Introduction to Geology and Geological Materials

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Introduction to Geology and Geological Materials

What is Geology?

Geology, according to Webster’s Dictionary, is “a science that deals with the history ofEarth and its life, especially as recorded in rocks.” It is a hybrid science, borrowing from manyfields. Some geologists study the chemistry of rocks, minerals or other Earth materials. Othersstudy Earth physics or the biology of plants and animals, ancient and modern. Still others studydistant planets to understand the origin of the solar system or even of the universe. Geologistsmay stray into the fields of oceanography, meteorology, biology or astronomy, butfundamentally, geology is the study of the Earth, as the dictionary says, and most geologistsstudy materials and processes at or near the Earth’s surface. The Earth has changed and evolvedduring its long history, and the changes are chronicled by Earth’s geology.

The Earth is unique among the planets. We enjoy running water, a hospitableatmosphere and cool temperatures. Rocks and minerals are constantly being recycled, albeitover long periods of time. Material, once at the surface, is carried deep into the Earth, only to bebrought up to the surface again. The planet’s surface is teeming with abundant life, and the lifezone extends 8-10km up into the atmosphere and an equal distance down into the Earth’s crust. No other planet has this fascinating combination of characteristics (Figure 1).

Figure 1. The top photo here showsbasalt from western Ontario. The bottomleft figure shows equivalent rocks fromMars (photo taken by the Pathfindermission). The dark areas on the photo ofthe moon in the lower right are alsobasalt. The similarity of rock typessuggests commonality in geologicalprocesses. However, Earth has anatmosphere and, most importantly,running water at the surface. Thus, Earthis a fascinating and dynamic planetcompared with Mars and the Moon.

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Geology is unique among the sciences because many geological processes are extremelyslow, and geological time is extremely long. So, much of a geologist’s time is spent collectingand analyzing evidence about things that happened a long time ago over long periods of time. The Earth is billions of years old, and we have scant evidence regarding some of the earliestevents in Earth history. Answering some geological questions is like trying to put a jigsawpuzzle together with many pieces missing. Geologists collect new evidence, and add newpieces, as they try to come up with complete pictures. Like all scientists, geologists apply thescientific method (Figure 2) when trying to answer complex questions. Because science cannotproduce absolute answers, this means that geological science, just like the Earth, is constantlyevolving. A key assumption made by geologists is that the laws of physics and Earth processesare the same today as they were in the past. Thus, if geologists find a 560 million year oldmassive sandstone, they may infer that it formed as a beach deposit in ancient seas, since that iswhere such sandstones are forming today. This allows geologists to infer the presence of oceansand continents over half a billion years ago (Figure 3).

Figure 2. The scientific method is one thatinvolves proposing a hypothesis to explainobservations, followed by subsequent testing. Generally the testing involves makingpredictions and then checking to see if thepredictions are correct. Often this requiresmaking new observations and collectingadditional data. Eventually, if a hypothesispasses enough tests, it evolves into an acceptedtheory. In reality, all of science is theory. Thismeans it can change as new information and newtests become available.

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Figure 3. The three views show Montara Beachin California. The top view shows typical beachsand; the middle view shows beach sand andsome sandstone. The bottom view shows ageologists examining the sandstone. Thesandstone itself is the fossilized remains of anancient beach. Sandstones of this sort areevidence that beaches, similar to modern daybeaches, existed in the ancient past.

Besides the academic interests, Geologyhas many practical aspects. Earth resources arefundamental to our existence (Table 1). Energyresources, including coal, oil, gas, uranium, andgeothermal, are keys to our modern societies. Copper, nickel, iron and other metals have forseveral hundred years been the basis ofindustries. Nitrates, borax and other chemicalsfrom the Earth are in heavy demand. Sand,gravel and other building materials are usedworld wide.

Geologists play important roles in manyengineering projects, including roads, buildings,and dams. They are also concerned about naturalhazards such as earthquakes, volcanoes, tidalwaves or landslides. Increasingly over the lastseveral decades, geologists have become moreand more concerned with environmentalproblems. Water supplies, water pollution, wastedisposal, and many other problems have becomemore acute as population and urban areasexpand.

