenote On the Yucatán Peninsula of
Mexico, a steep-walled collapse sinkhole



---

Glossary   615
that extends below the water table. Some
of these sinkholes are developed in fractured limestone on the perimeter of the
Chicxulub impact crater.
Channel pattern The shape of a flowing
stream as viewed from above (“bird’s-eye
view”); the three most common shapes or
patterns are straight, meandering, and
braided.
Channel restoration The process of returning a stream and adjacent areas to a
more natural state.
Channelization Engineering technique
to straighten, widen, deepen, or otherwise
modify a natural stream channel.
Chute In mass wasting and landslides, a
narrow path formed and maintained by
the rapid downslope movement of avalanches, rockfalls, or debris flows.
Cinder cone A conical volcanic hill
formed by an accumulation of coarse pyroclastic material erupted from the volcano; also known as a scoria cone.
Clay particles Very small particles less
that 1/256 mm in diameter.
Clear-cutting Forestry practice of harvesting all trees from a tract of land.
Cleavage In a mineral, the smooth, easy
fracture that takes place along parallel
surfaces lying in one or more planes; these
surfaces develop where bonds between atoms are weak or are especially long.
Climate Characteristic weather at a particular place or region during many seasons, years, or decades.
Climate forcing An imposed positive or
negative change of Earth’s energy balance
measured in Watts per meter squared
(W/m2).
Coarse grained soils Type of soil where
over half of the soil particles (by weight) is
composed of particles greater than 0.074
mm in diameter.
Coastal Zone bioengineering Coastal
defense that uses living or dead plants to
assist in controlling coastal erosion.
Cold front An atmospheric boundary
between two air masses of contrasting
temperature along which the cooler air
mass advances into the warmer air mass.
Collapse sinkhole Depression in sediment or bedrock caused by a subsurface
cave-in.
Collapsible soils Some windblown dust
deposits that are loosely bound with clay
particles and water-soluble minerals that
collapse when water percolates through

them, often resulting in land surface subsidence.
Colluvium Mixture of weathered rock,
soil, and other, usually angular, material
on a slope; produced by creep, landsliding, and other surface processes.

form a single super-continent, which he
called Pangea. The hypothesis was supported by geologic and paleontologic data.
Contour line On a topographic map, a series of connected points in which all points
are the same elevation above sea level.

Color In a mineral, the name associated
with the particular wavelengths of light
that are reflected from the entire specimen or from its powder.

Contour interval (CI) On a topographic
map, the vertical distance between each
line of equal elevation; generally expressed in meters or feet.

Combustion Second phase of a wildfire
that follows every ignition. Includes flaming, where fine fuel and volatile gases are
rapidly oxidized at high temperature, and
glowing or smoldering, which take place
later at lower temperature. Both flaming
and glowing liberate energy as heat and
light.

Convection Transfer of heat by a mass of
moving particles; for example, in boiling
water, hot water rises to the surface and
displaces cooler water that moves toward
the bottom. Atmospheric heat may be
transferred vertically. This is the primary
process in which heat is transferred within
the atmosphere and during wildfires.

Comet Particle in orbit in space composed of a spongelike rocky core surrounded by ice and covered by carbonrich dust with a tail of gas and dust that
glows, as its orbit approaches the sun;
ranges from a few meters to a few hundred kilometers in diameter.

Convection cell A self-contained circulation loop in a fluid in which hot, less
dense fluid rises and cooler, denser fluid
sinks; develops on a large scale in the atmosphere and within the Earth.

Community Internet Intensity Map
Online depiction of how people and
structures have experienced an earthquake on the basis of email reports.
Composite volcano Steep-sided volcanic cone produced by alternating layers of
pyroclastic debris and lava flows; also
known as a stratovolcano.
Conchoidal fracture In a rock or mineral, a broken surface that is smoothly
curved, like the inside of a clamshell.
Conduction With respect to physics, the
transfer of heat through a substance by
molecular interactions.
Constructive interference In an ocean
or a lake, combining two or more waves
on the water surface to form a resultant
higher wave.
Contact Interface or boundary between
two geologic mapping units that is generally shown as a thin line on a geologic map.
Continental collision boundary The interface between two tectonic plates of similar density that are moving toward each
other. Deformation and mountain formation occur along the interface because neither plate is able to slide below the other.
Continental Drift Hypothesis proposed
by Alfred Wegener, in the early twentieth
century, that states the continental landmasses of the world “drift” about the surface of the globe and were once united to

Convergence Coming together.
Convergent boundary Interface between two tectonic plates in which one
plate generally descends below the other
in a process known as subduction.
Core With respect to the interior of
Earth, the central part of Earth below the
mantle, divided into a solid inner core
with a radius of approximately 1300 km
and a molten outer core with a thickness
of about 2000 km; the core is thought to
be metallic and composed mostly of iron.
Coriolis effect An apparent deflection in
the path of a moving object. This ­apparent
deflection, to the right in the ­Northern
Hemisphere and to the left in the ­Southern
Hemisphere, results from Earth’s rotation.
The magnitude of this effect increases
with increasing latitude from the equator
to the poles.
Crater A type of volcanic vent that forms
a semicircular depression near the summit
of most volcanoes, particularly cinder
cones and stratovolcanoes. Volcanic craters form in response to the forceful ejection of volcanic debris and gasses.
Creep Slow downslope movement of soil
and other weakly consolidated earth materials; characterized by slow flowing,
sliding, or slipping.
Crevasse On a glacier, a large, deep fracture in the surface of the glacial ice caused
by stretching of ice at and near the glacier
surface.



---

616   Glossary
Crown fire With respect to wildfires,
flaming combustion that carries through
canopies of trees; commonly driven by
high wind and aided by a steep slope.
Crust The thin outermost rock layer of
Earth that is differentiated from the underlying mantle by having a lower density.
Crustal assimilation A process where
crustal rocks are melted by and incorporated into a body of magma that is rising
toward the surface. Addition of felsic
crustal melt to the original magma body
causes progressive increase of the silica
content of the magma as it rises.
Crystal form In a mineral, the characteristic geometric shape assumed by that
mineral because of its orderly, internal
atomic structure.
Crystallization Process by which atoms
precipitate from solution such that they organize themselves into the orderly internal
atomic arrangement of a mineral. The mineral may or may not possess crystal faces.
Cumulonimbus Thunderstorm cloud
that grows vertically until it reaches a stable layer of air; generally with an anvilshaped top.
Cumulus Any cloud that develops vertically; generally has a flat base with a
­cauliflower-like top; one of the three basic
cloud types.
Cumulus stage First phase of thunderstorm development characterized by the
growth of domes and towers on the top of
a cumulus cloud as it becomes a cumulonimbus cloud.
Cutbank Steep or nearly vertical slope on
the outside of a bend in a stream channel;
typically forms by stream erosion.
Cyclone An area of low atmospheric
pressure characterized by rotating winds
that, in the Northern Hemisphere, circulate in a counterclockwise direction.
Cyrosphere The portion of a planet consisting of ice; on Earth this includes permafrost in soil and below the seafloor,
glaciers, ice sheets, ice caps and fields, ice
shelves, and icebergs.
Daughter product A stable isotope that
forms by radioactive decay of an unstable
parent isotope.
Debris flow Rapid downslope movement
of unconsolidated, water-saturated earth
material that became unstable because of
torrential rain, rapid melting of snow and
ice, or sudden drainage of a pond or lake;
sometimes restricted to flows of this type
that contain mainly coarse material.

Decadal cycles Cycles of storm tracks
and thus climate and weather in North
America and Western Europe that may
last several decades; also known as Decadal
oscillations.
Decompression melting The process
by which solid rock melts due to a decrease in pressure that is caused by upwelling of hot rocks within the interior of
the planet.
Delta Low, nearly flat area of land formed
near the mouth of a stream where it enters
a lake or the ocean; commonly triangular
or fan-shaped and crossed by branching
distributary channels of the stream that
created it.
Delta plain Generally flat land surface that
develops where a stream enters an ocean or
lake; commonly covered with marsh or
swamp and cut by distributary channels.
Dendroclimatology Study of tree rings,
sets of which usually form each year, to
establish the absolute age of a tree or
piece of wood from a tree.
Deposition In geology, the laying down
or accumulation of earth material by processes that include the mechanical settling
of sediment, the chemical precipitation of
mineral matter, or the in-place accumulation of organic matter.
Derecho A windstorm with straight-line
winds exceeding 90 km (~56 mi.) per hour
along a line that is at least 400 km (~250
mi.) in length; damage produced by this
windstorm is often equivalent to that
caused by a tornado.
Desert An arid or semiarid region typified by mean annual precipitation less
than 250 mm (~10 in.); native animals and
vegetation adapted to conserve limited
water and soil that may be rich in salts.
Desertification Conversion of land from
a more biotically productive state to one
that more closely resembles a desert.
Desiccation crack A rupture in finegrained sediment, typically mud, produced
by complete or nearly complete drying.
Differentiated With respect to ­meteoroids
and asteroids, rock that has undergone igneous and sometimes metamorphic processes.
Digital elevation model (DEM) Threedimensional computer depiction of a land
surface as viewed from an oblique angle.
Constructed with a computer program
that samples land-surface elevation data
on a specific spacing grid, such as every
30 m (~100 ft.).

Direct effect A change that follows an
event without any intervening factors. In a
natural accident, disaster, or catastrophe,
such effects could include people killed,
injured, or dislocated and property damaged or destroyed; also referred to as a
primary effect.
Directivity With respect to an earthquake, the increased intensity of shaking
in the direction toward which a fault
­ruptures; observed in some moderate-tolarge earthquakes.
Disappearing stream A brook, creek, or
river that flows from a surface channel
into the ground and continues flowing
underground.
Disaster One possible effect of a hazard on
society. Usually a sudden event that causes
great damage or loss of life during a limited
time and in a limited geographic area.
Disaster preparedness With respect to
a natural hazard, actions of individuals,
families, communities, states, provinces,
or entire nations to minimize losses from
a hazardous event before it occurs.
Discharge Quantity of water flowing
past a particular point on a stream, usually
measured in cubic feet per second (cfs) or
cubic meters per second (cms).
Dissipative stage Final phase of a thunderstorm in which the supply of moist air
is blocked by downdrafts in the lower levels of the cloud. Without a source of moisture, precipitation decreases and the
storm cloud begins to disappear.
Distant tsunami A series of gravity
waves in the ocean originating from a
source typically thousands of kilometers
away from the shoreline that is inundated.
These waves are produced by the displacement of the entire water column of
the ocean by an underwater earthquake,
landslide, volcanic eruption, or extraterrestrial impact.
Distributary channel A waterway that
branches from another waterway in a
downstream direction. Typically found on
stream deltas and some alluvial fans.
Divergence Spreading apart.
Divergent boundary Interface between
tectonic plates characterized by production
of new lithosphere; found along mid-ocean
ridges and continental rift zones. Equivalent reference points on each plate will
move away from each other with time.
Doppler effect Change in wave frequency that takes place when either the emitter
or the detector is moving toward or away



