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Chapter 2 - The Rock Cycle & Basic Geologic Principles |
The science of geology is founded on basic principles that are useful for making observations about the world around us. Everything around us is made of chemical compounds that have testable and indentifying characteristics, allowing them to be classified. This applies to rocks, minerals, and derivative materials (such as sediments and soil). In addition, basic laws and principles can be applied to resolving the order of events leading to the formation of rocks and landscape features. This section presents many basic concepts universal to the physical sciences.
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Physical Properties of Earth Materials
1. Define the terms "rock" and "mineral" and explain the differences
2. Describe the structure of an atom and essential concepts of chemistry related to earth materials.
3. Explain how atoms are involved with the structure of minerals.
4. List and explain the physical properties of minerals.
5. Describe the structures of silicate minerals.
6. List some common silicate and non silicate minerals.
7. Describe basic geologic principles for interpreting landscape forming processes
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Keywords and Essential Concepts
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1. Define the terms "rock" and "mineral" and explain the differences
"Every Rock Has A Story"
Rocks are composed of chemical compounds naturally occurring in nature. Rocks are composed of particles ranging from microscopic grains to full sized crystals and crystal grains of different kinds of minerals. and containing many different identifiable physical characteristics. It is conceptually important that each rock has an origin in concepts of place, time, and physical and chemical conditions. Once rocks form, they are subject to change. These changes may be rapid (such as a volcanic explosion) or gradual, taking place over millions or billions of years, and involving movement over great distances, both at the surface or to deep within the Earth's crust below us. Trying to explain the what, how, and when of a rock's journey is fundamental to explaining why rocks are significant to resolving questions about our Earth's history and conditions within the physical environments where we live. |
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See Rock Cycle discussion page.
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Gypsum crystals from a cavern wall in Jewel Cave, South Dakota |
mineral—A naturally occurring, homogeneous inorganic solid substance having a definite chemical composition and characteristic crystalline structure, color, and hardness. Minerals are chemical compounds.
rock—Relatively hard, naturally formed mineral or petrified matter; stone; a naturally formed aggregate of mineral matter constituting a significant part of the earth's crust. Rocks consist of one or more minerals.
The mineral composition of a rock reflects the physical environment and geologic history where a rock formed. |
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| Rock Cycle |
Rock Cycle Illustrated |
Rocks & minerals |
Rocks are classified into three groups: igneous, sedimentary and metamorphic
igneous rocks—a rock or mineral that solidified from molten or partly molten material (referring to magma underground or lava on the surface). The word igneous also applies to the processes related to the formation of such rocks. Examples of igneous rocks include granite, gabbro, and basalt.
sediments—solid fragments of inorganic or organic material that come from the weathering of rock and soil erosion, and are carried and deposited by wind, water, or ice. Examples of sediment include gravel, sand, silt, clay, mud (mix of sand, silt, and clay), soil, lime mud, and ooze. Sediments are not rocks, but they may become rocks through heating, compaction, and cementation.
sedimentary rocks—rock that has formed through the deposition and solidification of sediment, especially sediment transported by water (rivers, lakes, and oceans), ice ( glaciers), and wind. Sedimentary rocks are often deposited in layers, and frequently contain fossils. Examples of sedimentary rocks include shale, sandstone, conglomerate, limestone, and chert.
metamorphic rocks— Rock that was once one form of rock but has changed to another under the influence of heat, pressure, or fluids without passing through a liquid phase (melting). Examples of metamorphic rocks include slate, schist, gneiss, marble, quartzite, and serpentinite. |
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| igneous rocks |
sediments |
sedimentary rocks |
metamorphic rocks |
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2. Describe the structure of an atom and essential concepts of chemistry related to earth materials.