Table 1-1. Earth’s geological resources are thebasis for modern society.

Group Examples

metallic and semimetallic elements gold, silver, copper, iron, manganese, nickel,aluminum

nonmetallic elements potassium, sodium, phosphorous, sulfur

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gems diamond, sapphire, agate

industrial minerals sand, clay, building stone, asbestos, mica, gravel

fertilizer and chemicals limestone, phosphate, salt, nitrates, borates

energy resources coal, oil, gas, uranium

Evolution of Geological Science

Early peoples, as far back as 2900 B.C. in Egypt and Greece, used coal and flint, andprocessed metals and minerals from the Earth. Herodotus, in 500 B.C., studied the flooding ofthe Nile River, and Aristotle described fossils in 350 B.C. Theophrastus wrote the firstmineralogy book Concerning Stones about 300 B.C. However, geology, like most sciences, didnot really advance rapidly until the renaissance of the 1400's-1500's. During that time, scholars(mostly people associated with churches) began talking to practitioners such as miners oralchemists. Great advances soon followed, ultimately leading to the industrial revolution.

Although we could debate which events in the development of modern geological sciencewere most important, several key ones stand out. In the late 1700's, James Hutton espoused hisprinciple of uniformitariansim, sometimes summarized as “the present is the key to the past.” Itstated that the geological processes taking place in the present operated the same in the past. This, and other ideas presented by Hutton, formed the basis for subsequent work by JohnPlayfair, Charles Lyell and others several decades later. Playfair, for example, was the first topropose that rivers cut their own valleys over very long times, and the first to describe the wayglaciers can move boulders and polish the Earth. He published several important booksincluding, in 1802, Illustrations of the Huttonian Theory of the Earth. Lyell further promotedthe ideas of Hutton. He studied geological formations in Europe, North America and Englandand concluded that the Earth must be millions of years old. Lyell tentatively accepted the theoryof Darwinian evolution and applied some of Darwin’s principles to fossils in his The GeologicalEvidence of the Antiquity of Man, published in 1863. He is, however, probably best known forhis Principles of Geology (1830).

The early British geologists were on the right track, and the discoveries spawned byHutton were significant. Scientists studied erosion and other processes taking place around themand estimated how long it took for different kinds of geological features to form. Theyconcluded that the Earth must be much older than estimates based on the Bible, which placed theage of the Earth at several thousand years. The true magnitude of geological time eluded themhowever. A major breakthrough came at the end of the 19th century when Marie and PierreCurie, Wilhelm Roentgen, and others discovered and applied radioactivity in their scientificquests. Soon, scientists were using radioactive age dating to estimate the age of rocks andminerals. Over just a few decades, the estimated age of the Earth went from 10's or 100's ofmillion to billions of years, and geologists found that explaining the evolution of the Earth wasmuch easier. They concluded that the Earth is always changing and evolving, but the processeswere much slower than originally envisioned.

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Earth Materials

regolith: general term for layer of fragmental,loose material of any origin and type thatoverlies bedrocksoil: unconsolidated earth material, rich inorganic components, that provides the naturalmedium for plant growthsediment: solid fragmental material, eitherorganic or inorganic, that originates fromweathering of rocksmineral: naturally occuring inorganic solidsthat are crystalline, that have a specificchemicl composition, and that have fixedphysical propertiesrock: naturally forming aggregates composedprimarily of inorganic Earth materials

Figure 4. This view shows the relative positionof continents about 130 million years ago. Since then, they have drifted apart, at ratesequivalent to how fast your fingernails grow. Evidence for continental drift includes (1) theway the continents seem to fit together, (2)matching fossils, rocks, mountain chains ondifferent sides of modern oceans, (3) theexistence of a mid ocean ridge and measurableseafloor spreading that takes place there.

