# Appendices > Extracted from the fifth-edition PDF Markdown source. APPENDIX A: Minerals Characteristic Properties of Minerals Minerals are naturally occurring substances with defined physical and chemical properties. Physical properties are commonly used for identification, and chemical composition is the basis for mineral classification. Mineral properties can also have an aesthetic or utilitarian value. For example, native copper, when discovered in the Great Lakes region by Native Americans, was first valued for its metallic properties of luster and malleability and was used for jewelry and tools. Many ancient cultures, in addition to our own, have valued gemstones such as rubies and sapphires for their beauty. Historically, one of the most valued minerals has been halite (NaCl), or common table salt, because all animals, including people, need it to live. Halite has been mined and obtained from coastal evaporation ponds for thousands of years. Some clay minerals have been highly valued because they can be molded into useful pots and decorative statues and painted with black, red, and orange paint made from still other minerals. Today, specific physical properties of a wide variety of minerals are used for everything from ceramics to electronics, metallurgy, agriculture, water treatment, and personal jewelry. An understanding of the physical properties will help us identify common minerals. Identifying Minerals Identification of mineral specimens that are large enough to fit in the palm of your hand, referred to as hand ­specimens, is a combination of pattern recognition and testing for particular properties or characteristics. These TABLE A.1 Mohs Hardness Scale | Relative hardness | Hardness of common materials | Mineral | Comment | |------------------:|-----------------------------------|------------|------------------------------------------------------------------------------------------------------------| | 1 | Graphite, "lead" in pencils (1–2) | Talc | Softest mineral known; used to make powder | | 2 | Fingernail (~2.2) | Gypsum | Used to make wall board | | 3 | Copper penny² (~3.2) | Calcite | Main mineral in marble and limestone | | 4 | | Fluorite | Mined for fluorine, used in glass and enamel | | 5 | Pocketknife (~5.1); glass (~5.5) | Apatite | Enamel of your teeth | | 6 | Hard steel file or needle (~6.5) | Orthoclase | Common rock-forming mineral | | 7 | | Quartz | When purple is gemstone amethyst, birthstone for February | | 8 | | Topaz | When transparent is gemstone, birthstone for November | | 9 | | Corundum | When red is ruby, gemstone, birthstone for July; when blue is sapphire, gemstone, birthstone for September | | 10 | | Diamond | Hardest known mineral; gemstones are extremely brilliant | 1 Note: The scale is relative, and unit increases in hardness do not represent an equal increase in true hardness. 2 U.S. penny minted before 1982 or Canadian penny minted before 1997. 592 --- Identifying minerals   593 properties include hardness, specific gravity, fracture, cleavage, crystal form, color, luster, as well as other properties that are diagnostic of a particular mineral or group of minerals. Cleavage plane HARDNESS Most physical properties of minerals reflect characteristics of their internal atomic structure. For example, a mineral’s hardness, that is, its resistance to scratching, is related to the size, spacing, and strength of bonding of the atoms within the mineral. Absolute hardness is not easy to measure, so in 1822 Austrian mineralogist Friedrich Mohs came up with an ingenious 1–10 scale of relative hardness (H) that is still widely used. On the Mohs scale the softest mineral is talc (H = 1) and the hardest mineral is diamond (H = 10) (Table A.1). The change in absolute hardness from one number to the next is not the same; any mineral or substance will scratch a mineral with a lower hardness on the Mohs scale. You can use common objects, such as a fingernail (H about 2.2), a copper penny (a U.S. penny minted before 1982, or a Canadian penny minted before 1997, H about 3.2), and a glass plate (H = 5.5) to approximate the relative hardness of minerals. For example, you can determine if a mineral is harder or softer than 5.5 by taking a fresh, unweathered surface of the mineral and attempting to scratch a glass plate. You must be certain that the mineral is actually making a scratch and not just leaving a line of powdered mineral on the surface. Do not attempt to use a glass test plate if it becomes fractured or broken; be sure to follow proper procedures in disposing of broken or damaged glass. If the mineral is softer than glass, then you can determine if it is harder than a copper penny. By this process you can bracket the relative hardness (e.g., 73.2 and 65.5) of the unknown mineral sample. Hardness tests can sometimes be misleading, so never rely on hardness alone to identify a mineral. SPECIFIC GRAVITY Some minerals have unusually light or heavy specific gravity. The specific gravity of a mineral is the density of a mineral compared with the density of water. Water has a specific gravity of 1. The specific gravity of minerals varies from about 2.2 for halite to 19.3 for gold. Most minerals have specific gravities of about 2.5 to 4.5. In practice, we take a sample of a mineral in hand, heft it, and estimate if it has a light, medium, or heavy specific gravity. CLEAVAGE Many minerals have a distinctive way in which they break (Table A.2). Those that break along smooth, even planar surfaces are said to have cleavage. Cleavage (a) One direction of cleavage. Mineral examples: muscovite and biotite micas (b) Two directions of cleavage at right angles or nearly right angles. Mineral examples: feldspars, pyroxene (c) Two directions of cleavage not at right angles. Mineral example: amphibole (d) Three directions of cleavage at right angles. Mineral examples: halite, galena (e) Three directions of cleavage not at right angles. Mineral example: calcite ![Source PDF page 2, containing Figure A.1](../figures/appendices-page-02.jpg) Figure A.1 Common Types of Cleavage Except for mica, where cleavage is indicated with solid lines, cleavage directions in this diagram are shown with dashed lines. (a) Mica has one cleavage direction and breaks into sheets. (b) Feldspar and pyroxene have two cleavage directions at nearly right angles. (c) Amphibole has two directions of cleavage that do not meet at right angles. (d) Halite and galena have three cleavage directions that meet at right angles. This relationship causes both minerals to break into cube-shaped fragments. (e) Calcite also has three cleavage directions, but they do not meet at right angles. This relationship causes calcite to break into rhombohedrons. develops because the forces that bond atoms together in a crystalline structure are rarely equal in all directions. Most cleavage will appear as a series of small, reflective steps with each parallel step breaking along the same direction of weak bonding within the mineral. One technique for recognizing cleavage is to rotate the specimen in front of you with a light source, such as the Sun or a ceiling light, over your shoulders. Each time you encounter a direction of cleavage, the small, parallel cleavage steps will reflect light all at once. --- 594 TABLE A.2 Properties and Environmental Significance of Selected Common Minerals | Mineral class | Mineral | Chemical formula | Color | Hardness | Other characteristics | Comment | |-----------------------|------------------------|--------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------|---------:|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | Silicates | Plagioclase feldspar | (Na, Ca)Al(Si, Al)Si2O8 | Usually white or gray, but may be others | 6 | Two good cleavages at approximately 90°; may have fine striations on one cleavage surface | One of the most common rock-forming minerals; a group of feldspars ranging from sodium to calcium rich; important industrial minerals | | Silicates | Alkali feldspar | (Na, K)AlSi3O8 | Gray, white to pink or salmon color | 6 | Two good cleavages at 90°; may be translucent to opaque with glassy luster | One of the most common rock-forming minerals; used in porcelain and a wide variety of industrial processes | | Silicates | Quartz | SiO2 | Varies from colorless to white, gray, pink, purple, and several others, depending on impurities | 7 | Often has good crystal shape with six sides; conchoidal fracture; coarse crystalline varieties have glassy luster, and microcrystalline varieties have dull to waxy luster | Very common rock-forming mineral; resistant to most chemical weathering; basic constituent of glasses and fluxes; commonly used as an abrasive; colored varieties, such as amethyst and agate, are semiprecious gemstones | | Silicates | Pyroxene | (Ca, Mg, Fe)2Si2O6 | Usually greenish