# Chapter 7 Study Guide Answers: Mass Wasting

This companion document answers the questions in `NaturalHazards.Chapter7.StudyGuide.v2.md`. Answers are organized to match the study guide: section review questions first, then the chapter-wide comparison questions and the practice questions. Wording and figures were checked against the textbook chapter `NaturalHazards.Chapter7.md`.

## Answers to Section Review Questions

### 7.1 An Introduction to Landslides

1. **What are slope segments, and how do the common types differ?** Slope segments are the straight or curved parts that make up most natural slopes. The five common types are the **free face** (a steep or nearly vertical cliff of resistant rock, such as El Capitan in Yosemite), the **talus slope** (a pile of broken rock accumulated below a free face), the **upper convex slope** (rounded, curving outward near the top), the **straight slope** (a relatively uniform incline), and the **lower concave slope** (becoming gentler toward the base). Free faces and talus are favored by hard rock and arid conditions; convex, straight, and concave slopes are more common on softer rock with thicker soil and vegetation.
2. **What are the three main ways materials on a slope may fail?** **Falling** (material moves freely through the air), **sliding** (a coherent block moves along a planar or curved slip plane), and **flowing** (particles move about and mix within the mass). Many landslides are **complex**, combining sliding and flowing — for example, a slump that picks up water and changes into an earthflow.
3. **What is the safety factor, and how is it defined?** The safety factor is the ratio of resisting forces to driving forces: `SF = resisting forces / driving forces`. If `SF > 1`, resisting forces exceed driving forces and the slope is considered stable. If `SF < 1`, driving forces exceed resisting forces and failure can be expected. `SF = 1` is the threshold of failure. The driving force is the downslope component of the weight of slope material, and the main resisting force is the shear strength of the material along the potential slip plane.
4. **How do slumps differ from soil slips and rock slides?** Slumps are **rotational** slides: cohesive material rotates and slides along a **curved** slip surface, commonly producing small benches and blocks tilted upslope, and they are most common in unconsolidated material and weak rock such as mudstone and shale. Soil slips and rock slides are **translational** slides: they move along relatively **planar** surfaces. A soil slip is a shallow slide of soil or weathered material over bedrock, usually within colluvium; a rock slide is a coherent block of bedrock sliding on a planar surface such as bedding, foliation, or a fracture.
5. **How and where do debris flows occur?** A debris flow is a water-rich, fluid mixture of rocks, sand, mud, and water, intermediate between a landslide and a water flood (mudflows and lahars are examples). They develop when slopes become saturated during rainstorms and can move down established stream valleys, flow from channels filled with colluvium, or follow steep narrow tracks or chutes. They can be small or enormous (carrying cubic kilometers of material) and are capable of moving house-size boulders.
6. **What variables affect driving and resisting forces on slopes, and how are they interrelated?** The main variables are **type of earth material, slope angle and topography, climate, vegetation, water, and time**. They interact: steeper slopes increase the driving force; water adds weight (raising the driving force) and raises underground water pressure (lowering the resisting force); vegetation adds weight but also root strength; weathering over time reduces cohesion; and seasonal moisture changes alter both driving and resisting forces.
7. **What are the three ways vegetation affects slope stability?** (1) It provides a protective cover that cushions rainfall and reduces surface erosion. (2) Plant roots add strength and cohesion, anchoring soil. (3) Vegetation adds weight to a slope and can increase infiltration, sometimes lowering stability on shallow, steep slopes (for example, ice plant and chaparral in Southern California).
8. **What is the relationship between downslope force and normal force?** They are the two perpendicular components of gravity acting on a potential slide mass. The downslope (driving) force is parallel to the slip plane, while the normal force is perpendicular to it and adds frictional resistance. As slope angle increases, the downslope force increases and the normal force decreases, so the safety factor falls; as slope angle decreases, the opposite happens.
9. **What are the two types of snow avalanches, and how do they differ?** **Loose-snow avalanches** begin at a point and widen as they move downslope. **Slab avalanches** begin as cohesive blocks of snow and ice and are far more dangerous and damaging; they are generally triggered by overloading of a slope or by zones of weakness in the snowpack.