Much of the early work by geologistswas descriptive. Explaining why things hadhappened was more difficult. One majorbreakthrough, which allowed scientists toanswer some of the unanswered “why”questions, came in the early 1960's. J.T. Wilsonis often given credit for developing andpopularizing the idea of continental drift at thistime, although others had proposed the conceptpreviously. Anyone could see that SouthAmerica and Africa fit together, much like twopuzzle pieces (Figure 4). Fossil and otherevidence, too, suggested that the continents had once been joined and subsequently had “drifted”apart. Before 1960, however, geologists could not understand why continents would moverelative to each other and, without knowingthe mechanism, the theory of continental driftstalled. In the 1960's, however, much betterseafloor mapping, better seismic networks,and many other things all came together. Geophysicists proposed credible hypothesesto explain continental drift. Within a decade,the theory of plate tectonics was firmlyestablished and it has passed all scientifictests since then. Today it forms thefundamental basis for much of moderngeology, and has provided us with many ofthe missing puzzle pieces.

Earth MaterialsRegolith, Sediment and Soil

An unconsolidated layer of materialcalled regolith covers most of Earth’s land

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surfaces. Regolith is mostly rock and mineral debris produced by mechanical or chemicaldecomposition (weathering) of preexisting rocks. We call such debris sediment. Regolith mayalso contain humus, organic material derived from decomposed plants and animals, volcanic ash,or a number of other components. If regolith contains sufficient organic material, we term it soil. Besides inorganic and organic solid material, soils also contain important amounts of water andair.

In most places, regolith forms a relatively thin layer at the Earth’s surface; it is underlainat some depth by bedrock. The thickness of the regolith depends on many factors, mostimportant, perhaps, climate and topography. The composition of regolith also depends on thesefactors but, most significantly, on the type of bedrock from which it was derived. We willdiscuss regolith and sediment in more detail later; for now we will focus on minerals and rocks.

Minerals

According to a glossary, minerals

“...are naturally occurring inorganic solids that are crystalline, that have aspecific chemical composition, and that have fixed physical properties.”

Crystalline means that each mineral has a specific, orderly and repetitive, internal arrangementof atoms. Some crystals have flat crystal faces, but many do not; they are still consideredcrystals. Minerals, just like all matter, consist of atoms of specific elements. They arecompounds, which among other things, means that we can describe them with a chemicalformula (Table 1-2). Some minerals, such as graphite or sulfur, contain only one element. Mostcontain several or many elements. Having a specific chemical composition means that allsamples of a specific mineral contain the same key major elements in the same proportions. Halite, common table salt, for example, has the composition NaCl. There are an equal numberor sodium (Na) and chlorine (Cl) atoms in a sample of halite, whatever its size. Halite iscrystalline, and each sodium atom is surrounded by six chlorine atoms and vice versa (Figure 5). Additional elements, besides sodium and chlorine, may be present in halite, but only at very lowlevels. Because all halite has the same atomic arrangement and composition, all halite has thesame physical properties. Two different minerals may have the same composition. For example,graphite and diamond are both essentially pure carbon (C). They are both crystalline but havedifferent arrangements of carbon atoms in their atomic structure. Consequently they havedifferent physical properties. Diamond is the hardest mineral known, graphite is one of thesoftest.

Mineral Formula

graphite or diamond C

quartz SiO2

feldspar KAlSi3O8

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Figure 5. Model showing atoms inhalite. Halite has formula NaCl,meaning there is one sodium (Na)atom for every chlorine (Cl) atom. In this drawing, the large spheresrepresent sodium and the small onesrepresent chlorine.

garnet Fe3Al2Si3O12

mica KAl2(AlSi3)O10(OH)2

Table 1-2. Examples of minerals and their formulas. The elemental symbols are C = carbon, Si = silicon, O = oxygen, K = potassium, Al = aluminum, Fe = iron, H = hydrogen.

Mineralogists have named and described more than3,000 minerals, but most are rare; less than 200 can beconsidered common. Plagioclase is the most abundantmineral in the Earth’s crust (Figure 6). It is abundant forseveral reasons. Most important, plagioclase contains theelements, oxygen, silicon, aluminum, calcium, sodium andpotassium, six of the seven most common elements in thecrust. Minerals vary widely in their compositions andproperties; we will talk about them more later.