to black | 5–6 | Crystals are commonly short and stout; two cleavages at about 90° | Important group of rock-forming minerals; particularly common in igneous rocks; weathers rather quickly | | Silicates | Amphibole | (Na, Ca)2(Mg, Al, Fe)5Si8O22(OH)2 | Generally black to green | 5–6 | Distinguished from pyroxene by the cleavage angle being 120° rather than 90°; generally has better cleavage with higher luster than pyroxene | Important group of rock-forming minerals, particularly for igneous and metamorphic rocks; relatively nonresistant to weathering | | Silicates | Olivine | (Mg, Fe)2SiO4 | Generally green, but also may be yellowish | 6.5–7 | Conchoidal fracture; commonly occurs in aggregates of small glassy grains | Important rock-forming mineral, particularly for igneous and metamorphic rocks; relatively nonresistant to chemical weathering | | Silicates | Clay | Various hydrous aluminum silicates with elements such as Ca, Na, Fe, Mg, and K | Generally white, but may vary due to impurities | 1–2 | Generally found as soft, earthy masses composed of very fine grains; may have an earthy odor when moist; often difficult to identify the particular clay mineral from hand specimen | Clay minerals are very important from an environmental viewpoint; many uses in society today; clay-rich soils often have many engineering geology problems | | Silicates | Chlorite | Hydrous Mg, Fe, Al silicate | Green to dark green | 2–2.5 | Commonly in layered or scaly masses with one direction of cleavage; thin sheets are flexible and nonelastic; glassy to pearly luster | Important group of rock-forming minerals in metamorphic rocks | | Silicates | Talc | Mg3Si4O10(OH)2 | Pale green to white or gray | 1 | In layered or compact masses with one direction of cleavage; thin sheets flexible and nonelastic; soapy feel with pearly to greasy luster | Used to make ceramics, paint, paper roofing, cosmetics, and ornamental stone | | Silicates | Biotite (black mica) | K(Mg, Fe)3AlSi3O10(OH)2 | Black to dark brown or dark green | 2.5–3 | Breaks apart in parallel sheets as a result of excellent cleavage in one direction | Important rock-forming mineral common in igneous and metamorphic rocks | | Silicates | Muscovite (white mica) | KAl2(AlSi3O10)(OH)2 | White, light yellow, brown, pink, or green; colorless in cleavage sheets | 2–3 | Breaks apart into thin sheets due to excellent cleavage in one direction | Important rock-forming mineral, particularly for igneous and metamorphic rocks; used for several purposes, including industrial roofing materials, paint, and rubber | | Carbonates | Calcite | CaCO3 | Colorless to white, but may have a variety of colors resulting from impurities | 3 | Effervesces strongly in dilute hydrochloric acid; often breaks apart to characteristic rhombohedral pieces as a result of two good cleavages at 78°; transparent varieties display double refraction, where a single dot on white paper looks like two dots if viewed through the mineral | Main constituent of the important sedimentary rock limestone and the metamorphic rock marble; associated with a variety of environmental problems with these rocks, including the development of sinkholes; chemically weathers rapidly; used in a variety of industrial processes including asphalts, fertilizers, insecticides, cement, and plastics | | Carbonates | Dolomite | Ca, Mg (CO3)2 | Generally white, but may be a variety of other colors, including light brown and pink | 3.5–4 | When powdered, will slowly effervesce in dilute hydrochloric acid; two cleavages at 78°; may be transparent to translucent and have a vitreous to pearly luster | Common mineral in the sedimentary rock dolostone and dolomitic limestone | | Carbonates | Malachite | Cu2CO3(OH)2 | Bright to emerald green, may be dark green | 3.5–4 | Effervesces slightly in dilute hydrochloric acid and turns the acid solution green; often displays a swirling structure of light and darker green bands | Valued as a decorative stone; used in jewelry and as a copper ore | | Oxides and hydroxides | Hematite | Fe2O3 | Usually various shades of reddish brown to red to dark gray | 1–6.5 | Streak is dark red | A common mineral found in small amounts in many igneous rocks, particularly basaltic rocks | | Oxides and hydroxides | Magnetite | Fe3O4 | Black | 6 | Magnetic | The most important ore of iron | | Oxides and hydroxides | Goethite | FeO(OH) | Generally yellow to yellow-brown or black | 1–5.5 | Often present as earthy masses or in the form of a crust; streak is yellow-brown | Formerly referred to as limonite, which is now known to be relatively rare; often forms from the chemical weathering of iron minerals and is present as "rust" | | Sulfides | Pyrite | FeS2 | Generally a pale brassy yellow tarnishing to brown | 6–6.5 | Often present as well-formed cubic crystals with striations on crystal faces | Has been used in the production of sulfuric acid, but mostly known for adding sulfur content to coal and contributing to the formation of acid-rich waters that result from the weathering of the mineral | | Sulfides | Chalcopyrite | CuFeS2 | Dark brassy to golden yellow, tarnishing to iridescent films that are reddish to blue-purple | 3.5–4 | Easily disintegrates; greenish black streak lacks cleavage | An important ore of copper; often associated with gold and silver ores | | Sulfides | Galena | PbS | Silver gray | 2.5 | Gray to black streak; high specific gravity; general metallic luster; cubic cleavage | Primary ore of lead | | Sulfates | Gypsum | CaSO4 · 2H2O | Generally colorless to white, but may have a variety of colors due to impurities | 2 | Often is transparent to opaque with one perfect cleavage; may form fibrous crystals, but often is an earthy mass | Several industrial uses in making of plaster of Paris for construction material, fertilizer, and flux for pottery | | Sulfates | Anhydrite | CaSO4 | Commonly white or gray, but may be colorless | 3–3.5 | Commonly observed as massive fine aggregates; may be translucent to transparent | Used for the production of sulfuric acid, as a filler material in paper, and occasionally as an ornamental stone | | Halides | Fluorite | CaF2 | Variable colors, often purple, yellow, white, or green | 4 | Often observed as cubic crystals but may also be massive; four perfect cleavages | A number of industrial uses, including a flux in the metal industry, optical lenses, and in the production of hydrofluoric acid | | Halides | Halite | NaCl | Generally colorless to white, but may be a variety of colors due to impurities | 2.5 | Salty taste; often has a cubic form due to three perfect cleavages | Common table salt; makes up rock salt, which is used for ice removal and as a host for buried nuclear waste | | Native elements | Gold | Au | Gold yellow | 2.5–3 | Crystals are rare; extremely high specific gravity; very ductile and malleable | Primary use as a monetary standard; used in jewelry and in the manufacture of computer chips and other electronics | | Native elements | Diamond | C | Generally colorless, but also occurs in various shades of yellow, brown, blue, pink, green, and orange | 10 | Commonly cut to display brilliant luster | Used in industrial abrasives and, of course, jewelry | | Native elements | Graphite | C | Black to gray | 1–2 | Often occurs as foliated masses; black streak and marks paper as well; greasy feel | A variety of industrial uses including lubricants, dyes, and in pencils as the "lead" | | Native elements | Native sulfur | S | Usually yellow when pure, but may be brown to black from impurities | 1.5–2.5 | Characteristic sulfurous smell, similar to the smell of fireworks or gunpowder | Often a by-product of the oil industry; may be used to manufacture sulfuric acid | Modified after Davidson, J. P., Reed, W. E., and Davis, P. M. 1997. Exploring Earth. Upper Saddle River, NJ: Prentice Hall; and Birchfield, B. C., Foster, R. J., Keller, E. A., Melhom, W. N., Brookins, D. G., Mintt, L. W., and Thurman, H. V. 1982. Physical geology. Columbus, OH: Charles E. Merrill. 