---

Glossary   617
from the other. This effect can be heard in
the higher-pitched sound of an approaching ambulance and the lower-pitched
sound of one that is moving away.
Doppler radar With respect to weather,
an electronic system that emits electromagnetic waves to detect the velocity of
precipitation that is falling either toward
or away from the receiver. Used to detect
rotation within a thunderstorm that often
precedes formation of a tornado.
Downburst Localized area of very strong
winds in a downdraft; commonly beneath
a severe thunderstorm.
Downdrift Located in the direction toward which a current of water, such as a
longshore current, is flowing.
Downstream flood Condition in which
surface runoff from a relatively wide area
has caused a stream to overflow its banks—
more common in the lower part of a
drainage basin where tributary streams
have increased the discharge of the overflowing stream.
Drainage basin Area that contributes
surface water to a particular stream network.
Driving force With respect to mass wasting, an influence that tends to make earth
material move downslope.
Drought Extended period of unusually
low precipitation that produces a temporary shortage of water for people, other
animals, and plants.
Dryline An atmospheric boundary between two air masses of contrasting moisture content where the air mass with less
moisture advances into the air mass with
greater moisture. Common in the southcentral United States in late spring and
summer, it is a source of atmospheric instability for the development of severe
weather.
Dry ravel After a fire, the gravity-driven
process of moving a large volume of sand,
gravel, and organic debris downslope.
This debris was stored upslope of brush
vegetation before a fire.
Dry slot A nearly cloud-free area behind
a cold front in an extratropical cyclone or
a wall cloud in a severe thunderstorm that
is created by sinking dry air.
Duff A layer of compacted and partially
decomposed leaves, twigs, and other organic matter in a forest soil that accumulates below leaf litter and above mineral
soil; generally burns by glowing and
smoldering combustion.

Dust storm A weather event in which
visibility at eye level drops to less than 1
km (0.6 mi.) for hours or days because of
fine airborne particles of silt and clay.
Dynamic equilibrium With respect to
the behavior of streams and rivers, the
overall balance between the work that a
river does in transporting sediment and
the amount of sediment that it carries.
E-line Long, narrow band inland from
the shore that marks the distance to which
coastal erosion is expected to have washed
away dry land in a given number of years.
E soil horizon A light-colored horizon
underlying the A horizon that is leached
of iron-bearing compounds.
E-zone With respect to coastal erosion, a
hazard area between sea level and an eline that is expected to erode away within
a particular time period.
Earth climate system Emphasizes the
interactions among the atmospheric,
cryospheric, and terrestrial, and marine
biospheric processes that affect the climate of Earth.
Earth fissure A large, deep crack in loose
sediment at the land surface; formed by the
lowering of the water table and the consequent drying out of the earth material.
Earth’s energy balance Condition in
which incoming solar radiant energy generally equals outgoing radiant energy
from our planet; this equality exists despite changes in the energy’s form as it
moves through the atmosphere, oceans,
land, and living things before being radiated back into outer space.
Earth system science The study of
Earth as a system.
Earthquake Sudden movement of a
block of Earth’s crust along a geologic
fault; the movement releases accumulated
strain in the rocks.
Earthquake cycle A hypothesis that explains the periodic nature of earthquakes
on a given fault on the basis of a drop in
elastic strain after an earthquake and reaccumulation of strain before the next event.
Easterly waves North–south oriented
low-pressure troughs that migrate from
east to west at equatorial latitudes causing
local convergence and divergence of the
trade winds giving rise to persistent thunderstorms, which can develop into tropical storms and hurricanes.
Eddy With respect to ocean currents, the
circular movement of water that develops

behind an obstruction, where two currents pass in opposite directions or along
the edge of a permanent current; can have
a central core of either warm or cold water. Can be large enough to be detectible
with satellite sensors.
Eddy vortex A persistent, circular movement of warm water that develops on the
edge of the Loop Current in the Gulf of
Mexico that can affect the intensity of
hurricanes that cross it.
EF scale A graduated range of values
from EF0 to EF5 for describing tornado
intensity based on the maximum threesecond wind velocity inferred from damage to buildings, towers, poles, and trees.
A modification of the F-scale developed
by T. Theodore Fugita in 1971.
Effusive eruption A passive outpouring
of magma onto Earth’s surface generating
a lava flow. Effusive eruptions are typical
of low viscosity basaltic magmas, which
readily allow volatiles to escape non-explosively. Low-volatile Andesitic and
rhyolitic magmas may erupt effusively.
Ejecta blanket Layer of debris surrounding an impact crater; typically forms
an irregular terrain of low mounds and
shallow depressions.
El Niño An event in the atmosphere and
ocean during which trade winds weaken or
even reverse, the eastern equatorial Pacific
Ocean becomes anomalously warm, and
the westward moving equatorial ocean current weakens or reverses; this change in
ocean temperature can cause anomalous
weather in adjacent landmasses.
Elastic rebound A process where elastic
strain is replaced by permanent fault offset during an earthquake.
Elastic strain Deformation that returns
to its original shape when stress is released.
Electrical resistivity Degree to which
earth materials are unable to conduct an
electrical current. Measurements of this
property are used in the exploration for
energy, mineral, and water resources.
Electromagnetic energy Property of a
system that enables it to do work through
oscillations in an electric and magnetic
field.
Electromagnetic spectrum The collection of all possible wavelengths or frequencies of electromagnetic radiation.
Electromagnetic wave An oscillation in
an electric and magnetic field that transfers radiant energy.



---

618   Glossary
Energy A quantifiable physical property
describing the state of an object or a system.
The part of energy pertinent to this text and
natural hazard is the force. See Force.
Enhanced Fujita scale See EF scale.
Environmental unity A principle of environmental studies that states that everything
is connected to everything else.
Epicenter Point on Earth’s surface directly above the focus of an earthquake;
the focus is the area that first ruptures on
a geologic fault in an earthquake.
Eustatic sea level The worldwide average vertical position of the ocean surface.
Evacuation With respect to a natural
hazard, the movement of people away
from the location of a probable dangerous
or destructive natural phenomenon.
Expansive soil In-place, weathered
earth materials that, upon wetting and
drying, will alternately expand and contract, causing damage to foundations of
buildings and other structures.
Exponential growth A type of compound growth in which a total amount or
number increases at a certain percentage
each year, and each year’s rate of growth is
added to the total from the previous year;
characteristically stated in terms of a particular doubling time, that is, the time in
years it will take the original number to
double; commonly used in reference to
population growth.
Exposure dating Determining the
amount of time that sediment or rock has
been at Earth’s surface by measuring
quantities of isotopes in the material that
are produced by cosmic rays.
Extinction With reference to a wildfire,
the final phase in which all combustion,
including smoldering, ceases because of
insufficient heat, fuel, or oxygen. With
reference to organisms, the complete disappearance of a biological species.
Extratropical cyclone An area of low
atmospheric pressure, characterized by
r­otating winds, which generally develops
along a weather front between 30° and 70°
latitude; commonly associated with heavy
­precipitation, strong winds, and other severe
weather; can produce a storm surge and
coastal erosion when crossing a ­shoreline.
Eye With respect to hurricanes and related intense tropical cyclones, the central
circular area of light winds and partly
cloudy skies where subsiding dry air encounters rising moist air.

Eyewall The portion of a hurricane immediately adjacent to a central circular
area of nearly clear sky and light winds;
usually the region having the highest
winds and most intense rainfall.
F-scale See EF scale.
Falling With respect to mass wasting and
landslides, earth materials such as rocks
dropping through the air from steep slopes.
Fault A fracture or fracture system in
earth materials along which rocks on opposite sides of the fracture have moved
relative to one another.

Fissure Large fracture or crack in rock
along which the opposing side have separated; may become a volcanic vent if lava
or pyroclastic material is extruded.
Flash flood Overbank flow that results
from a rapid increase in stream discharge;
commonly occurs in the upstream part of
a drainage basin and in small tributaries
downstream.
Flashy discharge Stream flow characterized by a short lag time or response time
between rainfall and peak stream discharge.

Fault creep Slow, essentially continuous
movement of blocks of Earth’s crust principally along one side of a fracture; also
called tectonic creep.

Flood basalt A laterally extensive, basaltic lava flow that generally is part of a
thick sequence of similar flows. These
flows commonly build up a plateau; also
referred to as a plateau basalt.

Fault gouge A clay-rich zone of pulverized rock produced by movement along a
fracture or fracture system in Earth’s
crust; can act as an underground barrier
or dam to the flow of groundwater.

Flood discharge The stream flow volume per unit of time (represented by the
units cms, cubic meters per second, or cfs,
cubic feet per second) at which damage to
personal property is likely to begin.

Fault scarp Steep slope or small cliff that
was formed during an earthquake by rupture and displacement of a block of Earth’s
crust.

Flooding From a hazards perspective,
high water levels in a stream, lake, or
ocean that may damage human facilities.
As a natural process, overbank flow that
may construct a floodplain adjacent to a
stream channel or a higher-than normal
water level along a coast that extends inland beyond the beach.

Faulting The process of creating a rupture in Earth’s crust by the movement of
one crustal block in relation to another.
Felsic Refers to an igneous rock or melt
composition with more than about 65
percent silica rich in, which is characteristic of granitic rock and rhyolitic magma.
Fetch Distance that wind blows over a
body of water; a principal factor that determines the height of windblown waves;
in a cyclone, a contributing factor to the
height of the storm surge.
Fibrous fracture A distinctive pattern
for the breakage of minerals in which long
slivers of the mineral break away from the
specimen.
Fine grained soils Type of soil where
over half of the soil particles (by weight) is
composed of particles less than 0.074 mm
in diameter.
Fire management With respect to wildfires, the control of combustion to minimize loss of human life and damage to
property. Accomplished through understanding the fire regime, public education,
remote sensing of vegetation to determine fire potential, and prescribed burns.
Fire regime An ecosystem’s potential for
wildfire as inferred from fuel types, terrain, and past fire behavior.

Floodplain Flat topography adjacent to a
stream produced by overbank flow and by
lateral migration of the channel and associated sand or gravel bars.
Floodplain regulation Governmental restriction of land use in an area likely to be
inundated by a stream’s overbank flow that
could damage buildings and infrastructure.
Flood-proofing With respect to urban
flooding refers to practices such as raising
buildings above potential floodwater, constructing flood doors on buildings, placing flood walls around buildings or other
structures, or installing improved drains
to remove floodwater accumulation.
Flood stage Water level of a stream at
which damage to personal property is
likely to begin.
Flow With respect to mass wasting,
downslope movement of earth materials
that deform as a fluid, such as the fluid
movement of grains of sand, rock, snow or
ice, debris, and mud.
Flowage See Flow.
Flowstone General term for a mineral
deposit, most commonly of calcium car-



---

Glossary   619
bonate, formed by flowing water on the
wall or floor of a cave.

Cold front, Warm front, Stationary front,
and Occluded front.

Flux In a natural system, the rate at which
a substance moves from one part of the
system to another.

Frostbite Injury to body tissues caused
by exposure to extreme cold.