Figure 6. Plagioclase, a type of feldspar,is the most abundant mineral in Earth’scrust.

Rocks

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Returning to the glossary, we find that rocks

“...are naturally forming aggregates composed primarily of inorganicEarth materials.”

Most rocks contain one or more minerals but a few, such as obsidian (volcanic glass), contain nominerals at all. Some rocks contain large mineral grains, easily seen with the naked eye. Granite, for instance, contains visible grains of the minerals quartz and potassium feldspar. Some rocks contain visible fragments of preexisting rocks. Others are so fine grained that seeingwhat they are made of without a microscope is impossible. Geologists divide rocks into threecategories, each of which has a fundamentally different origin: igneous rocks, sedimentary rocksand metamorphic rocks.

Igneous rocks form by the cooling and solidification of magma, molten materialgenerally originating deep within the Earth. As magma moves upwards it cools and eventuallycrystallizes to form a solid rock. The race between upward movement and crystallizationdetermines the kind of rock. If the magma reaches the Earth’s surface, we get an extrusiveigneous rock. If it solidifies underground, we get an intrusive igneous rock. Sometimes magmasat the surface result in volcanoes, other times they produce flat lying lava flows that cover largeareas. Although we often think of volcanoes spewing lava that flows across the land, manyvolcanic rocks form from volcanic ash. Basalt, a fine grained dark colored rock, is the mostcommon rock formed from lava. Ash deposits are highly variable, but often result in cindercones associated with volcanoes.

Intrusive igneous rocks form when magmas cool and crystallize before reaching theEarth’s surface. Granite is one kind of intrusive rock, but there are many others. Intrusive rocksare composed of the same minerals as extrusive rocks, but generally have larger mineral grainsbecause they cooled more slowly.

Sedimentary rocks form when preexisting rocks are weathered, producing sediment thatsubsequently accumulates to produce a new rock. Most sedimentary rocks form from loose ordissolved material transported by water and deposited on river, lake or ocean bottoms. Theprocess that converts loose sediment to hard rock is lithification. Sedimentary rocks that formfrom detritus (sediment) transported by water, wind or gravity are detrital sedimentary rocks. Sandstone, for example, is a sedimentary rock that forms from quartz grains produced byweathering of preexisting rock. Pressure, recrystallization and chemical cements bind the loosegrains together to produce a rock from loose sediment. Siltstone and shale are detrital rockshaving smaller grains than sandstone. Sedimentary rocks formed by precipitation of dissolvedmaterial are chemical sedimentary rocks. Limestone is a chemical sedimentary rock that oftenforms when calcite (a mineral) precipitates from ocean water. Rock salt is another example of achemical sedimentary rock.

Metamorphic rocks form when heat, pressure or chemical reactions change themineralogy, texture or composition of a preexisting rock. Most metamorphism occurs whenrocks are buried deep in the Earth or when they are “baked” by heat given off from a magmabody. Because most metamorphic rocks form at great depths in the Earth, they are typicallyfound in mountain belts where they have been exposed by uplift and erosion. Gneiss and schistare two of the more common types of metamorphic rocks.

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Geological time is long and, although the process may be slow, rocks change over time. A rock of one type may be transformed into a different type. A sedimentary rock, for instance,may be metamorphosed to produce a metamorphic rock. That rock may melt to produce amagma that solidifies yielding an igneous rock. Geologists use the rock cycle (Figure 7) todescribe the ways one kind of rock may change into another. Some parts of the cycle occur atthe Earth’s surface, others require that rocks are buried to great depths within the Earth. Use ofthe word “cycle” is misleading. It implies that rocks continuously change from one kind toanother, following some sort of repetitive path. Some rocks may indeed cycle, but most rocksthat we see today have simpler histories and may have followed only one (or none) of the arrowsin Figure 7.