| Basic concepts of chemistry are essential to understanding the physical and chemical properties of earth materials (minerals, rocks, sediments, and organic matter). The chemical characteristics of earth materials are reflect the environments how and where they are formed, they also determine their potential fate when exposed to chemical changes. For instance, rocks and minerals formed deep underground may not be stable in the surface environment where they are exposed to water, air, temperature changes, and other physical and chemical conditions. |
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| Structure of an atom |
Periodical Table |
element—a chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Common examples of elements are iron, copper, silver, gold, hydrogen, carbon, nitrogen, and oxygen.
isotope—each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but not in chemical properties; in particular, a radioactive form of an element.
molecule—a group of atoms bonded together, representing the smallest fundamental unit of a chemical compound that can take part in a chemical reaction
chemical compound—a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together in a defined spatial arrangement by chemical bonds. |
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Chemical compounds can be molecular compounds held together by covalent bonds, salts held together by ionic bonds, intermetallic compounds held together by metallic bonds, or crystal complexes held together by coordinate covalent bonds. |
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—compounds with ionic bonds readily dissolve in water |
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—compounds with metallic bonds transmit electricity |
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—compounds with covalent bonds tend to be durable and do not transmit electricity |
Van der Waals forces—the weak forces which contribute to intermolecular bonding. Friction between objects are related to Van der Waal bonds, such as rocks sticking together along fault zones.
crystal—a piece of a homogeneous solid substance having a naturally geometrically regular form with symmetrically arranged plane faces.
mixture—solid, liquid, or gas composed of two or more substances, but each keeps its original properties. Note that earth materials (rocks and sediments), seawater in oceans, and atmosphere are all mixtures. |
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3. Explain how atoms are involved with the structure of minerals.
Examples of crystal structures
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Structure of Salt:
the mineral halite - NaCl |
Calcite crystal structure
calcium carbonate - CaCO3 |
Base crystal structure for the mineral fluorite - CaF2 |
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4. List and explain the physical properties of minerals.
| All minerals have unique properties that aide in their identification. However, these identifying characteristics may not be easy to determine without extensive testing. Fortunately, the most common minerals are fairly easy to identify by general appearance or with simple tests for hardness, crystal form, color, magnetism, and streak (does it leave a colored line when scratched on a piece of tile?). |
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crystal form
color
streak
hardness
odor
taste
feel
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smell
magnetism
double refraction
acidic reaction
malleability
elasticity
electrical resistivity
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luster (metallic, non-metallic)
diaphaneity (transparent, translucent, opaque)
striations
fluorescence
phosphorescence
radioactivity
thermoluminescence
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| Crystal forms of different minerals |
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| Common rock-forming minerals |
gemstones are minerals |
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5. Describe the structures of silicate minerals.
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| Structure of silicate minerals |
Structure of sheet silicates |
General composition of the Earth's crust.
felsic—minerals of silica and aluminum-rich composition, and the rocks that form from them. Felsic materials are typically less dense than mafic materials. Felsic minerals include quartz, feldspars, muscovite, and clays. Rocks include granite, rhyolite, sandstone, quartzite
mafic— A mnemonic term combining and “Ma” (for magnesium) and “Fe” (for ferric iron). The term is used to describe dark-colored igneous minerals rich in iron and magnesium, as well as the rocks that bear those minerals. Basalt is a rock of mafic composition.
ultramafic—a rock composed chiefly of mafic minerals (rich in iron and magnesium), and less than about 45 percent silica, such as the minerals olivine, pyroxene, or amphibole. Peridotite, pyroxenite and serpentinite are ultramafic rocks. |
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* Based on the composition of the Earth's crust, and how rocks form, the earth is composed of a dominant few number of minerals (called common minerals).
Currently there are about 4,000 known minerals of different composition and mineral arrangement. |
| Composition of the crust. |
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6. List some common silicate and nonsilicate minerals.