597 --- 598   Appendix A Minerals Two characteristics of cleavage are important in identifying minerals: the number of cleavage directions and the angle between the cleavage directions. Minerals may have one, two, three, four, or six cleavage directions (Figure A.1). When you count the number of cleavage directions, be sure not to count the same direction twice because cleavage surfaces commonly repeat themselves on opposite sides of a specimen. In determining the ­angle between two cleavages, it is generally sufficient to decide whether or not the angle is close to 90°. In some specimens there is the potential to confuse a cleavage surface with a crystal face that developed as the crystal grew. Unlike cleavage surfaces, crystal faces are generally not repetitive, nor do they form imperfect steps. C RYSTAL FO RM Crystal faces are important, however, in recognizing crystal form, which is the geometric shape taken on by the mineral as it crystallized. Unfortunately, most mineral specimens do not show crystal form; however, when it is present, crystal form can be a useful diagnostic property for identifying some minerals. For example, the pointed and elongate hexagonal crystal form of quartz is diagnostic, as are the cube-shaped crystals of pyrite. Fr act ur e Broken surfaces of a mineral that do not show cleavage are described as fracture. The type of fracture that is present is generally important only if the specimen has no cleavage. Although most fracture is uneven or irregular, there are two types of fracture that are distinctive: conchoidal fracture and fibrous fracture. In conchoidal fracture, the broken surface is curved and can be at least partially smooth. A broken glass bottle will exhibit conchoidal fracture. In many cases a conchoidal fracture surface will reflect light continually as a specimen is rotated. C olo r The color of a mineral can be misleading because a given mineral may have several different colors, or several different minerals may have the same color. For example, depending on the impurities present, normally clear quartz may be white, pink, purple, red, brown, yellow, green, or black. A more reliable way of using color to identify some minerals is to powder part of a specimen on an unglazed porcelain plate. The resulting streak made by the powder is especially useful in identifying metallic minerals. For example, the hand specimen of the mineral hematite (Fe2O3) may be dull black or a shiny, dark metallic silver color, but its streak is always red. Streak tests can be done only for minerals that are softer than the porcelain “streak plate,” which typically has a hardness of about 6.5. Lu ste r Luster refers to the way light is reflected from a mineral. Most minerals can be described as having either a metallic or nonmetallic luster. Minerals with metallic luster appear silver, brass, or shiny, but not an enamel-like black, and are opaque, even on thin edges. Nonmetallic luster can be quite variable with descriptive terms for luster such as glassy, pearly, greasy, earthy, and resinous (like pine sap or dried turpentine). Light will also pass through thin edges of a nonmetallic mineral. Some minerals have more than one luster. For example, some specimens of graphite have metallic luster and others are nonmetallic. S teps i n Ident i fy i n g a Mi ne r al 1. Decide if the mineral has a metallic or a nonmetallic luster and refer to Table A.3. If the mineral is metallic, determine the color of its streak. 2. Determine the relative hardness of the specimen by first testing to see if it will scratch glass or alternatively, if it can be scratched by a knife (Table A.1). Note that if the mineral has a hardness close to that of glass (H about 5.5) or a pocket knife (H about 5.1), determining hardness may be difficult and the mineral may appear in Table A.3 in either the “harder than glass” or “softer than glass” sections. 3. Decide whether the mineral has cleavage. If ­cleavage is present, determine the number of directions and whether the angle between the directions is close to 90°. If cleavage is not present, determine the type of fracture. 4. Once you think you have identified the mineral, test for other physical or chemical properties (e.g., taste, smell, reaction with dilute hydrochloric acid) listed for that mineral in Tables A.2 and A.3 to confirm your identification. The identification of minerals in the field or in the laboratory from small specimens, using simple tests and perhaps a hand lens to determine cleavage and other properties, is basically a pattern recognition exercise. After you have looked at a number of minerals over a period of time and have learned how particular minerals vary, you will become more proficient at mineral identification. When positive identification is necessary, mineralogists use a variety of sophisticated analytical equipment to determine the chemical composition and internal structure of a sample. --- Identifying Minerals   599 TABLE A.3 Key to Assist Mineral Identification The original is a branching identification key. This Markdown table preserves its diagnostic descriptions and identified minerals, but not the graphic branch layout. | Diagnostic description | Mineral | |-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------| | White, pink, or salmon-colored; 2 cleavage planes at nearly right angles; hardness, 6. | Orthoclase (alkali feldspar) | | White, gray, or green-gray; 2 cleavage planes at nearly right angles; hardness, 6; striations on one cleavage. | Plagioclase | | White, clear, or any color; glassy luster; transparent to translucent; hexagonal (six-sided) crystals; hardness, 7; conchoidal fracture. | Quartz | | Various shades of green and yellowish green; glassy luster; granular masses and crystals in rocks; hardness, 6.5–7 (apparent hardness may be much less in granular masses). | Olivine | | Colorless to white; salty taste; 3 perfect cleavages forming cubic fragments; hardness, 2.5. | Halite | | White, yellow to colorless; 3 perfect cleavages forming rhombohedral fragments; hardness, 3; effervesces with dilute hydrochloric acid. | Calcite | | Pink, colorless, white, or brown; rhombohedral cleavage; hardness, 3.5–4; effervesces with dilute hydrochloric acid only if powdered. | Dolomite | | White to transparent; 1 perfect cleavage; hardness, 2. | Gypsum | | Pale green to white; feels soapy; 1 cleavage; hardness, 1. | Talc | | Colorless to light yellow, brown, or green; transparent in thin elastic sheets; 1 perfect cleavage; hardness, 2–3. | Muscovite | | Colorless to yellow to light blue, green, or purple; 4 cleavages not at 90°; hardness, 4. | Fluorite | | Pale green to white; feels soapy; hardness, 1. | Talc | | White to transparent; hardness, 2. | Gypsum | | Yellow; resinous luster; smells like fireworks; hardness, 1.5–2.5. | Sulfur | | Black to dark green; 2 cleavage planes at nearly 90°; hardness, 5–6. | Pyroxene | | Black to dark green; 2 cleavage planes about 120°; hardness, 5–6. | Amphibole | | Various shades of green and yellow; glassy to dull or waxy luster; granular masses and crystals in rocks; hardness, 6.5–7 (apparent hardness may be much less). | Olivine | | White, clear, or any color; glassy luster; transparent to opaque; hexagonal (six-sided) crystals; hardness, 7; conchoidal fracture. | Quartz | | Steel gray to black; reddish brown streak; earthy appearance; hardness, 5.5–6.5. | Hematite | | Black to brownish or greenish black; cleavage, 1 direction; hardness, 2.5–3. | Biotite | | Violet, yellow, green, pink, white, and colorless; 4 cleavages other than 90°; hardness, 4. | Fluorite | | Various shades of green; cleavage, 1 direction; in layered or scaly masses; hardness, 2–2.5. | Chlorite | | Dull to bright red; reddish brown streak; earthy appearance; hardness, 1–6.5. | Hematite | | Lead-pencil black; smudges fingers; hardness, 1–2; 1 cleavage that is apparent only in large crystals. | Graphite | | Yellow-brown to dark brown; may be almost black; streak yellow-brown; earthy; hardness (usually soft), 1–5.5. | Goethite | | Black; strongly magnetic; hardness, 6. | Magnetite | | Lead-pencil black; smudges fingers; hardness, 1–2; 1 cleavage that is apparent only in large crystals. | Graphite | | Light brass yellow; black streak; cubic crystals, commonly with striations; hardness, 6–6.5. | Pyrite | | Dark brass yellow; may be tarnished; greenish black streak; hardness, 3.5–4; massive. | Chalcopyrite | | Shiny gray; black streak; very heavy; cubic cleavage; hardness, 2.5. | Galena | Modified after Birchfield, B. C., Foster, R. J., Keller, E. A., Melhom, W. N., Brookins, D. G., Mintt, K. W., and Thurman, H. V. 1982. Physical geology. Columbus, OH: Charles E. Merrill; and Hamblin, W. K. and Howard, J. D. 2005. Exercises in physical geology, 12th ed. Upper Saddle River, NJ: Pearson Prentice Hall. --- APPENDIX B: Rocks In Chapter 1 we stated that a rock is an aggregate of one or more minerals. In traditional geologic investigations, this is the most commonly used definition. Another definition is used by geologists and geological engineers who are concerned with the properties of earth materials that affect engineering design and environmental problems associated with natural hazards. In environmental and engineering geology, the term rock is reserved for earth materials that cannot be removed without blasting using explosives, such as dynamite; earth materials that can be excavated with normal earth-moving equipment, for example a shovel or a bulldozer, are called soil. Thus, loosely compacted, poorly cemented sandstone may be considered a soil, whereas well-compacted clay may be called a rock. This pragmatic definition of rock described