### 7.2 Geographic Regions at Risk from Landslides

1. **Describe the general distribution of regions at risk from mass wasting.** Landslides occur wherever significant slopes exist, but risk is greatest in mountainous, high-relief areas. In the contiguous United States the highest-risk areas are the coastal mountains of California, Oregon, Washington, and British Columbia; the Rocky Mountains; the Appalachian Mountains; and coastal cliffs and bluffs, with Alaska and other western mountain areas also at high risk. Flatter regions such as the Plains and Midwest have lower risk, as do parts of central Canada (Saskatchewan, Manitoba, and eastern Ontario).
2. **Why is there relatively little risk of mass wasting in the Midwest and Great Plains?** These regions are relatively flat with low topographic relief and few steep slopes. Gentle slopes mean small driving forces, and there are fewer free faces and unstable hillslopes to fail.
3. **How might urbanization increase or decrease slope stability?** Urbanization can **decrease** stability by grading (cut-and-fill), removing vegetation and root strength, adding fill and building weight, cutting into the toe of a slope or into a potential slip plane, and altering surface and subsurface drainage through irrigation, leaking pools, and septic systems. It can **increase** stability when development follows landslide hazard maps and grading codes and uses carefully designed drainage and slope-support structures, as in Los Angeles after its grading codes were adopted.

### 7.3 Effects of Landslides and Linkages with Other Natural Hazards

1. **How can large landslides be related to flooding and tsunamis?** A landslide can dam a river and cause flooding upstream; the natural dam can later fail and release a destructive flood downstream; and a slide entering a lake, reservoir, or bay can displace water and generate a tsunami. For example, in 1963 more than 2.5 km² of a hillside slid into the Vaiont reservoir in Italy, and the resulting flood killed more than 2,600 people.
2. **How large are the annual deaths and costs associated with landslides in the United States?** On average about **25 people** are killed each year, rising to **100 to 150** when collapses of trenches and other excavations are included. Annual damage exceeds **$2.8 billion** and may be as high as **$8.5 billion**.
3. **Explain how earthquakes, volcanoes, storms, fires, floods, and erosion can interact with landslides.** Earthquakes shake slopes and trigger rockfalls, soil slips, and debris flows. Volcanoes can produce landslides, debris avalanches, lahars, and mudflows. Storms and prolonged rain saturate slopes and raise pore-water pressure. Wildfires remove vegetation and roots, increasing erosion and the risk of later debris flows. Stream and wave erosion removes the toe of a slope, reducing stability and reactivating old slides. Landslides can in turn dam streams and cause floods, or generate tsunamis when they enter water.
4. **Use Oso or the I-70 rockfalls to explain why past events and site history matter.** **Oso (Washington, 2014):** the hillside above the community had long been called "Slide Hill," historic landslides were visible across the valley, and a 2010 study identified the slope as one of the most dangerous in the county. Much of the 2014 slide mass came from the 2006 Hazel landslide, and more than 200 percent of normal precipitation fell in the preceding 20 days. The slide killed 43 people and destroyed 49 homes and structures. **I-70 rockfalls (Colorado):** heavy rain and ice wedging in cracks dislodged large granite boulders from a free face more than 100 m above the highway with little warning; in the survivor account, a truck driver was struck by a boulder and survived. In both cases, the geologic setting and history pointed to the hazard before it struck.

### 7.4 Natural Service Functions of Landslides

1. **What natural service functions can landslides provide?** They create new habitats in forests and aquatic ecosystems (landslide-dammed lakes provide habitat for fish and other organisms), expose new rock and soil surfaces, and transport and concentrate mineral grains. These benefits do not remove the danger — a landslide-dammed lake can also fail and cause a flood.
2. **How do mass wasting and erosion help expose, transport, and concentrate valuable earth materials such as gold and diamonds?** Weathering frees mineral grains from rock; mass wasting transports those grains downslope; and heavier minerals become concentrated at the base of slopes and in adjacent stream deposits. Gold and diamonds have been mined from colluvium and debris-flow deposits, though they are less abundant there than in stream deposits.