Common silicate minerals
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Silicate minerals are the dominant group of minerals that make up the rocky crusts of the Earth, Moon, and other stony planets (Mercury, Venus, Mars, and many other moons and asteroids within the Solar System. Silicate minerals chemically consist of compounds that contain the geometric arrangement of silicon-oxide tetrahedrons contained within simple to complex crystalline structures. |
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quartz—a hard colorless or white mineral consisting of silicon dioxide (silica-SiO2), found widely in igneous, metamorphic, and sedimentary rocks. Pure silica forms clear quartz crystals in unconfined spaces, such as geodes or open fissures in rock; inclusion of traces of other element in quartz's crystalline structure produces varieties of semiprecious gems varieties including amethyst, citrine, rose quartz, and smoky quartz. Microcrystalline varieties of sedimentary rock composed dominantly of quartz include chert, jasper, flint, agate, and chalcedony. |
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feldspar—an abundant rock-forming group of minerals typically occurring as colorless or pale-colored crystals. Feldspars are aluminosilicate minerals with varieties:
orthoclase or K-spar—a variety of feldspar that rich in potassium (KAlSi3O8),
plagioclase—a varieties of feldspar rich in feldspar which include sodium-rich Albite (NaAlSi3O8), and calcium-rich Anorthite (CaAlO2SiO2O8). |
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| crystal structure of feldspars |
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olivine—a mineral silicate of iron and magnesium, principally (Mg,Fe)2SiO4, found in igneous and metamorphic rocks occurring in basalt, peridotite, and other basic igneous rocks.
pyroxene—Any of a large class of rock-forming silicate minerals, generally containing containing two metallic oxides combining magnesium, iron, calcium, sodium, or aluminum and typically occurring as prismatic crystals.
amphibole—Any of a class of rock-forming silicate or aluminosilicate minerals typically occurring as fibrous or columnar crystals consisting of hydrated double silicate minerals, such as hornblende, containing various combinations of sodium, calcium, magnesium, iron, and aluminum.
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Micas are sheet silicate (phyllosilicate) minerals including:
biotite—a common rock-forming mineral occurring in black, dark-brown, or dark -green sheets and flakes: an important constituent of igneous and metamorphic rocks; a mafic variety of mica.
muscovite—a silver-gray form of mica (platy sheet silicate mineral) occurring in many igneous and metamorphic rocks. |
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clay minerals—any of a group of minerals that occur as microscopic sheet-like or fibrous crystals in clay. Clay minerals are a primary component of many soils and form from the weathering decay of other silicate and aluminum-rich minerals, such as feldspars, micas, and other minerals. |
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Common non-silicate minerals
rock salt—a rock dominantly composed of sodium chloride (NaCl - the mineral halite). Rock salt is an evaporite formed in restricted basins with an inflow of seawater located in an arid environmental setting.
gypsum—a mineral composed of hydrous calcium sulfate (CaSO4-2H2O); an evaporite mineral used in the manufacture of plaster.
calcite—a common rock-forming mineral consisting of calcium carbonate—CaCO3. Calcite can be white, colorless (transparent), or slightly colored, commonly yellow, by other inclusion of traces of other elements. Calcite is a major constituent of sedimentary rocks such as limestone, chalk, and travertine, and in the metamorphic rock form, marble.
dolomite—A white or light-colored mineral, essentially CaMg(CO3)2, commonly found in association with limestone or marble. Dolomite is a common replacement mineral in limestone that has been exposed the high magnesium content brine fluids.
hematite—A reddish, steel gray, or black mineral consisting of ferric oxide (Fe2O3).
limonite—An amorphous orange to brownish mineral consisting of a mixture of hydrated ferric oxides, important as an iron ore. Rust on iron vehicles is essentially limonite.
magnetite—a gray-black magnetic mineral that consists of iron oxide (Fe3O4) and is an important form of iron ore.
pyrite—a brass-colored mineral, FeS2, occurring widely and used as an iron ore and in producing sulfur dioxide for sulfuric acid. Also called fool's gold, iron pyrites.