Focus In an earthquake, the point or location in Earth of the initial break or rupture, and from which energy is first
r­eleased; during an earthquake, seismic
energy radiates out from this point or location. Also known as the hypocenter.
Foliation plane With respect to mass
wasting, a surface or zone of weakness in
metamorphic rocks produced by the
alignment of minerals during metamorphism.
Footwall Block of Earth’s crust that is below a slanted geologic fault.
Fog A cloud that is in contact with the
ground.
Force With respect to physics, an influence that tends to produce or to change
the motion of a body, or one that produces stress within a body.
Forcing With respect to climate, a variable, such as solar radiation, volcanic
aerosols, or greenhouse gases, that influences the direction of climate change.
Forecast With respect to a natural hazard, an announcement that states that a
particular event, such as a flood or storm,
is likely to occur during a particular time
interval, often with some statement of the
degree of its probability.
Foreshock Small to moderate earthquake that occurs before and in the same
general area as the main earthquake.
Formation A body of rock or sediment
composed of one or more distinctive
types that can be mapped at the surface or
traced in the subsurface—commonly given a geographic place name (e.g., Potsdam Sandstone) and formally defined in a
scientific publication.
Fracture With respect to minerals, the
breakage of a mineral in an irregular,
curving or fibrous manner.
Free face A steep cliff of bare rock.
Frequency The number of waves passing
a point of reference per second; ­commonly
expressed in cycles per second or hertz (Hz);
the inverse of the wave period.
Front With respect to weather, a boundary between two air masses that are distinguished by differences in temperature
and, commonly, moisture content. See

Frost heaving The upward and lateral
movement of soil particles and the land
surface as the result of ice formation in
the soil.
Frost-susceptible soil Surficial earth
material that is likely to move and expand
when frozen as the result of the accumulation of ice between particles of the material; commonly unconsolidated material
with a high content of silt.
Fugita scale See EF-scale.
Funnel cloud A narrow, rotating cloud
extending downward from a thunderstorm. This cloud may become a tornado
if it reaches the ground.
Gabion A metal wire basket, commonly
made out of chain-link fencing that is
filled with rocks. These baskets are
stacked along stream banks to prevent
erosion and are used as barriers to slow
water flow in retention ponds.
Geochronology Science of dating earth
materials, fossil organisms, and geologic
features and events; and of determining
the sequence of events in Earth history.
Geographic information system (GIS)
Computer technology capable of storing,
retrieving, transforming, and displaying
spatial information about Earth and of
making maps with these data.
Geologic cycle A group of interrelated
sequences of Earth processes known as
the hydrologic, rock, tectonic, and biogeochemical cycles.
Geologic map A two-dimensional representation of the age and composition of
earth materials that are found beneath the
soil within a given geographic area.
Geyser A particular type of hot spring
that ejects hot water and steam above
Earth’s surface; perhaps the most famous
is Old Faithful in Yellowstone National
Park.
Glacial interval A portion of Earth history during which a cooler, wetter climate
supported widespread continental glaciers.
Glacier A large mass of flowing ice that
moves downslope under its own weight
and persists from year to year; formed, at
least in part, on land by the compaction
and recrystallization of snow.

Global climate model (GCM) Linked
computer programs that use environmental data in mathematical equations to predict global change, such as increases in
mean temperature, changes in precipitation, or some other atmospheric variable.
Global positioning system (GPS) Interacting ground control, satellites, and
radio receivers that obtain extremely accurate locations and elevations on Earth’s
surface. Especially useful for detecting
land surface changes related to a volcanic
eruption, earthquake, tectonic creep, the
slow movement of a landslide, or land
subsidence.
Global warming The increase in the mean
annual temperature of the lower ­atmosphere
and oceans in the past 150 years, primarily
as a consequence of burning fossil fuels that
emit greenhouse gases into the atmosphere.
Gradient Slope of the land over which a
stream flows. Determined by calculating
the vertical drop in elevation through
some horizontal distance; commonly
­expressed as m per kilometer or ft. per
mile.
Granular structure A soil structure consisting of individual grains or clumps of
grains that form generally small semiround soil peds.
Great earthquake A sudden rupture
along a geologic fault in which the release
of energy has a moment magnitude of 8 or
greater.
Greenhouse effect Trapping of heat in
the lower atmosphere by the absorption
of infrared energy by water vapor, carbon
dioxide, methane, nitrous oxides, halocarbons, and other gases.
Greenhouse gas A substance such as
carbon dioxide, methane, nitrous oxides,
or halocarbons (e.g., Freon) that absorb
infrared radiation, and contributes to
global warming of the lower atmosphere.
Groin A long, narrow structure of rock,
concrete, wood, or other material generally constructed perpendicular to the
shore to protect the coastline and trap
sediment in the zone of littoral drift.
Groin field With respect to coastal processes, a group of long, narrow structures
of rock, concrete, wood, or other material
generally constructed perpendicular to
the shore to protect the coastline and trap
sediment in the zone of littoral drift.
Ground acceleration With respect to an
earthquake, the amount that the speed or



---

620   Glossary
velocity of the shaking of the Earth’s surface changes in a unit of time, such as a
second. Ground acceleration is commonly
given as a fraction or percentage of gravitational acceleration, which is 980 cm
(~380 in.) per second. The greatest (or
peak) ground acceleration during an
earthquake is a principal control on the
amount of damage to structures.
Ground fire With respect to wildfires,
combustion that takes place below the
surface; characterized by more smoldering and little flaming; fuels typically are
roots and partially decayed organic matter, such as peat.
Ground penetrating radar An ­electronic
system that uses 10 to 1000 MHz
e­lectromagnetic waves to image Earth’s
shallow subsurface to locate features such
as natural cavities and fissures, permafrost,
contaminated groundwater, and buried
pipes and tanks.
Groundwater Subsurface accumulation
of water that entirely fills all voids in earth
materials.
Groundwater mining The pumping of
subsurface water at a faster rate than it is
being replenished.
Gust front Leading edge of strong and
variable surface winds from a thunderstorm downdraft.
Habitat A particular set of environmental
conditions in which adapted plants and
animals thrive.
Hachure With respect to topographic
contours, a small mark drawn perpendicular to a contour line to indicate the downward sloping of the land into a closed
­depression; also referred to as a tic mark.
Hail Large rounded or irregular piece of
ice that has grown while moving up and
down numerous times within the clouds
of a severe thunderstorm.
Half-life The amount of time necessary
for one-half of the atoms of a particular
radioactive element to decay.
Halocarbon A chemical compound (e.g.,
Freon) containing a halogen element, such
as chlorine, fluorine, or bromine, that is
bonded to carbon; generally synthetic in
origin and in the atmosphere contributes
to global warming in troposphere and
ozone depletion in stratosphere.
Hanging wall A block of Earth’s crust
that is above a slanted geologic fault.
Hardness With respect to minerals, the
resistance of a mineral to scratching; see
Relative hardness.

Hardpan A soil layer of hard material
such as clay or silica oxide or calcium carbonate.
Hawaiian-type eruption An effusive to
mildly explosive volcanic eruption that
ranks between 0 and 1 on the volcanic explosivity index. This eruption type is characterized by fire fountains and lava flows
and is typical of shield volcano eruptions.
A central caldera, in contrast to an Icelandic-type eruption where lavas erupt from
a fissure, typifies the volcanic vent.
Hazard See Natural hazard.
Headland A small peninsula on an irregularly shaped shoreline that is more resistant to erosion than the surrounding earth
material.
Headwaters Tributaries of a river that
feed into it near its uppermost reach.
Heat energy The property of a system
that enables it to do work through random molecular motion. This property
may be transferred by conduction, convection, or radiation.
Heat index A value on a temperature
scale that describes the human body’s perception of air temperature, taking into account relative humidity.
Heat waves A prolonged period of
e­xtremely high air temperature that is
both longer and hotter than normal. The
criteria for establishing what constitutes
­longer and hotter than normal varies geographically with climate and living conditions.
High pressure center An area of elevated atmospheric pressure where the wind
blows clockwise in the Northern Hemisphere. Also called an anticyclone or a high.
Hot spot A hypothesized, nearly stationary heat source located below the lithosphere that feeds overlying volcanic
­processes near Earth’s surface.
Hot spring A natural discharge of
groundwater at a temperature higher than
that of the human body.
Hot tower A rain cloud that reaches the
top of the troposphere in a hurricane. The
release of latent heat in its formation
warms the air at the high altitude.
Hummocky An irregular land surface
marked by numerous small mounds and
depressions.
Hurricane Tropical cyclone characterized by sustained winds of 119 km (118
km in Canada or 74 mi. per hour); known
as a typhoon in the western North Pacific
and South China Sea, severe tropical cy-

clone in the southwest Pacific and southern Indian Oceans, and severe cyclonic
storm in the northern Indian Ocean.
Hurricane warning An alert issued by
weather forecasters for an area where hurricane conditions are expected in 24 hours
or less. In Canada this alert is also issued
for areas likely to experience high waves
and coastal flooding, but not necessarily
hurricane-strength winds.
Hurricane watch An alert issued by
weather forecasters for an area that may
experience hurricane conditions within 36
hours. In Canada this alert is not restricted to a 36-hour time frame.
Hydraulic chilling With respect to volcanoes, the use of water to cool and solidify flowing lava; a method used to divert lava flows threatening property.
Hydrogen sulfide (H2S) A toxic, flammable gas that smells like rotten eggs and
is emitted by volcanoes.
Hydrograph A graph of the discharge
(flow) of a stream with time.
Hydrologic cycle Circulation of water
from the oceans to the atmosphere and
back to the oceans by way of precipitation,
evaporation, runoff from streams, and
groundwater flow.
Hydrology The study of surface and subsurface water.
Hydrophobic layer Lower layer of debris on forest floors or brushland that has
accumulated water-repellant organic
chemicals; especially common in drier climates after very hot fires.
Hypocenter See Focus.
Hypothermia A medical condition characterized by rapid loss of core body heat
causing intense shivering, loss of muscle
coordination, mental sluggishness, and
confusion. This condition can develop in
an improperly dressed person in cool, wet,
windy weather or in one immersed in water that is 25°C (~77°F) or less; unless
treated, the condition leads to unconsciousness and eventually death.
Ice storm A prolonged period of freezing
rain during which thick layers of ice accumulate on all cold surfaces.
Iceberg Large block of floating glacial
ice that has broken from the front of a glacier, dropping into the water by a process
called calving.
Icelandic-type eruption An effusive to
mildly explosive volcanic eruption that
ranks between 0 and 1 on the volcanic explosivity index. This eruption type is char-