### 7.5 Human Interaction with Landslides

1. **How does clear-cutting affect landslide occurrence?** Clear-cutting removes roots and protective cover. A 20-year study in the Oregon Cascades found that timber harvesting did **not** significantly increase landslide erosion on geologically stable land, but logging on weak, unstable slopes increased landslide erosion **several times** compared with unlogged slopes. Logging roads also interrupt drainage, alter subsurface water movement, and change the distribution of material through cut-and-fill.
2. **How does road construction increase landslide occurrence?** Roads can interrupt surface drainage, alter subsurface water movement, cut into the toe of a slope (removing support), add or remove material through cut-and-fill grading, and concentrate runoff. As the roadcut example shows, removing material at the toe can expose a slip plane and shorten its length, reducing the resisting force even though the cut also removes some weight.
3. **How has human activity in Rio de Janeiro contributed to devastating landslides?** Steep slopes of fractured granite with thin soil, periodic torrential rain, cut-and-fill construction, progressive removal of vegetation (logging, fuel, and agriculture), roads, and dense hillside development (including filling to enlarge building sites and cutting the bases of slopes) combine to create a severe hazard. In February 1988, more than 12 cm (~5 in.) of rain fell in 4 hours, causing flooding and debris flows that killed at least 90 people and left 3,000 homeless.
4. **How can human activities sometimes reduce landslide risk?** Following landslide hazard maps and grading codes, planning drainage carefully (surface and subsurface), building well-anchored retaining walls and other supports, and siting critical facilities away from unstable slopes can reduce risk. In the Los Angeles area, grading codes in effect since 1963 and detailed engineering-geology studies have reduced landslide damage.

### 7.6 Minimizing the Landslide Hazard

1. **What surface features are associated with landslides?** Geologists look for: irregular (hummocky) ground at the base of a slope; crescent-shaped cracks or terraces on a hillside; a tongue-shaped area of bare soil or rock; large boulders or talus piles below a cliff; a linear strip of cleared or disturbed vegetation down a slope; exposed bedding parallel to the slope; tongue-shaped sediment deposits at the base of a slope or at the mouth of a valley; cracks in the ground, walls, foundations, or roads; leaning structures, tilted trees, or fences; and unusually wet or green areas and small springs.
2. **What are the main steps for preventing landslides?** The first step is **identification** (geologic study and mapping). Then avoid or carefully evaluate loading the top of a slope, cutting into sensitive slopes, placing fill on unstable slopes, and changing water conditions. Common engineering techniques include **drainage control**, **grading**, and **slope supports** such as retaining walls. On high, steep slopes, benches or steps with surface drains are used instead of a single cut-and-fill.
3. **How can a cut-and-fill operation increase a slope's safety factor?** In a single cut-and-fill operation, material is removed from the upper part of a slope and placed near the base. This reduces the overall gradient, removes weight from the upper slope (reducing the driving force), and adds material at the toe (increasing the resisting force), raising the safety factor. The method is not practical on a high, steep slope, where benches are used instead.
4. **How do landslide inventory, hazard, and risk maps differ?** A **landslide inventory map** shows landslides that have occurred and may show their age or activity. A **landslide hazard map** shows areas likely to experience landslides and may recommend appropriate land uses. A **landslide risk map** is more complex because it combines the probability of landsliding with an assessment of potential losses. All are valuable for planning but do not replace a detailed, site-specific evaluation.
5. **How can warning systems reduce landslide losses even though they do not stop movement?** They provide time to evacuate people and possessions, stop trains, and reroute traffic. Methods include visual surveillance and field inspection; rain gauges and precipitation thresholds; shallow wells that monitor water in an existing slide; tiltmeters and other deformation instruments; geophones that detect vibrations; slide fences tied to railroad signals; and acoustic-flow monitors that detect debris flows and lahars. At Mount Rainier, the automated debris-flow warning system can give communities roughly 40 minutes to 3 hours of warning.

### 7.7 Perception of and Adjustment to the Landslide Hazard

1. **List five signs that could indicate a potential landslide when inspecting property.** Any five of: hummocky ground or old slide deposits; cracks in walls, foundations, roads, or the ground; retaining walls that lean or are cracked; doors or windows that stick or uneven floors; tilted trees or fences; springs or unusually wet or green areas; leaking pools or septic systems; taut or sagging utility wires; a tongue-shaped area of bare soil or rock; or a linear strip of disturbed vegetation down the slope.
2. **Except for buying property on low-relief ground, what is the best way to minimize the probability of a landslide on your land?** Keep water away from the slope and maintain/repair drainage systems, because increased underground water pressure is the most common cause of failure and drainage is usually the most effective stabilizer. In addition, do not cut into, load, or add fill to a slope without a professional evaluation. (The single best adjustment overall is to avoid developing or buying on a landslide-prone site in the first place.)
3. **Why may people resist technical information about landslide risk?** Large slides are infrequent, evidence of past movement may be hidden or not readily visible, and residents tend to think a slide "could happen on other hillsides, but never this one." Hillside property is highly desirable and valuable, so people may accept the risk or be unable to sell, and it often takes a catastrophic event to bring the problem to their attention.
4. **What precautions should people take when traveling in avalanche country?** Obtain forecasts from the nearest avalanche center; remember that changing weather affects the avalanche hazard; learn to evaluate snow conditions; travel with trained people; and learn avalanche rescue procedures, because most people who survive avalanches are rescued by others in their party.