above also applies to the use of the term soil in engineering and environmental geologic reports. To an engineer, soil is earth material that can be excavated without blasting, regardless of whether it is on land or under water. In traditional geologic investigations the term soil is restricted to surficial earth material that has developed on land by in-place weathering. Other loose earth material not formed by in-place weathering is generally considered sediment, distinguished from sedimentary rock, which requires a hammer to break it apart. These differences in definitions are more than academic; they can affect communication between engineers, scientists, architects, planners, and emergency responders. metamorphic. Sometimes this decision is not as easy as you might think. It is particularly difficult to identify rocks that are fine grained. A specimen is considered fine grained if you cannot see individual mineral grains with your naked eye. Even if a specimen is not fine grained, it is advisable to use a magnifying glass or hand lens to examine a mineral or a rock. Some general rules of thumb will assist you in getting started: (1) If the sample is composed of bits and pieces of other rocks, it is most likely sedimentary, especially if the pieces have rounded corners; (2) if the rock specimen has minerals that are all oriented in the same direction, such as parallel sheets of mica, then it is most likely metamorphic; (3) if individual mineral crystals are relatively coarse grained, meaning you can see them with the naked eye or easily with a hand lens, and the crystals are interlocking and composed of ­minerals such as quartz and feldspar, it is probably an igneous rock that cooled underground, such as granite; (4) if the rock is mostly fine grained but contains some larger crystals, it is probably an igneous rock that cooled on or just below Earth’s surface; (5) if the rock is relatively soft and effervesces (fizzes) when dilute hydrochloric acid is applied, it is probably a carbonate sedimentary rock, such as limestone, or a metamorphic rock, such as marble; (6) if it is a really grungy, soft, highly weathered, or altered rock, as many in nature seem to be, you are going to have some problems identifying it! Identifying Rocks Physical properties of rocks include color, specific gravity, relative hardness, porosity, permeability, texture, and strength. Rocks are composed of minerals, and in some cases other substances such as natural glass and pieces of organisms. Whereas a few rocks contain essentially only one mineral, most contain several. As with minerals, the identification of hand specimens of rocks is primarily through the recognition of patterns. However, there are some useful hints that will assist you. The first task is to decide if the rock is igneous, sedimentary, or 600 Physical Properties of Rocks COLOR The color of a rock varies depending upon the minerals present and the amount of weathering that has occurred. Rocks encountered in their natural environment are various shades of light gray to brown to black. --- Physical Properties of Rocks   601 Pore space (a) Cement between grains Sand grain Porosity 25%–30% Porosity 10%–15% 0 2 mm Sand poorly sorted, Coarse sand, well sorted pore spaces of larger grains Porosity 5% Coarse sand, well most pore spaces (c) (b) ![Source PDF page 10, containing Figure B.1](../figures/appendices-page-10.jpg) FIGURE B.1 Sorting and Cementation Affect Porosity (a) Porosity of a well-sorted coarse sand. (b) Porosity of a poorly sorted sand. (c) Porosity of a cemented, ­well-sorted coarse sand; examples include calcium carbonate (CaCO3) and silica (SiO2). Chemical weathering can produce surface stains of black iron or manganese oxides, and brown to orange stains of iron oxides and hydroxides. SPECIFIC GRAVITY AND RELATIVE HARDNESS The same definition of specific gravity we gave for minerals applies to rocks and refers to the weight of the rock relative to the weight of water. Some rocks are lighter than water and will float. For example, pumice, a light-colored volcanic rock with many holes produced by gas bubbles, is lighter than water. Rocks become heavier as iron- and ­magnesium-bearing minerals become more abundant. There is no scale for relative rock hardness similar to the Mohs scale for minerals. Soft rocks can be broken with your ­fingers, and hard rocks require a sledgehammer to break apart. POROSITY AND PERMEABILITY Porosity is the percentage of a rock’s volume that is empty space, that is void space between grains, or open space in fractures. Permeability is the capacity of a porous rock to transmit a fluid, such as oil or water. The properties of porosity and permeability are important in understanding many geologic hazards, such as landsliding and earthquakes. TEXTURE The texture of a rock refers to the size, shape, and arrangement of the crystals or grains within it. Crystal or grain size can be measured directly or described qualitatively as fine or coarse grained. A rock with crystals or grains that are visible with the naked eye or easily with a magnifying hand lens is considered coarse grained; one that is not is fine grained. The degree to which grains in a rock are similar in size is referred to as sorting. For example, a sandstone in which most sand grains are similar in size is described as well sorted (Figure B.1a, c), whereas one that contains a variety of grain sizes is poorly sorted (Figure B.1b). Crystal or grain shape can vary from spherical with rounded corners to irregular with sharp, angular corners. Likewise, the arrangement of crystals or grains can be variable. Crystals can be interlocking, such as in granite or marble; aligned, such as mica grains in schist; or small grains may occupy spaces between large grains, such as in poorly sorted sandstone (Figure B.1b). Other rock textures are characterized by vesicles, or cavities, formed by gas expansion during their formation. For example, vesicular rocks commonly occur near the surface of a lava flow where gas is escaping. The texture of a rock can also influence its porosity. Rocks that are poorly sorted or well cemented generally will have lower porosity than those that are well sorted and only partially cemented. For example, in sandstone, porosity decreases from about 30 percent to 5 percent as pore spaces are filled with smaller grains or cementing material such as calcite (CaCO3) or quartz (SiO2) (Figure B.1). STRENGTH The strength of a rock is commonly reported in terms of its compressive strength, which is the mechanical compression necessary to break or fracture the specimen. As measured in the laboratory, the strength of a rock is given as a force per unit area, such as Newtons per meter squared (N/m2). Ranges of compressive strength for some of the common rock types are shown in Table B.1. --- 602   Appendix b Rocks TABLE B.1 Strength of Common Rock Types | Rock group | Rock type | Range of compressive strength (10^6 N/m²) | Comments | |-------------|-----------|------------------------------------------:|----------------------------------------------------------------------------------------------------------------| | Igneous | Granite | 100 to 280 | Finer-grained granites with few fractures are the strongest; generally suitable for most engineering purposes. | | Igneous | Basalt | 50 to greater than 280 | Zones of broken rock, holes, or fractures reduce the strength. | | Metamorphic | Marble | 100 to 125 | Solution openings and fractures weaken the rock. | | Metamorphic | Gneiss | 160 to 190 | Generally suitable for most engineering purposes. | | Metamorphic | Quartzite | 150 to 600 | Very strong rock. | | Sedimentary | Shale | Less than 2 to 215 | May be a very weak rock for engineering purposes; careful evaluation is necessary. | | Sedimentary | Limestone | 50 to 60 | May have clay layers, solution openings, or fractures that weaken the rock. | | Sedimentary | Sandstone | 40 to 110 | Strength varies with nature and extent of fractures, composition of grains, degree and type of mineral cement. | Data primarily from Bolz, R. E., and Tuve, G. L. eds. 1973. Handbook of tables for applied engineering science. Cleveland, OH: CRC Press. --- APPENDIX C: Maps and Related Topics Topographic Maps A topographic map depicts the shape and elevation of the land surface as well as the location of human-made features, such as roads, schools, and political boundaries. Elevations and some other natural features are shown by contour lines, which on topographic maps are lines of equal elevation above sea level. Individual contour lines on a topographic map are a fixed vertical distance apart, known as the contour interval (CI). Common contour intervals are 5, 10, 20, 40, 80, or 100 meters or feet. The contour interval selected for a particular map depends upon the topography being