---

Glossary   621
acterized by fire curtains and lava flows
typical of Icelandic shield volcano eruptions. An elongate fissure, in contrast to a
Hawaiian-type eruption where lavas erupt
from central caldera, typifies the volcanic
vent.
Igneous rock An aggregate of minerals
formed from the solidification or explosion of magma; extrusive if it crystallized
on or very near the surface of Earth, and
intrusive if it crystallized well beneath the
surface.
Impact With respect to hazardous events,
the effect or influence of an action.
Impact crater A circular-to-elliptical depression produced by collision of a large
object, such as an asteroid, with a planet,
moon, or other asteroid.
Impervious cover With respect to urban
hydrology, surfaces covered with concrete, roofs, or other structures that impede the infiltration of water into the soil.
In general, as urbanization proceeds, the
percentage of the land that is under impervious cover increases.
Inactive fault A fracture in the Earth’s
crust along which rock has not moved in
the past 2 million years.
Indirect effect A change that depends
upon intervening factors. In a natural accident, disaster, or catastrophe, such effects could include emotional distress; the
donation of money, goods, and services; or
the payment of taxes to finance recovery;
also called a secondary effect.
Injection well An artificial, generally cylindrical, hole excavated for pumping fluids, such as liquid hazardous waste, underground.
Inner core See Core.
Instrumental intensity With respect to
earthquakes, the direct measurement of
ground motion during an earthquake with
a high-quality seismograph. Used to immediately produce a map showing perceived shaking and potential damage.
Insurance The guarantee of monetary
compensation for a specified loss in return
for payment of a premium.
Integrated coastal zone management Management that promotes ecological function of the coastal zone
through sustainable solutions that minimizes conflicts from development in the
coastal zone.
Intensity With respect to earthquakes, a
measure of the effects of an earthquake at
a specific place; measured as the effects on

people and structures using the Modified
Mercalli Scale or the direct measurement
of ground motion by a sensitive seismograph; see Instrumental intensity. With respect to cyclones, the strength of the
storm as indicated by sustained wind
speeds and lowest atmospheric pressure.
Interglacial interval A portion of Earth
history when continental glaciation was
minimal or absent.
Intraplate earthquake Sudden movement along a fault caused by the abrupt
release of accumulated strain in the interior of a lithospheric plate, far away from
any plate boundary.
Ion An electrically charged atom or molecule formed by a loss or gain of electrons.
Iridium A platinum-group metal that is
more abundant in meteorites than in
earth materials. Its high concentration in
certain clay deposits is attributed to their
formation during or after impact of a
large extraterrestrial body.
Isostasy The principle stating that thicker, more buoyant crust is topographically
higher than crust that is thinner and denser. Also, with respect to mountains, the
weight of rocks of the upper crust is compensated by buoyancy of the mass of
deeper crystal rocks; that is, mountains
have “roots” of lighter crustal rocks extending down into the denser mantle
rocks, like icebergs in the ocean.
Isotope A variety of a chemical element
distinguished by its number of neutrons;
may be stable or radioactive.
Jet stream A concentrated flow of air
near the top of the troposphere, characterized by strong winds.
Jetty A very long, narrow structure of
rock, concrete, or other material generally
constructed perpendicular to the shore;
commonly constructed in pairs at the
mouth of a river or inlet to a lagoon, estuary, or bay; designed to stabilize a channel,
control deposition of sediment, and deflect large waves.
Joule The basic unit of energy in the
metric system; equivalent to the work
done by a force of one Newton when its
point of application moves one meter in
the direction that the force is acting.
Karst See Karst topography.
Karst plain A generally flat land surface
containing numerous sinkholes.
Karst topography A landscape characterized by sinkholes, caverns, and diversion of surface water to subterranean

routes that results from dissolution of
subsurface rocks by groundwater.
Kinetic energy The property of a system
that enables it to do work through motion.
K soil horizon A calcium carbonate-rich
horizon in which the carbonate often
forms laminar layers parallel to the surface; carbonate completely fills the pore
spaces between soil particles.
K-T boundary The interface in the geologic record where earth materials that
formed during the Cretaceous Period are
in contact with earth materials that
formed during the Tertiary Period; dated
as approximately 65 million years old.
Kyoto Protocol International agreement
to reduce emissions of greenhouse gases
in an effort to manage global warming;
now legally binding in at least 175 countries, including Canada, Russia, and the
European Union. The United States has
not honored its original agreement to
implement this protocol.
Lag time The time interval between the
greatest amount of precipitation and peak
discharge of a stream. Urbanization generally decreases the lag time.
Lahar A type of volcanic mudflow or debris flow that is triggered by melting of
surface snow and or ice, potentially in the
form of large glaciers, due to heating by
volcanic gasses or ash during an eruption.
Landslide Specifically, rapid downslope
movement of rock or soil; also a general
term for all types of downslope movement.
Landslide hazard map A two-dimensional representation of the land surface
that identifies areas that are likely to experience landslides and the appropriate land
uses in those areas.
Landslide risk map A two-dimensional
representation of the land surface that
identifies areas that are likely to experience a rapid mass movement of earth material, states the probability that one of
these events will occur, and assesses potential losses.
Land-use planning The preparation of
an overall master plan for future development of an area; the plan may recommend
zoning restrictions and infrastructure
both practical and appropriate for the
community and its natural environment;
based on mapping and classification of existing human activities and environmental
conditions, including natural hazards.



---

622   Glossary
Lapilli Gravel-size pyroclastic debris
ranging in size from 2 to 64 mm (~0.08 to
2.5 in.).
Latent heat Energy created by random
molecular motion that is either absorbed
or released when a substance undergoes a
phase change, such as during evaporation
or condensation.
Latent heat of vaporization Energy absorbed by water molecules as they evaporate.
Lateral blast Type of volcanic eruption
characterized by explosive activity that is
more or less parallel to the surface of
Earth. A lateral blast may take place when
the side of a volcano fails.
Lava Molten material that flows onto the
land surface from a volcano, or rock that
solidifies from such molten material.
Lava dome Relatively small volcano type
composed of highly viscous magma, usually rhyolitic in composition, that is often
restricted to the vent region. Lava domes
often found plugging the vent within the
volcanic crater of a recently erupted stratovolcano.
Lava flow Eruption of magma at Earth’s
surface that generally moves as a liquid
mass downslope from a volcanic vent.
Lava tube A natural enclosed conduit or
tunnel through which magma moves down
the slope of a volcano (sometimes many
kilometers); eventually the magma may
again emerge at the surface; after the volcanic eruption, a tube commonly remains
as an open void and forms a type of cave.
Levee A mound or embankment parallel
to a stream channel; it may consist of fine
sediment deposited from overbank flow
during a flood or be an earthen embankment constructed by humans to protect
adjacent land from flooding.
Lichenometry An absolute dating technique using measurements of the diameter of lichens on a rock surface to
­determine how long the surface has been
exposed to the atmosphere and sunlight.
Lightning A natural, high-voltage electrical discharge between a cloud and the
ground, between clouds, or within clouds.
The discharge takes a few tenths of a second and emits a flash of light that is followed by thunder.
Linkage With respect to natural hazards,
a relationship between two phenomena.
Liquefaction Transformation of watersaturated granular material from the solid
state to a liquid state.

Lithification The hardening of loose
earth material into rock, commonly by cementation and compaction.
Lithosphere Outer layer of Earth, approximately 100 km (~60 mi.) thick, which
comprises the tectonic plates that contain
the ocean basins and continents; includes
both Earth’s crust and the solid, upper
part of the mantle.
Littoral transport Movement of sediment in the nearshore environment as a
result of return flow from waves that have
washed up on the shore; sediment is
moved by both the longshore current and
by beach drift.
Local tsunami A series of gravity waves
in the ocean originating from a source
that is close to the shoreline that is inundated. These waves are produced by the
displacement of the entire water column
by either an underwater earthquake, landslide, volcanic eruption, or extraterrestrial
impact.
Loess Deposit of windblown silt and clay.
Longitudinal profile With respect to
streams, a graph that shows the decrease
in elevation of a stream bed between its
head and its mouth. This graph of elevation versus distance generally produces a
line that is concave up.
Longshore bar and trough Elongated
depression and adjacent underwater ridge
of sand roughly parallel to shore; produced by wave action.
Longshore current Waterflow parallel
to shore that develops in the surf zone as
the result of waves that strike the land at
an angle. Responsible for longshore drift.
Longshore drift The sediment transported in the nearshore environment parallel to the shore by a wave-generated
current.
Longshore trough See Longshore bar and
trough.
Loose-snow avalanche A rapid downslope flow of granular snow and ice.
Love wave A type of earthquake-generated surface oscillation in which molecular vibrations within the Earth are
t­ransverse to the direction the oscillation is moving.
Low pressure center A parcel of the atmosphere that has relatively low air pressure, often referred to as a low. see Cyclone.
Luster With respect to mineralogy, the
way that light is reflected from a mineral;
may be catagorized as metallic or nonmetallic.

Magma Naturally occurring molten rock
material formed deep within the crust or
in the mantle.
Magma chamber A large pool of molten
rock that accumulates in the subsurface
and is the primary source of lava, and volcanic gasses and debris that are extruded
to the surface during an eruption.
Magma evolution Refers to the increase
in silica content of a magma as it ascends
toward the surface. The change results
from crustal assimilation and chemical
processes within the magma as it rises.
Magnetic reversal Involves the change
of Earth’s magnetic field between normal
polarity and reverse polarity; also sometimes known as geomagnetic reversal.
Magnitude With respect to natural hazards, especially earthquakes, the amount
of energy released.
Magnitude-frequency concept The
idea that the intensity and extent of an
event are inversely proportional to how
often it occurs.
Mainshock The primary, highest magnitude earthquake in a series of earthquakes in
a given area during a limited time interval.
Major earthquake A sudden rupture
along a geologic fault in which the release of
energy has a moment magnitude of 7 to 7.9.
Managed retreat With respect to coastal erosion, a strategy involving the landward movement of buildings and associated infrastructure away from a shoreline
to remove the property from an area susceptible to flooding by seawater.
Mantle An internal layer of Earth approximately 3000 km (~1900 mi.) thick composed of rocks that are primarily iron and
magnesium-rich silicates. The lower
boundary of the mantle is with the core,
and the upper boundary is with the crust.
The boundary is known as the Mohorovičić
discontinuity (also called the Moho).
Mass extinction Sudden loss of large
numbers of plant and animal species relative to new species being added.
Mass wasting A comprehensive term
for any type of downslope movement of
earth materials.
Material amplification Increase in the
amplitude of seismic shaking caused by
some earth materials. Such increase is
generally associated with soft sediments,
such as silt and clay deposits.
Mature stage With respect to thunderstorm development, the second phase in



---

Glossary   623
which downdrafts move and precipitation
falls from the base of a cumulonimbus
cloud. With regard to tornado development, the phase in which a visible funnel
extends from the thunderstorm to the
ground.
Meander With respect to streams, a bend
in the channel that migrates back and
forth across the floodplain, depositing
sediment on the inside of the bend on a
point bar, and eroding the outside of the
bend into a cutbank.
Meandering A stream channel pattern
that is sinuous and is characterized by
gentle bends that migrate back and forth
across a floodplain.
Meandering river See Meandering stream.
Meandering stream A single, sinuous
channel of flowing water that moves
through gentle bends that migrate back
and forth across a floodplain.
Megafloods Extremely large floods capable of causing a catastrophe that may be
expected to reoccur every few hundred
years.
Megathrust earthquake A term used to
describe large earthquakes (Magnitude
8.0 and greater) that occur along low-angle faults at subduction zones.
Mesocyclone A region of rotating
clouds, typically around 3 to 10 km (~2 to
6 mi.) in diameter on the flank of a supercell thunderstorm.
Mesoscale convective system (MCS)
A large, circular, long-lived and interacting group of thunderstorms that are commonly identifiable on satellite images;
generally contain severe thunderstorms
and often produce tornadoes.
Mesosphere A zone in the upper atmosphere from about 50 to 80 km (~31 to 50
mi.) where the temperature decreases with
altitude; directly above the stratosphere.
Metamorphic rock An aggregate of
minerals that formed beneath Earth’s surface from preexisting rock by the effects
of heat, pressure, and chemically active
fluids. In foliated metamorphic rocks,
mineral grains are preferentially aligned
parallel to one another or light and dark
minerals are segregated into bands; nonfoliated metamorphic rocks have neither
characteristic.
Meteor Extraterrestial particle from dust
to several centimeters in size that is burned
up from frictional heat upon entry into
Earth’s atmosphere; light emitted from the
burning meteor forms a “shooting star.”