## Answers to Chapter-Wide Comparison Questions

 1. **How do free faces, talus slopes, upper convex slopes, straight slopes, and lower concave slopes differ?** A free face is a steep or vertical cliff of resistant rock; a talus slope is broken rock accumulated below a free face; an upper convex slope curves outward near the top; a straight slope is a uniform incline; and a lower concave slope becomes gentler toward the base. Hard rock and arid conditions favor free faces and talus; softer rock, thicker soil, and vegetation favor the convex–straight–concave sequence.
 2. **How do falling, sliding, flowing, and complex movements differ?** In falling, material moves freely through the air (rockfall). In sliding, a coherent block moves along a planar or curved slip plane. In flowing, particles move about and mix throughout the moving mass (earthflow, debris flow, avalanche). A complex movement combines two or more mechanisms or changes from one type to another (for example, a slump that becomes an earthflow).
 3. **How do rockfalls, rock slides, slumps, soil slips, earthflows, debris flows, creep, and avalanches differ?** A rockfall is blocks of bedrock falling and accumulating as talus. A rock slide is a coherent bedrock block sliding on a planar surface. A slump is a rotational slide of cohesive material on a curved surface, forming benches. A soil slip is a shallow translational slide of soil over bedrock. An earthflow is a wet, partly cohesive, internally deformed mass. A debris flow is a water-rich mixture of rock, sand, mud, and water, intermediate between a landslide and a flood. Creep is very slow downslope movement that tilts poles and curves tree trunks. An avalanche is a rapid granular flow of snow, ice, and sometimes debris.
 4. **How do rotational and translational slides differ?** Rotational slides (slumps) move along curved slip surfaces and typically produce benches and blocks tilted upslope; they are common in unconsolidated material and weak rock. Translational slides move along relatively planar surfaces such as bedding planes, foliation planes, fractures, or weak clay layers, and include soil slips and rock slides.
 5. **How do driving force, resisting force, shear strength, normal force, and safety factor relate?** The driving force is the downslope component of the weight of the slope material. The resisting force opposes movement and comes mainly from the shear strength of the material along the potential slip plane. The normal force is the component of weight perpendicular to the slip plane, and it contributes frictional resistance. The safety factor is the ratio of resisting to driving forces (`SF > 1` stable, `SF < 1` failure expected).
 6. **How does increasing slope angle affect driving force, normal force, and stability?** As slope angle increases, the downslope (driving) force increases and the normal force decreases, which reduces frictional resistance; the safety factor therefore decreases and the slope becomes less stable. Decreasing the slope angle does the opposite.
 7. **How do earth material, permeability, daylighting beds, climate, vegetation, water, and time affect slope failure?** Earth material (mineral composition, cementation, consolidation, and zones of weakness) controls the type and frequency of failure. Permeability controls how water moves through a slope; permeable soil over less permeable bedrock traps water and promotes soil slips. Daylighting beds are bedding planes that intersect the surface on a slope and provide ready-made slip planes. Climate sets the amount, type, and timing of precipitation and the vegetation type. Vegetation both strengthens slopes (roots) and can weaken them (added weight and infiltration). Water adds weight and pore pressure and erodes slope toes. Time brings weathering and seasonal changes that steadily reduce resisting forces.
 8. **How do loose-snow and slab avalanches differ?** Loose-snow avalanches begin at a point and widen as they move downslope. Slab avalanches begin as cohesive blocks of snow and ice, are generally more dangerous and damaging, and are usually triggered by overloading of the slope or zones of weakness in the snowpack. Conditions that make avalanches more likely include slopes of about 35°–40°, wind loading on the leeward side of a ridge, rapid new snowfall, and rapid warming that makes snow wet.
 9. **Why are mountainous regions and coastal cliffs more vulnerable than low-relief plains?** Mountainous and coastal settings have steep slopes and high relief, producing large driving forces, abundant free faces and talus, exposure to stream and wave erosion at the toe, and greater landslide exposure. Low-relief plains have gentle slopes, few steep faces, and therefore small driving forces.
10. **How can earthquakes, volcanoes, storms, fires, and erosion trigger landslides?** Earthquakes shake slopes and trigger rockfalls, soil slips, and debris flows. Volcanoes can produce landslides, debris avalanches, lahars, and mudflows. Storms and prolonged rain saturate slopes and raise pore-water pressure. Wildfires destroy vegetation and roots, increasing erosion and later debris flows. Stream and wave erosion removes the toe of a slope, reducing support and reactivating old slides.