represented as well as the scale of the map. If there is relatively little difference in elevation, or relief, between the highest and lowest points on a map, then the contour interval selected may be relatively small; however, if there is higher relief, the map maker will choose a larger contour interval so that the reader can resolve individual contour lines on steep slopes. This brings up an important point: Where contour lines are close together, the inclination or slope of the land is relatively steep, compared to flatter areas where contour lines are farther apart. The scale of a topographic map, or of any other map, may be communicated in several ways. First, a scale may be given as a ratio, such as 1 to 24,000 (1:24,000), which means that 1 inch on a map is equal to 24,000 inches (2000 ft.) on the ground. Second, a topographic map may have several graphic or bar scales that are subdivided in feet, kilometers (kilometres), and miles. These scales are generally found in the lower margin of the map and are useful for measuring distances. Finally, the scale of some maps is stated in terms of specific units of length on the map, such as 1 inch equals 200 feet. This means that 1 inch on the map is equivalent to 200 feet (2400 in.) on the ground. In this example we could also state that the scale is 1 to 2400. The most common scale for U.S. Geological Survey topographic maps is 1:24,000 and for Natural Resources Canada maps is 1:50,000, but scales of 1:125,000 or smaller are also used. Remember, 1 ÷ 24,000 is a larger number than 1 ÷ 125,000, so 1:125,000 is the smaller scale. Generally, the smaller the map scale, the more area is shown. In addition to contour lines, topographic maps also show a number of cultural features, such as roads, houses, and other buildings. Features such as streams and rivers are often shown in blue. In fact, a whole series of symbols are commonly used on topographic maps. These symbols are shown in Figure C.1 for United States maps, and similar symbols are printed on the back of most Canadian maps. READING TOPOGRAPHIC MAPS Reading or interpreting topographic maps is as much an art as it is a science. After you have looked at many topographic maps that represent the variety of landforms and features found at the surface of Earth, you begin to recognize these forms by the shapes of the contours. This is a process that takes a fair amount of time and experience in looking at a variety of maps. However, there are some general rules for reading topographic maps: Valleys containing streams of any size have contours that form Vs, in which the apex of the V points in the upstream direction. This is sometimes known as the rule of Vs. Thus, if you are trying to draw the drainage pattern that shows all the streams, you should continue the stream in the upstream direction as long as the contours are still forming a V-pattern. The Vs will no longer be noticeable near the drainage divide, that is, the high point between two drainage basins. Where contour lines are spaced close together, the slope or inclination of the land surface is relatively steep, and where contour lines are spaced relatively far apart, the slope is relatively low and the land is flatter. As you travel up an incline or slope, you may notice that the contour lines are spaced relatively far apart and that at some point the angle of inclination or slope changes and the contour lines become closer together; this point is known as “break in slope.” A break in slope is commonly observed at the foot of a mountain or where a valley side meets the surface of a floodplain. Contours near the upper parts of hills or mountains may show closure; that is, the contour line meets itself rather than extending to the edge of the map. These closed contours may be relatively oval or round for a conical peak, or longer and 603 --- 604   Appendix C Maps and Related Topics Control data and monuments Vertical control Third order or better, with tablet Third order or better, recoverable mark Bench mark at found section corner Spot elevation BM × × BM × Rivers, lakes, and canals Intermittent stream Intermittent river Disappearing stream Perennial stream Perennial river Small falls; small rapids Large falls; large rapids Masonry dam 16.3 120.0 18.6 5.3 Contours Topographic Intermediate Index Supplementary Depression Dam with lock Dam carrying road Bathymetric Intermediate Index Primary Index primary Supplementary Perennial lake; Intermittent lake or pond Dry lake Narrow wash Wide wash Boundaries National State or territorial County or equivalent Civil township or equivalent Incorporated city or equivalent Park, reservation, or monument Dry lake Wide wash Well or spring; spring or seep Submerged areas and bogs Marsh or swamp Submerged marsh or swamp Wooded marsh or swamp Submerged wooded marsh or swamp Rice Surface features Levee Sand or mud area, dunes, or shifting sand Intricate surface area Gravel beach or glacial moraine Tailings pond Levee Land subject to inundation Max pool Strip mine Gravel Tailings pond Mines and caves Quarry or open pit mine Gravel, sand, clay, or borrow pit Mine dump Tailings Mine dump Tailings Vegetation Woods Scrub Orchard Vineyard Mangrove Mangrove Buildings and related features Building School; church Built-up area Racetrack Airport Landing strip Well (other than water); windmill Tanks Covered reservoir Gaging station Landmark object (feature as labeled) Campground; picnic area Cemetery: small; large Cem Roads and related features secondary, or light duty. They are all symbolized as light duty roads. Primary highway Secondary highway Light duty road Unimproved road Trail Dual highway Dual highway with median strip Contours and limits Form lines Marine shoreline Topographic maps Approximate mean high water Topographic-bathymetric maps Mean high water Apparent (edge of vegetation) Railroads and related features Standard gauge single track; station Standard gauge multiple track Abandoned M ud Coastal features ef Re Rock or coral reef Rock bare or awash Group of rocks bare or awash Exposed wreck Depth curve; sounding Breakwater, pier, jetty, or wharf Seawall 431 Sand * ********** 3 Transmission lines and pipelines Power transmission line; pole; tower Telephone line Aboveground oil or gas pipeline Underground oil or gas pipeline ![Source PDF page 13, containing Figure C.1](../figures/appendices-page-13.jpg) FIGURE C.1 Topographic Map Symbols This chart shows some of the common symbols that the U.S. Geological Survey uses on its topographic maps. Older maps may have slightly different symbols. Symbols on Canadian topographic maps are printed on the reverse side of the map. (From U.S. Geological Survey) Telephone Pipeline --- Topographic Maps   605 narrower for a ridge. Remember, the elevation of the top of a mountain or hill is higher than the last contour shown and may be estimated by taking half the contour interval and adding that value to the elevation of the highest contour line. That is, if the highest contour on a peak were 1000 m with a contour interval of 50 m, then the approximate elevation of the mountain top is 1025 m. Most topographic depressions are also shown with closed contours; the difference is that a depression contour has small hachure or tic marks on the contour lines that point toward the center of the depression. Sometimes the topography on a slope is unusually hummocky, that is, undulatory or uneven, when compared with the general topography of the area. Such hummocky topography may indicate mass wasting processes and landslide deposits. In summary, after observing a variety of topographic maps and working with them for some time, you will begin to see the pattern of contours as an actual landscape consisting of hills, valleys, and other features. LOCATING YOURSELF ON A MAP The first time you take a topographic map outdoors you may have some difficulty locating where you are. Determining where you are is crucial to mapping floodplains, landslides, or other features. One way to locate yourself on a map is to identify two or three distinctive features that you can see—such as a mountain peak, road intersection, or a prominent bend in a road or river—that are also shown on the map. You can then use a compass to determine your location. This is done by taking a compass direction, that is, a bearing, to each of the prominent features and drawing these bearings on the map; your location is where they intersect. Today we can also use orbiting global positioning system (GPS) satellites for locating ourselves and working with maps. Hand-held GPS satellite receivers are readily available at a very modest price and can generally locate your position on the ground with an accuracy of 5 to 10 m (16 to 33 ft.). GPS receivers work by receiving signals from at least four satellites and by calculating the distance from each