Figure 7. The rock cycle

Earth Terranes

Oceanic Terranes

If you came from some distant galaxy and saw Earth for the first time, the first thing youwould notice would be Earth’s oceans. Earth is sometimes called the blue planet because blueocean waters cover 71% of its surface. Oceans contain 98% of the world’s water, all of it saline(salty) to various degrees. The major oceans include the Pacific, Atlantic, Arctic and Indian, butthe Pacific is nearly as large as the other three together. It is also the deepest ocean, averagingaround 3900 meters deep (12,800 feet), and reaching nearly 7000 meters (23,000 feet) in someplaces. Oceans are the major source of water in the Earth’s atmosphere; nature maintains abalance and evaporation removes from the oceans about the same amount that enters by rain or

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runoff. Oceans also play a very significant role controlling Earth’s climate.

Coastal Terranes

Moving landward from ocean basins, up the continental shelf, ocean waters shallow untileventually you reach the zone of waves and coastal currents. Waves and currents are, in mostplaces, energized primarily by wind. In shallow areas, waves may form breakers as they interactwith the ocean bottom, but in most places they simply appear as wind driven swells. Swellsappear to travel across the ocean’s surface, although in reality most of the water movement is upand down or circular. Waves may travel 1000's of miles across an ocean. Major storms and highwinds in the South Pacific may result in huge (need a cool surfer term here) for surfers to enjoyin southern California. Besides waves, ocean tides can cause significant water movement inshallow areas. Tides result from the gravitational attraction of the moon and sun and by theEarth’s rotation.

Coastal areas are dynamic, constantly being changed by the energy concentrated there. Erosion is an ongoing process, and rocks along the shoreline may be ground into fine sediments. Rivers deliver additional sediments, derived from inland, to coastal areas. Currents, waves andgravity move and sort the unconsolidated material. Much is carried out to deeper waters, butsome may remain to form beaches. Coastal waters and continental shelves are also dynamicbecause they are often regions of reefs and other biological activity. Most of the worlds’ humanpopulation lives with 75 kilometers (45 miles) and, in many places, humans spend lots of timeand money trying to keep coastlines they way they want them, instead of the way nature tries toshape them.

Continental Terranes

Although continents account for less than a third of the Earth’s surface, they havereceived much more attention from geologists than the oceans. Due to ease of access and themany uses we make of continental resources, we have amassed much information and knowmany details about continental geology. If we do a simple inventory of the continents, we findthat continental surfaces are mostly covered by water, by regolith and other soft sediments, or byrocks. Most geologists do not study unconsolidated material, and are more interested in bedrockgeology. Although generally originally forming as horizontal layers, in many places the bedrockhas been deformed (folded, tilted, faulted, etc.), and geologists find it convenient to dividecontinents into two kinds of terranes:

a. areas where the bedrock is composed of generally undeformed flat lying rocksb. areas where the bedrock is composed of deformed rocks

Most continental crust fall into category “b.” Over geological time scales, ocean spreading anddrifting, and colliding continents, have caused the continental crust to be uplifted and deformed,especially near continental margins. The deformation may consist of uplifting, tilting, folding orfaulting. A general term for this deformation is tectonics, and if tectonic events are great enoughto produce mountains we call them orogenies. Some orogenies, such as the Himalayan orogeny

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Figure 8. Major mountain belts and shields of NorthAmerica.

are ongoing today. The Rocky Mountains were formed during the Laramide orogeny whichoccurred about 50 million years ago. The Appalachian Mountains formed during severalorogenies which took place between 250 and 600 million years ago.

Continents change in size by accretion (two or more continents joining), volcanic activity(which brings magma to the surface), or sea level changes (that expose more or less continent). These processes affect continental margins more than interiors, so most continents haverelatively young rocks at their margins compared with their centers. Some continents, such asSouth America, have orogenies taking place at one or more of their margins today. Mostorogenies result in long, relatively narrow and sinewy orogens, which may form mountain belts. Some, such as the Andes, are the sites of active volcanism. Others, such as the Himalayas, arenot.