11. **How can a landslide cause flooding or a tsunami?** A landslide can dam a river, causing flooding upstream; the natural dam can later fail and release a destructive flood downstream; and a slide entering a lake, reservoir, or bay can displace water and generate a tsunami. The 1963 Vaiont reservoir slide in Italy, which killed more than 2,600 people, is a classic example.
12. **How can logging, roads, grading, fill, reservoirs, drainage, and urbanization affect landslide risk?** Clear-cutting removes root strength and protective cover, significantly increasing landslide erosion on weak, unstable slopes. Roads interrupt drainage, alter subsurface water, cut into toes, and concentrate runoff. Grading cuts and fills slopes, removing support and adding weight. Fill placed on the upper slope increases the driving force. Reservoirs can alter groundwater conditions along their shores and contribute to failure. Proper drainage lowers water pressure and increases stability. Urbanization can increase risk (grading, vegetation removal, added water) or decrease it (hazard maps, grading codes, engineered drainage and supports).
13. **How do inventory, hazard, and risk maps differ?** An inventory map shows landslides that have already occurred and may show their age or activity. A hazard map shows areas likely to experience landslides and may recommend land uses. A risk map combines the probability of landsliding with an assessment of potential losses.
14. **What surface features can reveal past or potential landslides?** Hummocky ground at the base of a slope; crescent-shaped cracks or terraces; tongue-shaped bare soil or rock; boulders or talus piles below cliffs; a linear strip of cleared or disturbed vegetation; exposed bedding parallel to the slope; tongue-shaped sediment deposits at the base of a slope or valley mouth; cracks in the ground, walls, foundations, or roads; leaning structures, tilted trees, or fences; and unusually wet or green areas and small springs.
15. **How do drainage, grading, retaining walls, warnings, and land-use planning reduce risk?** Drainage diverts surface water and removes groundwater, lowering pore pressure and increasing resisting forces. Grading (cut-and-fill) reduces slope angle, removes upper-slope weight, and adds toe support; benches reduce gradient and catch falling rock. Retaining walls, piles, and gabions support the slope. Warning systems provide time to evacuate, stop trains, and reroute traffic. Land-use planning keeps development out of high-hazard areas and sites critical facilities safely.
16. **Why is avoiding development usually preferable to stabilizing a large slide?** Avoidance is the cheapest and most reliable adjustment. Large natural slides are difficult or impossible to stop, correction is expensive and cannot guarantee success, and even "stabilized" slides can reactivate when water conditions change — as at Portuguese Bend and La Conchita.
17. **What should be investigated before buying property on a slope?** Have a professional geologist evaluate the property and surrounding slopes; consult city or county engineering departments; review landslide inventory, hazard, and risk maps; avoid homes at the mouth of a valley or canyon where debris flows may originate; look for hummocky ground, old slide deposits, tongue-shaped bare areas, cracks, tilted trees or fences, springs, and unusually wet areas; inspect retaining walls for leaning or cracking; check walls, foundations, and roads for cracks; treat sticking doors/windows and uneven floors as possible warning signs; check for leaking pools or septic systems and taut or sagging utility wires; and never assume an old or quiet landslide is permanently stable.
18. **How do La Conchita, Oso, Portuguese Bend, Thistle, Rio de Janeiro, and Mount Rainier illustrate the interaction of natural processes and human decisions?** **La Conchita:** repeated slides in an ancient landslide area; cracks and prior slides warned of the hazard, but development and a limited retaining wall left the community exposed, and the 2005 earthflow killed 10 people. **Oso:** a known "Slide Hill" with historic landslides and heavy rain; logging and the decision to allow development in the runout zone contributed to 43 deaths. **Portuguese Bend:** road construction, fill, and altered drainage reactivated an ancient slide, destroying more than 150 homes; later groundwater pumping partly stabilized it, but future water conditions remain important. **Thistle:** a reactivated old slide blocked a river and caused more than $400 million in losses, though prevention would have cost only about $300,000–$500,000. **Rio de Janeiro:** steep geology, intense rain, deforestation, and dense hillside development combined to produce deadly debris flows. **Mount Rainier:** automated debris-flow monitoring now provides 40 minutes to 3 hours of warning, showing how science and planning reduce losses.