satellite to your location. This is done with extremely accurate clocks that determine the time delay in receiving an expected radio signal from each satellite; the longer the delay, the further the satellite is from the receiver. With signals from at least four satellites, the computer in your GPS receiver can calculate a three-dimensional position on Earth’s surface. Using correction signals transmitted from the Federal Aviation Administration Wide Area Augmentation System (WAAS), the accuracy of defining your position can be reduced to less than 3 m (10 ft.) (Figure C.2). Another way to increase positional accuracy is to average multiple measurements taken at the same location over a period of 10 minutes or more. Your GPS receiver can be linked directly to geographic information system (GIS) computer software, so when a position is known, it may be plotted directly on a map viewed on a computer display in the receiver, your vehicle, or a laptop computer. GPS and GIS technology have revolutionized the way we do mapping in the field. A note here concerning the expression “in the field”; this expression means “outside on the surface on Earth,” not the “field of geology.” For example, a geologist about to go out and study landslides triggered by heavy rains in the Appalachians would say to her colleagues, “I am going into the field.” AN EXAMPLE FROM A COASTAL LANDSCAPE In this example we will look at topographic and cultural features in a coastal landscape (Figure C.3). If we were flying along the coast in a plane, we would have a sideways or oblique view of this landscape consisting of two hills with an intervening valley (Figure C.3a). The coast along the eastern hill to the right is a sea cliff, and in the center is a peninsula that was formed as a sand spit by longshore coastal processes. A hook on the end of the peninsula suggests that the direction of littoral sand transport in the surf zone and beach is from the east (right) to the west (left). The topographic map for the area has a contour interval (CI) of 20 ft. (Figure C.3b). This map indicates that the elevation of the highest hill on the east side of the map is approximately 290 ft. above sea level; the estimated elevation is half way between 280 ft., the highest contour on the hill, and 300 ft., which would be the next contour if the hill were higher. Three streams drain the hill, two into the ocean and one into a river; notice that in all three streams the topographic contours create a V in the upstream direction toward the top of the hill. Other information that may be “read” from the topographic map includes the following: The landform on the western portion of the map (left side) is a hill with elevation of about 275 ft., a gentle slope to the west, and a steep slope to the east (right) toward the ocean and the river valley. The eastern slope is particularly steep near the top of the mountain where the contours are very close together. A river in the center part of the map is flowing into a bay protected by a hooked sand bar. The relatively flat land next to the river is a narrow floodplain. Along the west side of the river is a light-duty road --- 606   Appendix C Maps and Related Topics Global positioning satellites 2. 1. 1. 2. 3. 4. 5. 6. Wide area reference station WAAS geostationary satellite Field receiver (mobile) hand-held GPS unit ![Source PDF page 15, containing Figure C.2](../figures/appendices-page-15.jpg) FIGURE C.2 Using the Global Positioning System U.S. Air Force NAVSTAR Global Positioning System (GPS) satellites orbit approximately 22,200 km (13,800 mi.) above Earth. Each satellite has its own distinct signal, shown with #1 yellow lines, which can be recognized by a hand-held GPS receiver. Using extremely accurate timekeeping, the receiver calculates the distance to a satellite by determining how long its signal took to arrive. A receiver must process signals from at least four satellites to calculate a three-dimensional position on Earth’s surface. As long as the satellite signal is not altered, most GPS receivers will achieve a positional accuracy of within 5 to 10 m (16 to 33 ft.) of their true position on Earth’s surface. Increased accuracy can be obtained with a correction signal obtained from a Wide Area Reference Station that is part of the Federal Aviation Administration’s Wide Area Augmentation System (WAAS). The WAAS reference station receives signals from the same GPS satellites, shown as #2 yellow lines, at the same time as the mobile GPS receiver. The WAAS then compares the position calculated from the GPS satellites with its true position to establish a correction factor. The correction factor is then transmitted to a WAAS geostationary satellite, shown as the #3 wavy line, and then broadcast back to the mobile GPS receiver, shown as the #4 wavy line. A GPS receiver with WAAS correction typically has a positional accuracy of within 3 m (10 ft.) or less of the true position. that parallels the river and then extends along the coastline. A second, unimproved road crosses the river and extends out to the head of the sand spit, providing access to a church and two other buildings. The floodplain is delineated on both sides of the river by the 20-ft. contour, above which to the 40-ft. contour is a break in slope at the edge of the valley. The eastern and southern slopes of the 275-foot hill on the west (left) side of the map have a number of small streams flowing mostly southward into the ocean. These streams appear to be relatively steep gullies that are eroding into the hillslope. In particular, the stream directly south of the 275-foot elevation mark appears to be cutting headward into the steep hillside. To continue a study of this area, we might construct an east–west topographic profile across the area along line A–A′. In constructing a profile we must decide how much to ­exaggerate the vertical scale of the view (Figure C.3c). The resulting vertical exaggeration (VE) is determined by comparing the same unit on both the vertical and the horizontal scale; for example, the number of feet represented by 1 inch on both scales. After constructing topographic profiles, your next task might be to obtain aerial photographs and geologic maps to draw more conclusions concerning the topography and geology. --- Topographic Maps   607 (a) A´ 0 26 260 A 200 275 20 200 10 0 100 20 Ocean 0 CI = 20 ft. Elev (ft.) 1 mile Elevation in feet above mean sea level (b) W 300 E A´ 200 A River 100 VE ~ = 21 0 Elev (ft.) N 0 300 1 2 Distance (mi.) 3 4 W E A´ A 0 0 VE ~ =7 1 2 Distance (mi.) (c) 3 4 ![Source PDF page 16, containing Figure C.3](../figures/appendices-page-16.jpg) FIGURE C.3 Topography of a Coastal Landscape (a) An oblique view of the coastline mapped in part (b) of this figure. (b) Topographic map for the same area with a contour interval of 20 ft. (c) Topographic profiles along line A–A′ of the topographic map shown with vertical exaggeration of approximately 21 times and 7 times. Vertical exaggeration is the ratio of the vertical to horizontal scale for the topographic profile. The ratio for the upper profile is approximately 21, so it has a vertical exaggeration of approximately 21 times. For the lower profile, the vertical exaggeration is about 7 times. In the real world, of course, there is no vertical exaggeration (the vertical and horizontal scales are the same). As an experiment, you might try to make a topographic profile along line A–A’ with no vertical exaggeration. What do you conclude? (From U.S. Geological Survey) --- 608   Appendix C Maps and Related Topics GEOLOGIC MAPS Geologists are interested in the types of earth materials found in a particular location and in their spatial distribution. Producing a geologic map is a very basic step in understanding the geology of an area. In constructing a geologic map a geologist must make interpretations that are consistent with the Earth history of the mapped area and the geologic processes that have been at work. Like any other scientific endeavor, each new observation becomes a test of the geologist’s interpretations, sometimes referred to as a working hypothesis. The first step in preparing a geologic map is to obtain a good base map (usually topographic) or aerial photograph on which the geologic information may be transferred. A geologist then goes into the field and makes observations of earth materials where they are exposed at or near the surface. From these exposures, called outcrops, the geologist groups distinctive rock or sediment types into separate units called formations. Each formation must be thick enough to be depicted on the scale that is being mapped. On the geologic map, each formation is separated from adjacent formations by boundaries known as contacts, which appear on the map as thin lines between swaths of different colors. As the geologist maps formations, he or she also takes measurements of the three-dimensional orientation, or attitude, of the mapped units. These measurements, referred to as strike and dip, are generally made with a compass and a device for measuring vertical 40 41 31 1 30 30 29 0 5 km E (a) 500 m E E´ angles known as a clinometer. Each strike and dip measurement is commonly recorded directly on the map with a T-shaped strike and dip symbol. Strike is the compass direction of the line formed by the intersection of layering in the earth material with a horizontal plane, and dip is the maximum angle that the layering makes with the horizontal (Figure C.4). For example, a simple geologic map of an area of approximately 1350 km2 (520 mi.2) might show three rock types—sandstone, conglomerate, and shale (Figure C.5a). 