Meteorite Extraterrestrial particle from
dust to asteroid size that hits Earth’s surface.
Meteoroid Extraterrestrial particle in
space that is smaller than 10 m and larger
than dust size; may form from breakup of
asteroids.
Meteor shower Large numbers of meteoroids entering the atmosphere and producing light from frictional heating; commonly occurs when Earth’s orbit passes
through the tail of a comet.
Microburst A strong, localized downburst from a thunderstorm that affects an
area of 4 km (~2.5 mi.) or less.
Microearthquake Sudden movement
along a geologic fracture or fault in which
the release of energy has a moment magnitude less than 3. Patterns of these very
small quakes may be precursors to largermagnitude earthquakes; also called a very
minor earthquake.
Microzonation With respect to earthquakes, the detailed location and land-use
planning of areas having various earthquake hazards.
Mid-ocean ridge A seafloor mountain
range commonly found in the central part
of oceans characterized by seafloor spreading. An example is the Mid-Atlantic Ridge.
Milankovitch cycles Natural ­variations
in the intensity of solar radiation that
reaches Earth’s surface that recur at
a­pproximately 20,000-, 40,000-, and
100,000-year intervals.
Mineral A naturally occurring, inorganic
crystalline substance with an ordered
atomic structure and a specific chemical
composition.
Mitigation The avoidance of, lessening,
or compensation for anticipated harmful
effects of an action, especially with respect
to the natural environment.
Modified Mercalli Intensity scale A
scale with 12 divisions on which the
amount and severity of shaking and damage from an earthquake can be ranked.
Moho See Mohorovičić discontinuity.
Mohorovičić discontinuity Compositional boundary that separates the less
dense crust from the underlying higherdensity mantle.
Moisture content The amount of water
in a soil as a percent (weight of water to
weight of the solid soil particles as a percent). Also called the water content.
Moment magnitude scale A numerical
assessment of the amount of energy released by an earthquake on the basis of its

seismic moment, which is the product of
the average amount of slip on the fault
that produced the earthquake, the area
that actually ruptured, and the shear modulus of the rocks that failed.
Mudflow A slurry of fine, unconsolidated
earth materials and water that flows rapidly
downslope or down a channel; also referred
to as an earth flow or type of debris flow.
Multiple hypotheses Several proposed
explanations for a phenomenon that are
being considered concurrently; also referred to as multiple working hypotheses.
Natural hazard A potential danger that
poses a threat to people or property that
exists or is caused by nature; generally one
that is not made or caused by humans.
Natural service function A benefit that
arises from an event caused by nature that is
also a hazard to people or the environment.
Near-Earth Asteroid Tracking Project
(NEAT) NASA-supported program started in 1996 to study the size, distribution,
and dynamic processes associated with
near-Earth objects and to identify those
objects having a diameter of about 1 km
(~3300 ft.) or greater.
Near-Earth object (NEO) Asteroids that
reside and orbit between Earth and the
sun or have orbits that intersect Earth’s
orbit.
Newton Measure of force in the metric
system in which 1 Newton is the force
necessary to accelerate a 1 kg (~2.2 lb.)
mass 1 m (~3.3 ft.) per second each second
that it is in motion.
Nor’easter An extratropical cyclone that
tracks along the East Coast of the contiguous United States and Canada and which
has continuously blowing northeasterly
winds just ahead of the storm; commonly
characterized by hurricane-strength winds,
heavy snows, intense precipitation, and
large waves along the coast.
Normal fault A generally steep fracture or
fracture system along which the hanging
wall has moved down relative to the footwall in the direction that the fracture dips.
North Atlantic Oscillation A North Atlantic atmospheric pressure phenomenon
that fluctuates in intensity affecting the
path of westerly winds that flow from
North America toward northern Europe
and consequently the winter weather on
these two continents.
Nowcasting With respect to weather,
the near real-time prediction of the movement and behavior of weather systems,



---

624   Glossary
such as severe storms, that have formed or
are in the process of forming.
Nuée ardente See Pyroclastic flow
O soil horizon Soil horizon that contains
plant litter and other organic material;
found above the A soil horizon.
Ocean acidification An increase in the
acidity of seawater due primarily to dissolving additional atmospheric carbon dioxide. This condition results from higher
levels of carbon dioxide in the atmosphere
and is threatening the growth of corals
and other marine organisms that make
skeletal material from calcium carbonate.
Ocean conveyor belt A large-scale circulation pattern in the Atlantic, Indian,
and southwestern Pacific Oceans that is
driven by differences in water temperature and density; also referred to as a
­thermohaline current. Northward-flowing
warm water in this current maintains the
moderate climate of northern Europe; if
this current were to slow or stop, ice-age
conditions might return to portions of the
Northern Hemisphere.
Occluded front An atmospheric boundary between two air masses of contrasting
temperature along which the colder air
mass advances into a cool air mass and elevates warmer air above the boundary.
Organic soil A surficial, in-place accumulation of partially decayed plant material, such as peat.
Organizational stage With regard to
tornado development, the initial phase in
which a funnel cloud forms from rotating
winds within a severe thunderstorm.
Outer core See Core.
Outflow boundary A line separating
thunderstorm-cooled air from the surrounding air. This line behaves like a cold
front with both a wind shift and drop in
temperature. In large thunderstorm complexes this boundary may persist for more
than 24 hours and provide instability that
fosters development of new storms.
Overbank flow Moving floodwaters that
have exceeded the capacity of a stream
channel and have spread outward beyond
its sides.
Overwash Flooding of the beach, dunes,
and in some cases an entire barrier island
by a storm surge; commonly transports
sand from the beach and dunes inland; see
Washover channel.
Ozone (O3) A pungent, strongly reactive
form of oxygen gas that is most abundant

in the stratosphere, where it protects the
Earth surface from high levels of ultraviolet radiation. In the lower atmosphere it is
an air pollutant.
Ozone depletion Stratospheric loss of
triatomic oxygen; related to release of
halocarbons and other gases that destroy
triatomic oxygen in the atmosphere.
P wave The fastest type of earthquake
vibration within the body of Earth; also
called a primary or compressional wave, this
vibration oscillates in the direction that it
travels.
Pahoehoe A type of lava with a smooth
to ropy surface texture that forms when a
flow solidifies; most common in basalt.
Paleomagnetism Refers to the study of
magnetism of rocks and the intensity and
direction of the magnetic field of Earth in
the geologic past.
Paleoseismicity The occurrence of
earthquakes in space and time in the geologic past.
Parcel With respect to weather, an imaginary balloonlike blob of air several hundred cubic meters in volume that acts independently of the surrounding air.
Parent isotope An unstable variety of a
chemical element distinguished by its
number of neutrons and its potential for
radioactive decay. Measuring the proportions of parent and daughter isotopes in a
substance is the basis for most absolute
dating.
Particulate With respect to the atmosphere, a piece of dust, pollen, soot, ash, or
other very fine material that is suspended
in the air.
Peat Organic-rich sediment composed
primarily of partially decayed plant material, such as moss, leaves, and sticks; typically forms in bogs and swamps and has a
high water content.
Peds The natural soil fragment or clod,
the soil naturally breaks into these fragments that children may call dirt clods.
Peléan-type eruption A severe volcanic
eruption that ranks between 3 and 4 on
the volcanic explosivity index. This eruption type is characterized by a moderate
to large eruption column with plume
heights between 3 and 15 km (~2–9 mi.)
and are typical of lava dome eruptions
triggered by dome collapse.
Permafrost Earth material in the ground
with a temperature that is below freezing
continuously for at least two years; com-

monly cemented with ice; underlies about
20 percent of the world’s land area.
Permanent gas A substance in the atmosphere, such as nitrogen or oxygen,
that cannot be compressed with pressure
alone and is present in generally constant
proportions.
Permeability A measure of the ability of
an earth material to transmit fluids such as
water or oil.
Phreatomagmatic See Vulcanian-type
eruption.
Pineapple Express An atmospheric
flow of tropical moisture and extratropical
storms originating in the region of the
Hawaiian Islands and directed toward the
West Coast of the United States.
Plate tectonics An all-encompassing,
global scientific theory that explains the
characteristics and origin of most earthquakes, volcanoes, and other Earth features; based on the subdivision of the entire lithosphere into more than a dozen
large, rigid blocks that move horizontally
and interact to cause deformation, melting, and other changes.
Pleistocene Epoch Geologically recent
interval of Earth history characterized by
widespread continental glaciation and
commonly referred to as the “Ice Age”;
the subdivision of the Quaternary Period
before the Holocene Epoch.
Plinian-type eruptions A cataclysmic
volcanic eruption that ranks from 4 to 6
on the volcanic explosivity index. This
eruption type is characterized by a towering eruption column with a plume height
that may exceed 35 km (~22 mi.) and is
typical of stratovolcano eruptions.
Plunging breaker A type of water wave
in which the wave crest rises up and falls
into its own trough as it strikes a relatively
steep shoreline; tends to be associated
with beach erosion.
Point bar Accumulation of sand and other sediment on the inside of a meander
bend of a stream.
Polar desert An arid region near the
North or South Pole where the annual
precipitation averages less than 250 mm
(~10 in.) per year.
Polar jet stream A concentrated flow of
air near the top of the troposphere that, in
the Northern Hemisphere, typically crosses
southern Canada or the northern conterminous United States and is characterized by
strong winds blowing from west to east.