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| Halite and Gypsum |
Calcite |
Hematite and Limonite |
Magnetite and Pyrite |
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Properties of minerals
The following physical properties of minerals can be used to identify a mineral:
odor—few minerals have an odor. Sulfur-bearing minerals may put off a rotten-egg like smell. Many sedimentary rocks have the smell of petroleum.
taste—halite tastes like salt (because it is NaCl). (Note that tasting minerals and rocks isn't recommended.)
color—some minerals have very distinct colors, however, color is not a reliable indicator by itself.
crystalline structure—minerals have unique and obvious crystal structures, however, crystal structure alone may not be enough to identify a mineral.
cleavage—the tendency of a crystallized substance to split along definite crystalline planes, yielding smooth surfaces. mica, feldspar, calcite, and selenite gypsum have good mineral cleavage.
transparency—few common minerals are transparent. Quartz and calcite can have high transparency.
hardness—minerals have different durability properties. Mohs Hardness Scale is a list of harness of common minerals
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| Mohs Hardness Scale |
magnetism—iron (naturally pure iron) and magnetite (Fe3O4) are common magnetic rocks, iron-rich meteorites are magnetic.
luster—metallic sheen or shine, reflective surface. Galena and pyrite have high luster.
specific gravity—the ratio of the density of a substance to the density of water.
streak—soft minerals may leave a streak of color on a piece of tile. Hematite is red, pyrite is brown, magnetite is black, etc.
striations—some mineral crystals have narrowly-spaced lines (such as hornblende)
fluorescence—some minerals glow colors under a blacklight including some fluorite, calcite, and zinc minerals
phosphorescence—some minerals absorb light energy and release light when the light is turned off.
radioactivity—rocks containing uranium, radium, and thorium may display measurable radioactivity.
thermoluminescence—some minerals will glow in colors when heated.
double refraction—light passing through calcite will transmit a double image.
acidic reaction—calcite fizzes when exposed to mild acid. Dolomite will fizz in hot acid.
malleability—gold, copper, iron, and silver is able to be hammered into objects.
elasticity—soft minerals may be bendable (like mica)
electrical resistivity—some metallic minerals transmit electricity |
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Summary comments about rocks and minerals
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igneous rocks: rocks formed from the cooling of molten material (magma, lava) are rich in silicate mineral. Molten material derived from deep in the mantle is typically enriched in iron- and magnesium-rich silicate minerals (called mafic or ultramafic). |
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weathering of minerals: water (H2O) occurs in rocks within the earth and is a primary chemical agent on the earth surface. Water is called the "universal solvent," dissolving and transporting material in solution, altering the chemical composition of mineral, and transporting sediment (erosion and deposition). |
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durability of quartz: because quartz is an extremely durable mineral (with a Mohs hardness of 7.0) and because is is an extremely abundant mineral in the Earth's crust, quartz is concentrated by erosional processes in the form of "sand." |
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minerals in sedimentary rocks: most sedimentary rocks are enriched in the minerals quartz, calcite, and clay minerals. Minerals with high hardness and low solubility are transported by erosion and deposited in sedimentary basins. Typically soft minerals that are highly soluble are dissolved and carried by surface and groundwater where most contributes to the saltiness of seawater. However, dissolved components in water can precipitate to form mineral cements, including like calcite (CaCO3), iron-rich minerals (hematite and limonite), and silica (quartz). |
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metamorphic processes cause changes in the mineral composition in rocks: changes in heat, pressure, and exposure to fluids, over time, will change the mineral composition of earth materials, such as converting sediments into sedimentary rocks, changing sedimentary and igneous rocks into metamorphic rocks. Conversely, exposing rocks to fluids—at or below the surface—degrade rocks of any kind into sediments. |
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7. Describe basic geologic principles for interpreting landscape forming processes
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Basic geologic principles are used to interpret the geologic history of the earth. These basic rules have many basic applications to interpreting the order of events from small scale activities like interpreting the order of footprints along a lakeshore, to large scale like interpreting the order of event in the formation of features like mountain ranges or the sequence of events exposed in a region where oil exploration is taking place. To illustrate try interpreting the order of events in the diagram involving animal trackways. |
| relative dating tracks |
Cross Sections
geologic cross section—an interpretation of a vertical section through the Earth's surface, most usefully a profile, for which evidence was obtained by geologic and geophysical techniques or from a geologic map.