40 Strike and dip of sedimentary rock units (showing angle of dip); for example, at 1 on the map. The strike is northwest and dip is 40° to the northeast. Sandstone Shale (b) Horizontal plane ![Source PDF page 17, containing Figure C.4](../figures/appendices-page-17.jpg) FIGURE C.4 Strike and Dip Conglomerate Sea level Dip angle tion Idealized block diagram showing the strike and dip of inclined sedimentary rocks. Strike is the long direction of a T-shaped symbol, and the dip direction is shown with the short line perpendicular to strike. 40 N 30 Inclined rocks direc (a) A very simple, idealized geologic map showing three formations, each consisting of a different type of sedimentary rock. (b) Geologic crosssection and topographic profile along line E–E’. The cross-section shows that the pattern of the formations on the geologic map is one made by an arch-like geologic structure known as an anticline. 39 38 Dip ![Source PDF page 17, containing Figure C.5](../figures/appendices-page-17.jpg) FIGURE C.5 Geologic Map E´ 38 20 ike Str --- Summary   609 Mojave Desert Sa n Santa Monica Mountains San Fernando Valley San Gab riel M An dr ea sF au lt ounta ins Palos Verdes Hills Los Angeles Basin ![Source PDF page 18, containing Figure C.6](../figures/appendices-page-18.jpg) FIGURE C.6 Digital Elevation Model This digital elevation model (DEM) of the Los Angeles area is constructed as an oblique view from the ­southwest with a fair amount of vertical exaggeration. The flat, light blue area in the lower half of the image is the Los Angeles Basin. The smaller basin above the Los Angeles Basin is the San Fernando Valley, site of the epicenter of the 1994 Northridge earthquake. It is separated from the Los Angeles Basin by the Santa Monica Mountains. To the left of the Los Angeles Basin are the Palos Verdes Hills on the Palos Verdes Peninsula, the site of the Portuguese landslide discussed in Chapter 6. Finally, on the right side of the map is the San Andreas Fault, a strike-slip fault that is one of the major geologic hazards in southern California. (Courtesy of Robert Crippin, NASA, Jet Propulsion Laboratory) On this map the arrangement of the strike and dip symbols suggests the presence of an arch-like underground geologic structure known as an anticline. The nature of this structure becomes apparent in a geologic cross-section and topographic profile constructed along the line E–E′ (Figure C.5b). Geologists often make a series of cross-sections of a geologic map to better understand the geology of the area. Geologic maps at a variety of scales from 1:250,000 to 1:24,000 are generally available from a number of sources, including the U.S. Geological Survey. Digital Elevation Models Topographic data for the United States and many other parts of the world are now available as digital files. These data sets contain arrays of elevation values at a specific spacing, for example, the surface ground elevations on a 30 m grid for a 900 m2 (9700 ft.2) area. Computer programs are then used to synthesize and view the data; color shading may be added to show the topography. The resulting graphic representation of Earth’s surface is known as a digital elevation model (DEM). DEMs can be examined from a variety of angles using software such as Google Earth, and you may view the topography obliquely from any compass direction. The vertical dimension may also be exaggerated so that minor topographic differences may become more apparent. A DEM for the Los Angeles Basin clearly illustrates that Los Angeles is nearly surrounded by mountains and hills, which have been uplifted by recent tectonic activity (Figure C.6). DEMs are becoming important research tools in evaluating the topography of an area. They may be used to delineate and map features such as fault scarps, landslides, karst topography, floodplains, and volcanic and impact craters. Summary Several types of maps and images are useful in evaluating geology and Earth processes. Of particular importance are topographic maps and profiles, and geologic maps and cross-sections. Digital elevation models may be constructed from topographic data, and a variety of other special purpose maps are also available. Examples include maps of recent landslides, floodplain and coastal flood hazard maps, and engineering geology maps that show engineering properties of earth materials. --- How Geologists ­Determine Geologic Time APPENDIX D: In order to understand the history of Earth, we must determine the actual age of earth materials with the science of geochronology. In geochronology, scientists combine relative and absolute dating techniques to reconstruct Earth history. Relative dating establishes a chronological order of events using fossil evidence and geologic relationships, the use of which is governed by geologic principles and laws. The oldest of these laws, the law of superposition, was proposed by Nicholas Steno in 1668, and has been applied and tested by scientists for over 300 years. This law states that in any succession of sediment or sedimentary rock that has not been deformed, the oldest layer lies at the bottom, with successively younger layers above. Although a relative geochronology can be established based upon superposition and other laws and principles of relative dating, it is absolute dating that provides the information necessary to establish rates of geologic processes and the numeric ages of earth materials. From a natural hazards perspective, it is important to establish rates of geologic processes and the timing of past geologic events such as volcanic eruptions, earthquakes, tsunamis, floods, and landslides. The chronology of these events is critical in estimating their return period or recurrence interval. Knowing how frequently an event takes place helps us better understand the hazard and predict when a similar event is likely to occur in the future. Geologic time is much different, in a way, than our normal time framework. Although geologic time and “normal” time use the same units of measure—years— they differ vastly in duration and in the instruments we use to measure duration.1 Normal time is counted in hours, days, seasons, or decades, and the instrument used to measure time is a clock. In contrast, geologic time— sometimes called “deep time”—is measured in tens of thousands to hundreds of millions to several billion years. To measure this vast amount of time, geologists take ­advantage of naturally occurring forms of uranium (U), potassium (K), carbon (C), and other chemical elements to date the earth materials in which they are found. The natural forms of these chemical elements are called isotopes and are distinguished from each other by 610 their weight. The weight of an isotope is the sum of the weight of smaller, subatomic particles called protons and neutrons that are found in the center of the atom, the nucleus. Each proton and neutron has a weight of 1, so the number of protons plus the number of neutrons determines the atomic weight of an isotope. All isotopes of a chemical element have the same number of protons, because it is the number of protons that defines a chemical element. Where isotopes vary is in their number of neutrons. For example, carbon has three common isotopes, carbon-12, carbon-13, and carbon-14. All three isotopes have 6 protons because they are carbon, but the number of neutrons varies from 6 to 7 to 8, respectively. Isotopes also differ in their stability. Some isotopes, such as carbon-12 and carbon-13, have a stable number of neutrons and protons in their nucleus and do not split apart. Most isotopes, however, are ­unstable and will undergo spontaneous splitting, sometimes called decay, at some point in the future. Unstable isotopes are commonly called radioactive ­isotopes because radiation is released when they decay. Although it is impossible to predict when an individual unstable atom will decay, laboratory studies can determine the rate at which a large number of atoms of a radioactive isotope will decay. Each radioactive isotope