## Answers to Practice Questions

 1. **Define mass wasting and explain how it differs from the chapter's narrower use of landslide.** In its broad sense, mass wasting is any downslope movement of earth materials driven by gravity. In its restricted sense — the sense used for "landslide" in this chapter — it is the rapid downslope movement of rock or soil as a more or less coherent mass. The chapter also uses "landslide" loosely to include earthflows, debris flows, rockfalls, and avalanches.
 2. **Describe the five common slope segments and the conditions that favor each.** Free face (steep or vertical cliff of resistant rock) and talus (broken rock below a free face) are favored by hard rock and arid conditions; the upper convex, straight, and lower concave segments are more common on softer rock with thicker soil and vegetation. Local conditions can produce exceptions.
 3. **Compare falling, sliding, flowing, and complex mass movements.** Falling is free fall through the air (rockfall); sliding is movement of a coherent mass along a planar or curved slip plane; flowing is movement in which particles mix within the mass (earthflow, debris flow, avalanche); complex movements combine two or more mechanisms or change from one to another.
 4. **Compare rockfalls, slumps, soil slips, rock slides, earthflows, debris flows, creep, and avalanches.** Rockfall: bedrock blocks fall and accumulate as talus. Slump: cohesive material rotates on a curved slip surface, forming benches (rotational slide). Soil slip: shallow translational slide of soil over bedrock. Rock slide: coherent bedrock block sliding on a planar surface. Earthflow: wet, partly cohesive, internally deformed mass of soil and weathered rock. Debris flow: water-rich rock/sand/mud mixture, intermediate between a landslide and a flood. Creep: very slow movement that tilts poles and curves tree trunks. Avalanche: rapid granular flow of snow, ice, and debris.
 5. **Explain the difference between rotational and translational slides.** Rotational slides (slumps) move along curved slip surfaces and typically form benches and upslope-tilted blocks. Translational slides move along planar surfaces such as bedding, foliation, fractures, or clay layers and include soil slips and rock slides.
 6. **Describe the variables used to classify downslope movements.** Four: the mechanism of movement (fall, slide, flow, or complex), the type of earth material (solid rock, soft consolidated sediment, or loose unconsolidated material), the amount of water present, and the rate of movement (rapid if visible to the naked eye, otherwise slow).
 7. **Explain the difference between driving and resisting forces on a slope.** Driving force is the downslope component of the weight of the slope material (including vegetation, fill, and buildings). Resisting force opposes movement and comes mainly from the shear strength of the material along the potential slip plane; the normal force adds frictional resistance and helps the material resist failure.
 8. **Calculate or interpret a safety factor and explain what values above or below 1 mean.** The safety factor is `SF = resisting forces / driving forces`. If resisting forces are 200 units and driving forces are 100, `SF = 2` (stable). If driving forces rise to 250, `SF = 0.8` (failure expected). `SF > 1` means stable, `SF < 1` means failure can be expected, and `SF = 1` is the threshold.
 9. **Explain how slope angle changes downslope and normal components of gravity.** Gravity can be divided into a downslope (driving) force parallel to the slip plane and a normal force perpendicular to it. As slope angle increases, the downslope component increases and the normal component decreases, reducing frictional resistance and lowering the safety factor; decreasing the angle does the opposite.
10. **Describe how rock type, bedding, foliation, fractures, clay, and permeability affect stability.** Weak or easily dissolved rocks (shale, mudstone, weak pyroclastic material, limestone) fail as creep, earthflows, debris flows, or slumps, whereas resistant rocks (well-cemented sandstone, granite) support steeper slopes. Bedding planes, foliation planes, fractures, faults, old slide surfaces, and weak clay layers are pre-existing zones of weakness; they are especially hazardous when they intersect (daylight) or parallel the slope. Permeability controls water movement: permeable soil over less permeable bedrock traps water, breaks down cohesion, and promotes soil slips. Quick or expansive clays can liquefy when disturbed.
11. **Explain the three ways vegetation can affect slope stability.** (1) It cushions rainfall and reduces surface erosion. (2) Roots add strength and cohesion. (3) It adds weight and can increase infiltration, sometimes lowering stability on shallow, steep slopes.