---

Glossary   625
Pool Common bed form produced by
scour in meandering and straight stream
channels with relatively low channel
slope; characterized at low flow by slowmoving, deep water; generally, but not
exclusively, found on the outside of meander bends.
Porosity The percentage of void or empty space in earth material such as soil or
rock.
Potential energy The property of a system
that enables it to do work by virtue of an
object’s position above some reference level.
Potentially active With respect to earthquakes, a term applied to a geologic fault
that shows evidence of movement in the
Pleistocene Epoch, but not in the past
10,000 years.
Power With respect to physics, the rate
of doing work.
Precursor With respect to natural hazards, a physical, chemical, or biological
phenomenon that occurs before a hazardous event such as an earthquake, volcanic
eruption, or landslide.
Precursor event See Precursor.
Prediction With respect to a hazardous
event, such as an earthquake, the advance
determination of the date, time, and size
of the event.
Preignition Initial phase of a wildfire during which fuel is brought to a temperature
and water content that favors ignition; involves both preheating and pyrolysis.
Preparedness A state of readiness
achieved by an individual or group of
people, such as for a natural hazard or
war; see Disaster preparedness.
Prescribed burn A fire purposely set and
contained within a designated area to reduce the amount of fuel available for a
wildfire; also called a controlled burn.
Primary effect See Direct effect.
Prismatic structure A soil structure
consisting of clay-rich columns.
Process With respect to natural hazards,
the physical, chemical, or biological ways
by which events, such as volcanic eruptions, earthquakes, landslides, and floods,
affect Earth’s surface.

mountain building, canyon erosion, and
landscape construction was periodically
interrupted by catastrophic events that
caused rapid changes to topography or
biota.
Pyroclastic activity Eruptive or explosive volcanic activity in which all types of
volcanic debris, from ash to very large
particles, are physically blown from a volcanic vent.
Pyroclastic debris Volcanic material
that is explosively erupted from a volcano.
The material ranges in size from fine ash
to large blocks and may be composed of
low-density magma, which cooled in the
atmosphere during the eruption or of cold
dense rock ripped from the interior of the
volcano.
Pyroclastic deposit An accumulation of
particles forcefully ejected from a volcanic
vent, explosive in origin, containing volcanic ash particles and those of larger size
such as blocks and bombs.
Pyroclastic flow Extremely hot and
rapid flow of eruptive material down the
flank of a volcano; consists of volcanic
gases, ash, and other materials that move
rapidly; commonly forms upon the collapse of an eruption column; may also be
known as an ash flow, fiery cloud, or nuée
ardente.
Pyroclastic rock Lithified igneous earth
material composed of fragments of volcanic glass, mineral crystals, and fragments
of other rocks that were ejected during a
volcanic eruption.
Pyrolysis In a fire, a group of chemical
processes by which heat divides or splits
large fuel molecules into smaller ones.
Products from these processes include
volatile gases, mineral ash, tars, and carbonaceous char.
Quick clay Deposit of very fine sediment
that when disturbed, such as by seismic
shaking, may spontaneously liquefy and
lose all shear strength.
R soil horizon Consolidated bedrock
that underlies the soil.

Proxy data With respect to climate,
proxy data refers to data that is not strictly
climate in nature, but can be used to infer,
indirectly, changes in the climate record.

Radiation With respect to physics, the
transfer of energy as electromagnetic
waves or as moving subatomic particles. A
primary way that heat is transferred from
the sun to Earth, the Earth to the atmosphere, and to a lesser extent from a wildfire to the atmosphere.

Punctuated uniformitarianism Concept that the geologic record of gradual

Radioactive isotope Unstable variety of
a chemical element distinguished by its

number of neutrons and its emission of
ionizing radiation.
Rain band Precipitation-producing
clouds that spiral around a hurricane.
Reactive With respect to natural hazards, a person’s action or response to a
hazard that occurs after an event, such as a
natural accident, disaster, or catastrophe.
Recurrence interval The time between
natural events, such as floods or earthquakes. Commonly given as the average
recurrence interval, which is determined
by averaging a series of intervals between
events.
Red flag warning With respect to wildfires, a notice from the National Weather
Service or other governmental agency
that an extreme fire condition is occurring
or is likely to occur within 24 hours.
Refraction Bending of a waveform. With
respect to coastal processes, the bending
of surface waves as they enter shallow water. This takes place where part of the
wave “feels the bottom” and slows down
while the remainder of the wave continues to move forward at a faster velocity.
Relative dating Placing geologic features, objects, earth materials, or events in
chronological order in the geologic time
scale without determining absolute ages.
Relative hardness With respect to minerals, the ability of one mineral or object
to scratch another. Generally determined
on the Mohs Hardness Scale in which diamond is assigned the highest value of 10
and talc the lowest value of 1. Other minerals, such as corundum, quartz, and calcite, have intermediate values of relative
hardness.
Relative humidity A measure of the
amount of water vapor in the air compared
to the amount that would saturate the air at
a given temperature and pressure; commonly expressed as a percentage.
Relative sea level The local or regional
position of the sea at the shore that is influenced by uplift or subsidence of the
land and changes in global eustatic sea
level.
Relief With respect to topography, the
difference in elevation between a higher
point (as on a hilltop) and a lower one (as
on a valley floor).
Residence time In natural systems, the
duration that a substance spends at a specific place or in a specific compartment. A
measure of how long a chemical element,



---

626   Glossary
compound, or other substance stays in a
specific part of a system.
Resisting force With respect to mass
wasting, an influence that tends to oppose
downslope movement.
Resonance With respect to earthquakes,
the matching of the frequency of shaking
with the natural vibrational frequency of
an object. With respect to storm surge
and tsunamis, the superposition of waves
reflecting from one side of a narrow bay
or lake with waves of water moving in another direction.
Resurgent caldera Uplift by rising
magma of the central collapsed block of a
giant volcanic crater; the giant crater resulted from an earlier explosion or collapse of the volcano summit; see Caldera.
Retreat With respect to glaciers, an interval of time when the area occupied by
glacial ice decreased.
Retrofitting With respect to earthquakes,
the renovation and physical modification
of existing structures to withstand ground
shaking and other effects of an earthquake.
Return interval The recurrence interval
of wildfires; see Recurrence interval.
Return stroke In cloud-to-ground lightning, the component that involves the
downward flow of electrons. Electron
flow starts near the ground and is then
initiated at successively higher points in a
channel of ionized air. This flow produces
the first bright light in a lightning flash.
Reverse fault A generally steep fracture
or fracture system in which the hanging
wall has moved up relative to the footwall
in a direction opposite to the direction of
fault dip. A low-angle reverse fault is
called a thrust fault.
Rhyolite A fine-grained, light-colored
volcanic rock consisting of feldspar, ferromagnesian minerals, and quartz with a
relatively high silica content of about 70
percent; associated with volcanic events
that may be very explosive.
Richter scale A semi-qualitative measurement of the size of an earthquake
based on the amount of ground shaking as
determined by maximum S-wave amplitude measured on a seismogram. The
scale is not absolute and is affected by local geologic conditions and distance from
the earthquake epicenter.
Ridge With respect to weather, an elongated area of high atmospheric pressure.
Riffle A section of stream channel, which
at low flow is shallow, fast-flowing water
moving over a gravel streambed.

Rift In tectonics, a long, narrow trough
that is bounded by normal faults; a surface
rupture produced by extension of the
lithosphere.
Ring of Fire A popular name given to the
chain of volcanoes on islands and on the
continents surrounding the Pacific Ocean;
actual combustion is much less common
than the red-hot glow of lava fountains
and flows.
Rip current A seaward flow of water in a
confined narrow area from a beach to beyond the breaker zone.
Riprap Layer or assemblage of large broken stones placed to protect an embankment or shoreline against erosion by running water or breaking waves.
Risk From a natural hazards viewpoint,
risk may be considered as the product of
the probability of an event times the consequences.
Risk analysis An evaluation that estimates
the probability that an event will occur.
River A large, natural stream that carries a
considerable volume of flowing surface
water.
River system Consists of three zones from
headwater to river mouth known respectively as the zone of sediment and water
production, zone of transport of water and
sediment, and zone of sediment deposition.
Rock In geology, a solidified natural aggregate of one or more minerals, natural
glass, or solid organic remains. In engineering or design applications, a solidified
natural material that requires blasting for
excavation or removal.
Rock cycle Group of interrelated processes by which igneous, metamorphic,
and sedimentary rocks can each be produced from the others.
Rogue wave A single crest of an oscillation of the surface of the ocean that is
much higher than usual. Commonly
caused by the constructive interference of
smaller waves.
Rope stage With regard to tornado development, the final decaying phase in
which the upward-spiraling air in the funnel contacts down-drafts from the thunderstorm, and the tornado begins to move
erratically.
Rotational With respect to landslides, a
type of sliding downslope movement that
develops on a well-defined, upward-curving
slip surface in homogeneous earth material.
Rule of Vs As applied to topographic
maps, the principle that all contour lines

must form a shape similar to the letter V
when they cross a stream valley and that
the apex of the V points upstream.
Runoff Water moving over the surface of
Earth as overland flow on slopes or as
stream flow; that part of the hydrologic
cycle represented by precipitation or
snowmelt that results in overland flow to
a stream.
Runup With respect to waves, the up-rush
of the wave along the shore; also, the combined vertical and horizontal distance that
a tsunami moves inland from the shoreline.
S wave One type of earthquake vibration
that occurs within the body of Earth; also
called a secondary or shear wave, vibrations
from this type of oscillation are perpendicular to the direction that it travels.
Safety factor With respect to landsliding, the ratio of resisting to driving forces;
a safety factor greater than 1 suggests that
a slope is stable.
Sand storm High winds, generally in the
desert, that carry fine (~0.05 to 2 mm)
mineral particles to heights less than 2 m
(~7 ft.) above the ground.
Saturated flow Type of flow in soil
where all pores are filled with water.
Scale In maps, the relationship between
the distance between features on the map
and their actual distance apart on Earth’s
surface. Expressed either as a ratio, such as
1:24,000, or as a bar scale, a segmented
line on the map.
Scarp Steep slope or cliff at the head of a
landslide or along a geologic fault; produced by the downslope movement of a
slump or slide or by the displacement of
earth materials in an earthquake.
Scoria cone See Cinder cone.
Sea cliff Steep, commonly nearly vertical
bluff adjoining a beach or other coastal
environment; produced by a combination
of wave erosion and erosion from the
land, such as landsliding and runoff of surface water. Groundwater seepage may also
contribute to its development.
Seafloor spreading The plate tectonics
concept that new crust is continuously
added to the edges of lithospheric plates
at divergent plate boundaries as a result of
upwelling of magma along mid-ocean
ridges.
Sea wall A structure, commonly made of
concrete, large stone blocks, wood or steel
pilings, or cemented sand bags, built parallel to the coastline to protect buildings
and infrastructure from wave and flood
damage.