has a constant rate of decay that is unaffected by physical forces. Absolute dating is possible if a measurable quantity of a radioactive isotope is incorporated into a mineral or substance when it forms. The decay of the unstable radioactive isotope is irreversible and becomes a clock that is running at a constant rate. In absolute dating, the unstable radioactive isotope is known as the parent and the stable isotope that eventually forms from the decay of the parent is known as the daughter product. Radioactive isotopes, particularly those of very heavy elements, undergo a series of radioactive decay steps that finally ends when a stable, nonradioactive isotope is produced. For example, the isotope uranium-238 (U-238) undergoes 14 nuclear transformations in different steps to --- APPENDIX D TABLE D.1 Half-Lives of Radioactive Isotopes Commonly Used in Absolute Dating | Parent radioactive isotope | Daughter stable isotope | Half-life | |----------------------------|-------------------------|----------------| | Uranium-238 | Lead-206 | 4.5 billion yr | | Uranium-235 | Lead-207 | 700 million yr | | Potassium-40 | Argon-40 | 1.3 billion yr | | Carbon-14 | Nitrogen-14 | 5730 yr | How Geologists ­D etermine Geologic Time   611 100 Percentage 75 finally decay to stable lead-206 (Pb-206), which is not radioactive. An important characteristic of a radioactive isotope such as U-238 is its half-life, which is the time required for one-half of a given amount of the isotope to decay to another form. Every radioactive isotope has a unique and characteristic half-life (Table D.1). As decay proceeds over time, the amount of parent radioactive isotope in a substance decreases and the amount of the stable daughter isotope proportionally increases (Figure D.1). Without knowing the actual quantity of radioactive isotope that was initially present, it is often possible to obtain a numeric date by comparing the relative proportions of parent and daughter isotope in a substance. In most cases, two important conditions must be met to obtain an accurate numeric date. First, no new atoms of the parent isotope have been added to the substance since it formed; this is true for many minerals in igneous rocks where the radioactive isotope becomes part of the atomic structure when a mineral crystallizes from magma. Second, all atoms of the daughter isotope produced by decay remain trapped in the substance; again this occurs for minerals in igneous rocks where atoms are strongly bonded in the crystal structure. Should any daughter product “leak” from the ­sample, it would give a numeric date that was too young. Because radioactive isotopes such as U-238, U-235, and K-40 have relatively long half-lives (Table D.1), their decay is useful in dating rocks on the order of millions to billions of years. For example, the ratio of Pb-207/Pb-206, the daughter products of U-235 and U-238, has been used to date the oldest piece of continental crust, a crystal of the mineral zircon from sandstone in Western Australia, at 4.4 billion years before the present. The actual methods for measuring amounts of parent and daughter isotopes and calculating numeric dates are complex and tedious, although the concept is easy to grasp. These methods have been successfully used to assign numeric dates to the geologic time table and to delineate important Earth history events such as mountain building, ice ages, and the appearance of life forms. ter gh u Da 50 Pa re 25 0 1 nt 2 3 4 5 6 Number of half-lives ![Source PDF page 20, containing Figure D.1](../figures/appendices-page-20.jpg) FIGURE D.1 Relative Proportion of Parent and ­Daughter Isotopes Over Time This graph shows the reduction of a parent radioactive isotope to a daughter stable isotope with each additional half-life. Physical anthropologists and archeologists concerned with human history are interested in developing a geochronology for the last 8 million years of Earth history; that is the period from which human and ancestral human fossils are found. From a natural hazards perspective, we are often interested in establishing the geochronology of the past few hundred to few hundred thousand years of Earth’s history, and for this we have several potential methods. For example, the radioactive isotope U-234 undergoes decay to thorium (Th)-230 at a known rate, and the ratio of these two isotopes is useful in numerically dating a variety of materials, such as coral, back to several hundred thousand years. For sediment younger than about 40,000 years, C-14 is used extensively for numeric dating. The most common form of carbon is stable C-12, but C-14, which is radioactive, occurs in small quantities and undergoes radioactive decay to the stable daughter isotope nitrogen-14 (N-14). The half-life of the decay of C-14 to N-14 is 5730 years. The C-14 method works because C-14 is only incorporated into organic matter while an organism is alive; when the organism dies, the C-14 in its tissue undergoes radioactive decay without being replaced with new C-14. Common materials dated with C-14 include wood, bone, charcoal, and other types of buried organic material. C-14 has a relatively short half-life; as a result, after about 40,000 years, the amount of C-14 remaining is very small and difficult to measure. Consequently, use of this technique is limited. --- 612   APPENDIX D How Geologists ­D etermine Geologic Time The field of geochronology is expanding rapidly, and new techniques are constantly being developed. For example, it is now possible to directly date the amount of time that a landform has been exposed at Earth’s surface. This is known as exposure dating. The basic idea is that certain isotopes, such as beryllium-10, aluminum-26, and chlorine-36, are produced when cosmic rays interact with Earth’s atmosphere and accumulate in measurable quantities in surface materials such as soil, alluvium, and exposed rock surfaces. Thus, the amount of these isotopes that has accumulated is a measure of the minimum time of exposure to the surface environment. Another innovative technique, known as lichenometry, uses the slow, but constant growth of lichens on rock surfaces to determine a minimum time that a rock is exposed. Lichens are plant-like growths of photosynthetic algae or bacteria with a fungus. These organisms tend to grow in circular patches with a rate of growth that is known for a particular species. Careful measurements of the size of lichen patches, coupled with known growth rates, provide minimum numeric ages for exposure of the rocks on which they are found. This method has been successfully used in California and New Zealand to date regional occurrences of rockfalls generated by large earthquakes, thus dating past seismic activity.2,3 Lichenometry can provide dates to about 1,000 years before present. Sediment may also be accurately dated through the use of dendrochronology, which is the analysis of annual growth rings of wood. Because of variations in annual precipitation, the width of tree rings differs from year to year. This produces a distinctive sequence of wide and thin rings similar to a bar code on merchandise at a store. Once a regional chronology is developed using growth rings in both living and dead trees, it then becomes possible to determine the age of a growth ring pattern from a piece of buried wood. This method has been used extensively by archeologists to date prehistoric sites of human habitation and also by climatologists as a method of reconstructing prehistoric patterns of precipitation. Growth rings record climate because during dry years, tree rings are narrow relative to wet years. Very accurate numeric geochronology may also be obtained from counting varves. A varve is a layer of sediment representing one year of deposition, usually in a lake or an ocean. Careful counting of varves may extend the chronology back several thousand years. Although the most accurate chronology is the historical record, its length varies from only a few hundred years in the United States and Canada to several ­thousand years in China. Given the brevity of human history compared with the great length of geologic history, we resort to numeric dating methods to establish geochronology. There are more than 20 methods useful in establishing geochronology that yield numeric dates. These methods are crucial to understanding rates of geologic processes and the recurrence interval of natural hazards. R efere nc e s 1\. Ausich, W. I., and Lane, G. N. 1999. Life of the past, 4th ed. Upper Saddle River, NJ: Prentice Hall. 2\. Bull, W. B., and Brandon, M. T. 1998. Lichen ­dating of earthquake-generated regional rockfall events. Southern Alps, New Zealand. Geological Society of America Bulletin 110: 608–84. 3\. Bull, W. B. 1996. Dating San Andreas fault ­earthquakes with lichenometry. Geology 24: 111–14. ---