12. **Describe the three major ways water contributes to landslides.** (1) Rain saturates slopes and triggers shallow soil slips and debris flows. (2) Deep infiltration can cause slumps months or years later. (3) Stream, lake, and wave erosion removes the toe of a slope, steepens it, and can reactivate an old slide. Water also adds weight and raises underground water pressure, and can cause quick clay to liquefy.
13. **Compare loose-snow and slab avalanches and identify conditions that make avalanches more likely.** Loose-snow avalanches start at a point and widen; slab avalanches start as cohesive blocks and are more dangerous, usually triggered by overloading or snowpack weakness. Avalanches are more likely on slopes of about 35°–40° (generally steeper than 25°, while snow continually falls off steeper than about 50°–60°), when wind loads snow on the leeward side of a ridge, when rapid snowfall adds weight, and when rapid warming makes snow wet and unstable.
14. **Identify major geographic regions at risk from landslides in the United States, Canada, and worldwide.** In the United States: the coastal mountains of California, Oregon, and Washington; the Rocky Mountains; the Appalachian Mountains; coastal cliffs and bluffs; and Alaska and other western mountain areas. In Canada: the coastal mountains of British Columbia, while parts of central Canada (Saskatchewan, Manitoba, eastern Ontario) have lower risk. Worldwide: mountainous, high-relief, wet, and seismically active regions, with activity expected to increase as urbanization expands into landslide-prone areas, tree cutting continues, and climate change brings regional increases in precipitation.
15. **Describe direct and indirect effects of landslides.** Direct effects include people being struck or buried by debris; damage to homes, roads, utilities, and structures; blocked roads and railroads; blocked shipping lanes and river channels; and altered soil, vegetation, valleys, and coastlines. Indirect effects include upstream flooding from a landslide dam, later failure of that dam with downstream flooding, tsunamis generated by slides entering water, disease from disturbed fungal spores, and delays to commerce, rescue, and emergency response.
16. **Trace linkages among landslides, earthquakes, volcanoes, storms, fires, floods, and tsunamis.** Earthquakes trigger rockfalls, soil slips, and debris flows. Volcanoes produce debris avalanches, lahars, and mudflows. Storms and prolonged rain saturate slopes. Fires remove vegetation and roots, increasing erosion and debris flows. Floods and erosion remove the toe of slopes and reactivate old slides. Landslides can dam rivers to cause floods, and can generate tsunamis when they enter lakes, reservoirs, or bays.
17. **List the natural service functions of landslides.** They create new forest and aquatic habitats; landslide-dammed lakes provide fish habitat; they expose new rock and soil; and they transport and concentrate heavy minerals such as gold and diamonds at the base of slopes and in streams.
18. **Explain how clear-cutting, road construction, grading, fill, reservoirs, and urbanization affect landslide risk.** Clear-cutting removes roots and cover (significant on weak, unstable slopes). Roads interrupt drainage, alter subsurface water, cut toes, and concentrate runoff. Grading cuts and fills slopes, removing support and adding weight. Fill added to the upper slope increases the driving force. Reservoirs change groundwater conditions along the shore. Urbanization can increase risk through grading, vegetation removal, and water additions, or decrease it through hazard maps, grading codes, drainage, and slope supports.
19. **What evidence should a geologist look for when mapping potential landslides?** Sensitive earth materials such as clay and shale, plus surface features: hummocky ground, crescent-shaped cracks or terraces, tongue-shaped bare soil or rock, boulders or talus piles, a linear strip of disturbed vegetation, exposed bedrock parallel to the slope, tongue-shaped sediment deposits, cracks in the ground or structures, leaning structures or tilted trees and fences, and unusually wet or green areas and springs.
20. **Distinguish a landslide inventory map, hazard map, and risk map.** Inventory: shows landslides that have occurred and may show their relative activity. Hazard: shows areas likely to experience landslides and may recommend land uses. Risk: combines the probability of landsliding with an assessment of potential losses.