---

Glossary   627
Secondary effect With respect to natural
hazards, phenomena that are indirectly the
result of a hazardous event, such as wildfires
that are ignited by lava flows or water pollution that occurs after flooding has damaged
a sewage treatment plant; applies to those
phenomena that are caused by the primary
or direct effects of a hazardous event.
Secondary succession With respect to
ecology, the regular pattern of colonization
and replacement of plant species in a biological community that was partially destroyed in a hazardous event, such as a
wildfire, flood, or lengthy volcanic ash fall.
Sediment Transported particulate material created by weathering, biological activity, or chemical precipitation.
Sediment flushing With respect to aftermath of wildfire, the transport of sediment from slopes to stream channels and
eventually downstream from even moderate storm events.
Sedimentary rock Hard, compacted,
and cemented particulate material that
was created by weathering, biological activity, or chemical precipitation.
Seismic gap Area along an active fault
zone that is capable of producing a large
earthquake but has not produced one recently.
Seismic source The point at which vibrations from an earthquake are generated.
Seismic wave A periodic disturbance of
particles of Earth by an earthquake or
other vibration that is propagated without
the net movement of the particles.
Seismogram A written or digital graphic
record made by a seismograph.
Seismograph Instrument that records
earthquakes, also known as a seismometer.
Sensible heat Energy created by random molecular motion that may be physically sensed, such as by a thermometer.
Shake map A near-real-time, two-dimensional representation of the ground
motion and potential damage from an
earthquake.
Shear strength With respect to mass wasting, the internal resistance in earth material
to shear stress; on slopes this strength resists
failure by sliding or flowing.
Shield volcano A broad, convex volcanic
mountain built up by successive lava flows;
the largest type of volcanic mountain.
Shrinking stage With regard to tornado
development, late phase in which the tornado thins and begins to tilt as its supply
of warm moist air is reduced.

Shrink-swell clay Earth material composed of very fine mineral particles that
decrease in volume (shrink) when they are
dry and increase in volume (swell) when
they are wet.

Soil chronosequence A series of soils
arranged from youngest to oldest.

Silt particle Small particles 1/256 to
1/16 mm.

Soil horizons Collection of distinct soil
layers parallel to the surface (for example,
A, B, and C) each produced by vertical and
horizontal movements of the materials in
a soil that creates the soil profile.

Sinkhole Surface depression formed by
the dissolution of underlying sedimentary
rock or collapse over a subterranean void
such as a cave.
Slab avalanche Downslope movement
of snow and ice that starts out as large cohesive blocks; generally more dangerous
and damaging than loose-snow avalanches.
Sliding With respect to mass wasting, the
deformation or downslope movement of a
nearly intact block of earth material along
a slip surface.
Slip rate Long-term rate of displacement
along a fault; usually measured in millimeters or centimeters per year.
Slope The slant or incline of the land
surface.
Slope stability map A two-dimensional
representation of the land surface that
shows its susceptibility to mass wasting;
commonly based on a survey of past landslides that includes a measurement of the
steepness of the slopes on which they occurred.
Slow earthquake An event in which
rupture and movement along a geologic
fault takes days to months to complete;
also referred to as a silent earthquake.
Slump Type of landslide characterized by
downward slip of a mass of earth material,
generally along a curved slip surface.
Slump block A mass of landslide material that has moved downslope along a
curved slip surface.
Slumping With respect to mass wasting,
the downslope movement of rock, sediment, or soil along a curved slip surface.
Smectite A group of clay minerals with
extremely small crystals that can absorb
large amounts of water; common constituent of expansive soil.
Snow avalanche Rapid downslope
movement of snow, ice, and rock.
Soil In geology, a generally loose accumulation of surface earth material that has
been altered by chemical weathering in
the place that it accumulated and can
serve as a medium for plant growth. In engineering and design applications, unconsolidated surface material that can be excavated or removed without blasting.

Soil erosion Physical processes such as
running water and blowing wind that removes soil particles.

Soil slip A type of mass wasting event in
which a thin layer of unconsolidated earth
material slides downslope along a narrow
path; the slip surface for this type of mass
movement is commonly in weathered
slope deposits that are below the soil, but
above unweathered bedrock; in California
shallow soil slips are commonly referred
to as mudslides.
Soil taxonomy A classification of soils
used by agriculture and soil science.
Solar nebula A flattened, pancake-like
rotating disk of hydrogen and helium
dust. Around 5 billion years ago a solar
nebula condensed to form the sun by accretion of matter from its flattened disk.
Solutional sinkhole A depression that
develops by the dissolution of underlying
bedrock, such as limestone.
Sorting In sediment, size distribution of
the particles; poorly sorted sediment has a
large range in particle size, and in wellsorted sediment most particles are close to
the same size.
Spacewatch A program started in 1981
to inventory the region surrounding
Earth for asteroids and comets with the
goal of identifying those whose orbits
cross Earth’s orbit.
Specific gravity The density of a mineral or rock as compared with the density
of water.
Spilling breaker A type of water wave in
which the wave crest tumbles down its
own forward slope as it strikes a gently
sloping shoreline; tends to be associated
with deposition of sand on a beach.
Spit A claw-shaped, sandy to gravelly
coastal landform at the end of a peninsula
or island formed by accretion of beach
ridges in the direction of longshore drift.
Spot fire A small fire that is commonly
ignited by wind-blown burning embers
ahead of the flaming front of a wildfire.
Spreading center Synonymous with
mid-ocean ridges where new crust is continuously added to the edges of lithospheric plates.



---

628   Glossary
Spring With respect to groundwater, the
natural outflowing of subsurface water
where the groundwater system intersects
Earth’s surface.
Squall line A line of thunderstorms often accompanied by high wind and heavy
rain; commonly forms in advance of a
cold front or dryline and can produce
tornadoes.
Stable isotope A variety of a chemical
element distinguished by its number of
neutrons and one that does not naturally
undergo radioactive decay. In absolute
dating, the product of radioactive decay,
the daughter isotope, is stable.
Stage With respect to flowing water, the
height of the water in a stream channel.
Typically used in reference to the water
level at which flooding begins to occur.
Flood stage is commonly the water level
that is likely to cause damage to personal
property.
Stalactite A cylindrical or conical mineral deposit, most commonly of calcium
carbonate, that has grown downward
from the roof of a cave or other overhanging feature.
Stalagmite A column or ridge of mineral
deposits, most commonly calcium carbonate, on the floor of a cave; forms as water
drips from stalactites on the cave roof.
Stationary front Transition zone between two different air masses, commonly
with contrasting temperature, that moves
very little.
Stepped leader In cloud-to-ground
lightning, a channel of ionized air that approaches the ground in a series of nearly
invisible bursts. This channel becomes the
path for the luminous return stroke.
Storm surge Wind-driven ocean waves,
usually accompanying a hurricane,
nor’easter, or similar storm, that pile water up on a coastline.
Strain Change in shape or size of a material as a result of applied stress.
Stratosphere Zone in Earth’s atmosphere
above the troposphere where the air temperature is either constant or warms with
increasing altitude; contains significant
quantities of ozone, which protects life
from high levels of ultraviolet radiation.
Stratovolcano See Composite volcano.
Streak The color of a mineral in powdered form; commonly produced by rubbing a mineral specimen on an unglazed
ceramic plate.

Stream With respect to surface water, a
body of water that flows in a channel; includes a brook, creek, and river.
Strength In soil, the ability to resist deformation; the resistance results from cohesive and frictional forces in the soil. In
rock, the mechanically applied compression necessary to break or fracture a specimen.
Stress A type of physical force. In geological terms, stress is the force applied to rocks
that causes them to deform. The stress results from relative movement of plates along
tectonic boundaries can either be compressional, tensional, or shearing, corresponding
to convergent, divergent, and transform
plate boundaries, respectively.
Strike-slip fault A fracture or fracture
system along which a block of Earth’s
crust moves horizontally relative to the
block on the opposite side of the fracture.
Strike and dip Two parameters measured by geologists to describe the
­three-dimensional orientation of ­features,
including layering in rocks, that intersect
Earth’s surface.
Strombolian-type eruption A mildly
explosive volcanic eruption that ranks between 1 and 2 on the volcanic explosivity
index. This eruption type is characterized
by ejection of incandescent cinders that
usually accumulate around the vent region, giving rise to the formation of a cinder cone volcano.
Strong earthquake Earthquake in
which the release of energy has a moment
magnitude of 6 to 6.9.
Subduction zone Convergence of tectonic plates where one plate dives beneath
another and is consumed in the mantle.
Submarine landslide An underwater
mass movement, often caused by an
e­arthquake that can occur on a sloping
seafloor, such as the wall of an underwater
canyon, the continental slope, or the
­seafloor surrounding an island.
Submarine trench A relatively narrow,
long (often several 1000s of kilometers),
deep (often several kilometers) depression
on the ocean floor that forms as a result of
convergence of two tectonic plates with
subduction of one.
Subsidence Sinking, settling, or other
lowering of parts of the crust of Earth.
Subtropical jet stream A concentrated
flow of air near the top of the troposphere
that, in the Northern Hemisphere, typi-

cally crosses Mexico and Florida and is
characterized by strong winds blowing
from west to east.
Suction vortices Small, intense whirls
within a larger tornado; these rapidly rotating winds may cause much of the intense damage done by tornadoes.
Sulfur dioxide (SO2) A colorless gas with
a pungent odor similar to a burnt match.
This gas is emitted from volcanoes and
burning of fossil fuels, and when dissolved
in rainwater produces acid rain.
Supercell An unusually long-lived thunderstorm that has a rotating updraft that
continually redevelops on the storm’s flank.
Supercell storm See Supercell.
Surface fire Combustion in a wildfire that
takes place primarily along the ground and
consumes fuels such as grass, shrubs, dead
and downed limbs, and leaf litter.
Surface wave One type of vibration
produced by an earthquake in which the
oscillation moves along the boundary between the air and land; generally strongest close to the epicenter where it causes
much of the damage to buildings and
other structures.
Surf zone The area between the breaking
waves and the swash zone along the shore.
Swash zone Area along the shore where
waves run up on the beach face and then
back again into the water.
Swell With respect to coastal processes,
the sorting out of storm-produced waves
into wave sets having more or less ­uniform
heights and lengths. This sorting allows
groups of waves to move long distances
from storms to coastal areas with relatively little loss of energy.
Talus Pieces of rock that have accumulated at the base of a cliff or steep slope;
may form individual piles or a continuous
slope composed of blocks of rock that
have fallen from above.
Tectonic Pertaining to forces involved in
large-scale crustal movement.
Tectonic creep Slow, more or less continuous, movement along a fault; also
called fault creep.
Tectonic cycle A repetitive sequence of
events and processes that create and destroy the Earth’s crust and its ocean basins
and mountain ranges.
Tectonic framework The geometry and
spatial patterns of faults and seismic
sources that define a specific region.