North America’s two main mountain belts, the Appalachian Orogen and the Cordillera,have had prolonged and complex histories. The Appalachian mountain belt formed in pulses ofmountain building that occurred over several hundred million years. The Appalachians containmany varied rocks. Deformed sedimentary rocks dominate in some places, metamorphic origneous rocks in others. Volcanism was locally important. Much faulting occurred in some

parts of the Appalachians, in otherplaces the rocks are folded, and inothers most of the mountainbuilding was caused by gentleruplift.

The Cordillera is a chain ofmountains that extends up the westcoast of South America, throughCentral America and the UnitedStates, and up the west coast ofCanada to Alaska. At the latitude ofSan Francisco, the Cordillera is1600 kilometers wide; it is muchnarrower in other places. In Southand Central America, mostCordillera mountains are volcanic. There, and in the Cascade Range ofWashington and Oregon, activevolcanism continues today.

Not all Cordillera mountainsare volcanic, however. TheLaramide Orogeny created theuplifted Rocky Mountains ofColorado. The Rockys form theeastern part of the North AmericanCordillera. To the west of theRockys, mountains of the Basin andRange Province in Nevada and

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western Utah are fault-block mountains. Faulting produced uplifted blocks of the crust separatedby downdropped valleys. The Sierra Nevada Mountains in California were formed from largebodies of intrusive igneous rock called batholiths. In western Canada, mountain buildingoccurred when “microcontinents” accreted onto a growing North America.

Formation of the Appalachian and Rocky Mountains of eastern and western NorthAmerica deformed ancient Earth Crust, called the North American Craton. Cratons make up the flat, tectonically stable interior portions of most continents (Figure 9). Most are composed oferoded flat rocks, sometimes called basement rocks, covered by sediments. The centers ofcratons generally contain shields, areas with extensive exposure of very old basement rocks. Often these ancient rocks are highly deformed; they are the roots of mountain ranges that longago were eroded smooth. In contrast with shields, the deformed rocks at present day continentalmargins are relatively young and mountains may still stand high. Much of what we know aboutthe evolution of the early Earth is based on studies of shield geology. The Canadian Shield ofNorth America, which extends into the United States in the New York, Michigan, Wisconsin andMinnesota, contains evidence of mountain building events that took place up to 3.2 billion yearsago. All major continents contain shields and rocks similar to those found in the CanadianShield.

Figure 9. Cratons and Orogens inNorth America. This figure showsthe deep basement rocks that wouldbe exposed if all the mountain belts,sedimentary rocks and sedimentwere stripped away from NorthAmerica. The oldest cratons andorogens are in the center of thecontinent. Together they make upthe Canadian Shield. See Figure 8.

South of the Canadian Shield, in the central part of North America, sediments coverrelatively flat lying sedimentary bedrock, many thousands of feet thick. This region is the NorthAmerican platform (Figure 10). If you were to drill holes in different places in the platform, youwould find that the sediments vary in thickness depending on where you are. Beneath thesediments, you would find generally flat lying bedrock. Near the surface the bedrock isrelatively young, but the deeper you drilled, the older the rocks get. Eventually you would drillinto the same kinds of ancient rocks that make up the Canadian Shield. Platform sediments vary,

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Figure 10. Landforms of the continental United States.

primarily due to climate variations. In some places they include rich soils, and in other placesthey are not. The bedrock beneath is also variable, but geologists interpret most of it asindicating that shallow seas covered the central portions of North America at various times in thepast.

Figure 10 shows the landforms of the United States. The Appalachian and Cordilleran Orogensshow well – they include many mountain belts. Between the two, the North American Platformis relatively flat.

Most National Parks in the United States are in the arid west, especially in the mountainous landof the Cordillera. This is primarily an accident of history – by the time the United States gotaround to designating parks, the eastern half of the country and the western coast were all readyhighly developed. Only the arid lands of the west, and some of the high mountains remainedundeveloped.

Today we have three National Parks in the Appalachian Mountains (Great Smoky,Shenandoah, and Acadia). There are also several in Florida. With the exception of Mammoth

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Cave and Isle Royale, all others are in the western part of the continental United States, inAlaska, or in Hawaii.