Note that geologic cross sections are made by using available mapable features found on the surface or interpreted from data about the subsurface. Natural cross sectional views are sometimes possible along canyon high walls or along steep mountain range front. How most subsurface data derive or imaged through geophysical methods, such as by seismic data (by earthquakes or manmade explosions), by measurements of gravity, magnetism, electrical resistivity, or information derived from wells such as core sample, radiation measurements, or other geophysical methods. |
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| Cross section of Little Rocky Mountains, Montana |
Cross section of the South Bay region, California |
Basic Geologic Principles
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Law of Original Horizontality—this law states that most sediments, when originally formed, were laid down horizontally. However, many layered rocks are no longer horizontal.
Law of Superposition—this law states that in any undisturbed sequence of rocks deposited in layers, the youngest layer is on top and the oldest on bottom, each layer being younger than the one beneath it and older than the one above it.
Law of Cross-Cutting Relationship—this law states that a body of igneous rock (an intrusion), a fault, or other geologic feature must be younger than any rock across which it cuts through.
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Unconformities: Gaps in the "geologic record"
unconformity—a surface between successive strata representing a missing interval in the geologic record of time, and produced either by an interruption in deposition or by the erosion of depositionally continuous strata followed by renewed deposition. |
| Types of Unconformities |
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nonconformity—an unconformity between sedimentary rocks and metamorphic or igneous rocks when the sedimentary rock lies above and was deposited on the pre-existing and eroded metamorphic or igneous rock.
angular unconformity—an unconformity where horizontally parallel strata of sedimentary rock are deposited on tilted and eroded layers, producing an angular discordance with the overlying horizontal layers.
disconformity—an unconformity between parallel layers of sedimentary rocks which represents a period of erosion or non-deposition.
conformable boundary—an arrangement where layers of sedimentary strata are parallel, but there is little apparent erosion and the boundary between to rock layer surfaces resemble a simple bedding plane
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| Conformable or gradational contact between sedimentary layers |
Nonconformity in the Grand Canyon (known as the "Great Unconformity") |
Disconformities between sedimentary formations in the Grand Canyon |
Angular unconformity between sedimentary rocks of different ages |
| Origin of Unconformities |
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Unconformities are caused by relative changes in sea level over time.
transgression—the migration of a shoreline landward as sea level (or lake level) rises.
regression—the migration of a shoreline seaward as sea level (or lake level) falls.
Sea level change my be caused by region uplift or global changes in sea level, such at the formation or melting of continental glaciers. Whatever the cause of sea level change, when sea level falls, sediments are eroded from exposed land. When sea level rises, sediments are typically deposited in quiet water settings, such as on shallow continental shelves or in low, swampy areas on coastal plains. |
| unconformities by sea level changes |
| sedimentary facies—sedimentary deposits that reflect environmental conditions at the time of deposition of sediments. Examples include offshore mud facies, reef facies, beach sand facies, terrestrial facies, etc. Facies reflect the character of a rock expressed by its formation, composition, and fossil content. |
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| Geologic Principles Illustrated |
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| Applying basic geologic principles: Laws of original horizontality, superposition, and cross-cutting relationships explain the order of this diagram with the order of formation: rock units C,B,A,D, then A. |
Example of a cross section through the Wind River Mountains, Wind River Basin, and Absaroka Mountains of Wyoming |
Seabed exploration produces cross-sectional seismic profiles. |
Geologists study cross sections created by geophysical exploration methods. This is an
example of a seismic profile showing the location of exploratory wells. |
| Cross Section Interpretation Exercise |
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Using the laws of original horizontality, superposition, and cross-cutting relationships interpret the order of the formation of features illustrated in this hypothetical cross section |
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Quiz Questions
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