---

Glossary   629
Tectonic plate A very large, fault-bounded block of Earth’s crust and underlying
upper mantle that moves as a coherent
mass on top of the asthenosphere; also
called a lithospheric plate. This type of block
commonly forms at a mid-ocean ridge and
is destroyed in a subduction zone.
Tectonic creep Slow, more or less continuous movement along a fault.
Tephra Any material ejected and physically blown out of a volcano; mostly ash.
Texture In rock, the size, shape, and arrangement of mineral grains and other
particles. In soil, the size distribution of
mineral and rock particles.
Thermal expansion With respect to the
ocean, the increase in volume of seawater
that occurs upon heating.
Thermal spring A place where heated
groundwater flows from rock, sediment, or
soil onto the land surface or into a body of
surface water such as a river, lake, or ocean.
Water temperature must be appreciably
higher than the mean annual air temperature; includes both warm springs and hot
springs.
Thermokarst An irregular terrain of
sinkholes and mounds formed by the
melting of permafrost.
Thermosphere With respect to weather,
the outermost part of the atmosphere
starting at about 80 km (~50 mi.) in altitude, characterized by an upward increase
in temperature and very little gas; directly
above the mesosphere.
Thrust fault A low-angle fracture or fractures in Earth’s crust where older rocks
are displaced over younger rocks.
Topographic map A two-dimensional
representation of the relief of Earth’s surface using contour lines of equal elevation.
Topographic profile A sideways view of
the land surface with a curving line drawn
to show how it changes in elevation from
one point to another point.
Tornado A destructive, commonly funnelshaped cloud of violently rotating winds
that extends downward from a severe
thunderstorm to reach Earth’s surface.
Total load With respect to stream processes, the sum of the dissolved, suspended,
and bed load that a stream or river carries.
Tower karst A landscape of steep-sided
hills rising above a plain or above sinkholes. Commonly developed by extensive
dissolution of limestone in a humid tropical climate.

Transform boundary Interface between
two tectonic plates where one plate slides
horizontally past another, such as along
the San Andreas fault in California; synonymous with a transform fault.
Transform fault Type of fracture of
Earth’s crust associated with offsets of
mid-ocean ridges where one plate is displaced horizontally; may form a plate
boundary, such as the San Andreas fault in
California.
Translational With respect to landslides,
a type of sliding downslope movement
that develops on a well-defined, planar
slip surface, most commonly in layered or
jointed rock or along a weak clay layer.
Triangulation The location of a point using distances from three points.
Tributary A small stream that flows into a
larger stream.
Triple junction Areas where three tectonic plates and their boundaries join.
Troglobite An organism, such as a blind
salamander, that lives its entire life underground in a cave.
Tropical cyclone General term for large
thunderstorm complexes rotating around
an area of low pressure that formed over
warm water in the tropics or subtropics;
includes low-intensity tropical ­depressions
and tropical storms and high-intensity
storms called hurricanes, typhoons, severe
tropical cyclones, or severe cyclonic
storms.
Tropical depression A circular atmospheric low-pressure center in the tropics
whose winds are less than 63 km (~39 mi.)
per hour; the threshold for a tropical
storm.
Tropical disturbance An area of disorganized, but persistent thunderstorms and
associated low-pressure trough that may
be in the formative stages of a tropical depression.
Tropical storm An organized system of
thunderstorms around a circular lowpressure center that derives its energy
from warm ocean waters and has winds
between 63 km (~39 mi.) and 119 km (~74
mi.) per hour; stronger than a tropical depression, but weaker than a hurricane.
Tropopause The boundary between the
troposphere and the stratosphere.
Troposphere Lowermost layer of the atmosphere defined by a general decrease in
temperature with increasing height above
Earth’s surface.

Trough With respect to weather, an elongated area of low atmospheric pressure.
Tsunami A series of gravity waves caused
by the large-scale displacement of the entire water column of the ocean or rarely a
large lake. Most commonly the result of a
large earthquake deforming the seafloor,
although may also be caused by a large underwater or seacliff landslide, collapse of
part of a volcano into the ocean, a submarine volcanic explosion, or the ocean impact of an asteroid. These waves are not
tidal waves and, in a few cases, are not seismic sea waves.
Tsunami ready A certification issued by
the National Oceanic and Atmospheric
Administration or other governmental
agency that a community has taken appropriate preparedness measures for future tsunamis.
Tsunami runup map A two-dimensional
representation of the level to which a tsunami has inundated or is projected to inundate a coastal zone.
Tsunami warning An emergency notice
from a governmental agency that a tsunami has been detected and is heading toward the area receiving the alert.
Tsunami watch An emergency notice
from a governmental agency that an event
has taken place—such as an earthquake,
landslide, or volcanic explosion—which
could cause a tsunami to strike the alerted
area.
Typhoon See Hurricane.
Ultra-Plinian-type eruption A supercolossal volcanic eruption that ranks between 6 and 7 on the volcanic explosivity
index. These eruptions occur when
s­tratovolcanoes or continental calderas
collapse during eruption causing the remaining subsurface magma to be forcibly
erupted.
Unified soil classification system
Classification of soils, widely used in engineering practice, based on amount of
coarse particles, fine particles, or organic
material.
Uniformitarianism Concept that the
present is the key to the past; that is, we
can read the geologic record by studying
present processes.
Unsaturated flow Type of flow in soil
where all pores are not filled with water.
Updrift Located in the direction from
which a water current, such as a longshore
current, is flowing.



---

630   Glossary
Uplift An increase in land elevation that is
commonly the result of tectonic or volcanic processes.
Urban heat island effect A local climatic condition resulting from various design
and land-use practices in a city, large
town, or other extensively developed area.
This condition can intensify heatwaves
and cause temperatures in a metropolitan
area to be up to 12°C (~22°F) warmer
than the surrounding rural area.

is shattered and expelled in clouds of very
fine particles into the atmosphere.
Volcanic dome Type of volcano characterized by very viscous magma with a high silica content; activity is generally explosive.
Volcanic Explosivity Index (VEI) A relative scale by which different volcanic eruptions can be compared based on quantitative
and qualitative measurement of explosivity,
including the plume height, volume of
ejected material, and frequency of eruption.

Valley fever A potentially fatal respiratory illness produced by fungal spores found
in desert soils. Mass wasting caused by
earthquakes can produce dust containing
these spores.

Volcanic vent Circular or elongated
opening through which lava and pyroclastic debris are erupted.

Variable gas With respect to the atmosphere, a substance such as carbon dioxide
or water vapor that cannot be compressed
with pressure alone and is present in
changing proportions.

Vulcanian-type eruption A mild to severe
explosive eruption that ranks between 2 and
3 on the volcanic explosivity index. These
eruptions are triggered by the interaction
between groundwater and rising magma,
which causes episodic violent explosions
when the water flashes to steam, in a process
known as a preatomagmatic eruption.

Varve A very thin sedimentary layer or
couplet of layers deposited in a single
year; in glacial lake deposits it consists of a
coarse layer deposited when ice melts in
the summer and a fine layer deposited
when very fine suspended sediment finally
settles out in the winter.
Vein In geology, the filling of a fracture
with earth material, especially minerals,
that differs from the surrounding rock.
Vertical exaggeration (VE) Vertical
scale of a diagram or model that is larger
than the horizontal scale; used in many
geologic cross-sections to show relationships more clearly.
Vertical wind shear A change in direction
or speed of the wind with changing altitude.
Very minor earthquake See Microearthquake.
Viscosity The ease of flow of a fluid; controlled by the magnitude of the internal
friction within a fluid. Thicker fluids have
greater viscosities.

Vortex With regard to weather, a spinning column of wind.

Wadati-Benioff Zone Inclined zone of
earthquakes produced as a tectonic plate
is subducted.
Wall cloud A localized, persistent, and
often abrupt lowering of condensed water
vapor from the rain-free base of a severe
thunderstorm. These clouds may rotate
and are commonly associated with tornado formation.
Warning With respect to natural hazards,
the announcement of a possible hazardous event, such as a large earthquake or
flood, that could occur in the near future.
Warm front An atmospheric boundary
between two air masses of contrasting
temperature along which the warmer air
mass advances into the cooler air mass.
Washover channel A narrow conduit of
water eroded through a beach or coastal
dunes by a storm surge.

Vog A volcanic fog or smog of sulfur dioxide and other volcanic gases combined
with aerosols of sulfuric acid and dust. In
Hawaii, vog is commonly an acrid blue
haze produced by gases emitted from
Kilauea volcano.

Watch With respect to weather, an alert
issued by a forecast office that meteorological conditions are favorable for severe
weather, such as a tornado, hurricane, severe thunderstorm, winter storm, or flash
flood.

Volatiles Chemical compounds, such as
water (H2O) or carbon dioxide (CO2),
that evaporate easily and exist in a gaseous
state at the Earth’s surface.

Water content The amount of water in a
soil as a percent (weight of water to weight
of the solid soil particles as a percent); also
called the moisture content.

Volcanic ash eruption An explosive volcanic event in which a tremendous quantity of fine-grained rock and volcanic glass

Water–energy–food nexus The concept that water, energy, and food are inextricably tied together such that changes in

one will effect change on one or both of
the other factors.
Watershed Land area that contributes
water to a particular stream system; generally used to describe the catchment area
for a smaller tributary stream; see Drainage basin.
Waterspout A weak, generally funnelshaped, wind vortex that most commonly
develops under fairweather conditions
and has all or part of its track over the sea
or a lake; some of these whirlwinds are
tornadoes that form or track over the
­water.
Water vapor The gas phase of water.
Watt The basic unit of power; generally expressed as units of energy per unit of time.
Wave front A long, continuous crest of a
single oscillation, such as one on the surface of a lake or ocean.
Wave height Vertical distance between the
crest and preceding trough of an oscillation,
such as one on the surface of a lake or ocean.
Wave normal An imaginary line perpendicular to the crest of an oscillation, such
as one on the surface of a lake or ocean.
Wave period The time in seconds for
successive crests of an oscillation, such as
one on the surface of a lake or ocean, to
pass a reference point; the inverse of the
frequency of the oscillation.
Wavelength With respect to physics, the
horizontal distance between two successive crests or troughs of an oscillation.
Used to describe characteristics of seismic, electromagnetic, water, and other
oscillations.
Weather Atmospheric conditions, such as
air temperature, humidity, and wind
speed, at any given time and place.
Weathering Changes that take place in
rocks and minerals at or near Earth’s surface in response to physical, chemical, and
biological activity; the physical, chemical,
and biological breakdown of rocks and
minerals.
Wicked Problem A natural hazard management problem that is resistant to resolution due to strong social, political, or
economic implications.
Wide Area Augmentation System
(WAAS) A network of global positioning
system (GPS) receivers at fixed locations
that act as reference points for transmitting positional corrections by satellite to
mobile GPS receivers. Operated by the
Federal Aviation Administration, this



---

Glossary   631
­system ­currently improves the accuracy of
locations obtained from WAAS-enabled
GPS receivers in the United States and
parts of Canada.
Wildfire Uncontrolled combustion or
burning of plants in a natural setting, such
as a forest, grassland, brushland, or tundra.
Wildland–urban interface The area where
natural undeveloped grassland, brushland, or
forest is adjacent to land developed for
homes, businesses, or recreation.

Wind chill The influence of wind on the
perception and actual effect of cold air.
Meteorological agencies in the United
States and Canada have developed a wind
chill index based on how the human body
loses heat in the cold and wind.
Wind shear A change in the speed or direction of the wind along a given direction.
Work In physics, the exertion of a force to
overcome resistance or to produce molecular change.

Younger Dryas A previously unexplained glacial interval from around
13,000 to 11,600 years ago that coincides with the extinction of many large
mammals in the Northern Hemisphere
and the disappearance of the Clovis culture. A recent hypothesis proposes that
these events were caused by the airburst
of a comet over the Canadian continental ice sheet.



---