21. **Describe warning methods for landslides, debris flows, rockfalls, and avalanches.** Methods include visual surveillance and field inspection; rain-gauge networks with precipitation thresholds for shallow soil slips; shallow wells monitoring water in existing slides; tiltmeters and other deformation instruments; geophones detecting vibrations from moving rock; slide fences tied to railroad signals; and acoustic-flow monitors for debris flows and lahars. At Mount Rainier these systems can provide roughly 40 minutes to 3 hours of warning.
22. **Explain why drainage control is central to landslide prevention and correction.** Increased underground water pressure is the most common cause of slope failure, because water adds weight and reduces the resisting force. Drainage diverts surface runoff and removes groundwater, lowering water pressure and increasing the resisting force, so it is often the most effective way to prevent or stop movement.
23. **Evaluate the advantages and limits of grading, benches, retaining walls, piles, gabions, and debris basins.** Grading (cut-and-fill) reduces slope angle and moves weight from top to toe, raising the safety factor, but is impractical on high, steep slopes. Benches reduce steepness, improve drainage, and catch falling rock. Retaining walls, piles, and gabions support the slope but must be anchored below the base, backfilled with permeable material, and provided with drain holes; without drainage, water pressure can still cause failure. Debris basins and geofabric fencing catch falling material. Overall, engineering correction is often cost-effective (benefit-to-cost ratios of about 10:1 to 2,000:1) but expensive and unable to guarantee that a large natural slide will stop.
24. **Explain why avoiding development on an unstable slope is usually the best adjustment.** Avoidance is the cheapest and most reliable option. Large natural slides are difficult to prevent or stop, correction is expensive, and even stabilized slides can reactivate if water conditions change, as at Portuguese Bend and La Conchita.
25. **List signs and questions to investigate before buying a house on a slope.** Have a professional geologist evaluate the property; consult local engineering agencies; review landslide maps; avoid valley/canyon mouths; look for hummocky ground, old slide deposits, tongue-shaped bare areas, cracks, tilted trees or fences, springs, and wet areas; inspect retaining walls for leaning or cracking; check walls, foundations, and roads for cracks; be wary of sticking doors/windows and uneven floors; check for leaking pools or septic systems and taut or sagging wires; and remember that an old slide is not permanently stable.
26. **Describe what people should do before and during heavy rain or a landslide warning.** Keep water away from unstable slopes and maintain drainage systems; do not cut into, load, or add fill to a slope without professional evaluation; stay out of valleys, channels, chutes, and areas below unstable slopes during heavy rain or warnings; follow evacuation orders; and do not return until officials determine the area is safe.
27. **Explain avalanche safety practices.** Obtain forecasts from the nearest avalanche center; remember that changing weather affects the hazard; evaluate snow conditions before traveling; travel with trained people; and learn avalanche rescue procedures, because most people caught in avalanches who survive are rescued by their own party (and most fatal avalanches are triggered by the victims themselves or their party).
28. **Use La Conchita, Oso, Portuguese Bend, Thistle, Rio de Janeiro, or Mount Rainier to explain how science and human decisions affect landslide losses.** **La Conchita:** geologic studies and the 1995 slide showed the hazard, but development continued and a limited retaining wall failed to hold back the 2005 earthflow (10 deaths, ~30 homes destroyed or damaged). **Oso:** geologists and residents knew the slope as "Slide Hill," yet development proceeded in the runout zone; record rainfall (over 200 percent of normal in 20 days) triggered the slide that killed 43 people. **Portuguese Bend:** road building, fill, and altered drainage reactivated an ancient slide that destroyed more than 150 homes; later groundwater pumping stabilized it, showing that human action can both cause and reduce the hazard. **Thistle:** an old, recognized slide reactivated and blocked a river, causing more than $400 million in damage even though prevention would have cost about $300,000–$500,000. **Rio de Janeiro:** geology, intense rain, deforestation, and hillside development combined to kill at least 90 people in 1988. **Mount Rainier:** automated debris-flow monitoring now gives communities 40 minutes to 3 hours of warning.