Introduction To Geology And Geological Materials

1-1 Introduction to Geology and Geological Materials What is Geology? Geology, according to Webster’s Dictionary, is “a science that deals with the history of Earth and its life, especially as recorded in rocks.” It is a hybrid science, borrowing from many fields. Some geologists study the chemistry of rocks, minerals or other Earth materials. Others study Earth physics or the biology of plants and animals, ancient and modern. Still others study distant planets to understand the origin of the solar system or even of the universe. Geologists may stray into the fields of oceanography, meteorology, biology or astronomy, but fundamentally, geology is the study of the Earth, as the dictionary says, and most geologists study materials and processes at or near the Earth’s surface. The Earth has changed and evolved during its long history, and the changes are chronicled by Earth’s geology. The Earth is unique among the planets. We enjoy running water, a hospitable atmosphere and cool temperatures. Rocks and minerals are constantly being recycled, albeit over long periods of time. Material, once at the surface, is carried deep into the Earth, only to be brought up to the surface again. The planet’s surface is teeming with abundant life, and the life zone 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 shows basalt from western Ontario. The bottom left figure shows equivalent rocks from Mars (photo taken by the Pathfinder mission). The dark areas on the photo of the moon in the lower right are also basalt. The similarity of rock types suggests commonality in geological processes. However, Earth has an atmosphere and, most importantly, running water at the surface. Thus, Earth is a fascinating and dynamic planet compared with Mars and the Moon.

1-2 Geology is unique among the sciences because many geological processes are extremely slow, and geological time is extremely long. So, much of a geologist’s time is spent collecting and 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 earliest events in Earth history. Answering some geological questions is like trying to put a jigsaw puzzle together with many pieces missing. Geologists collect new evidence, and add new pieces, as they try to come up with complete pictures. Like all scientists, geologists apply the scientific method (Figure 2) when trying to answer complex questions. Because science cannot produce absolute answers, this means that geological science, just like the Earth, is constantly evolving. A key assumption made by geologists is that the laws of physics and Earth processes are the same today as they were in the past. Thus, if geologists find a 560 million year old massive sandstone, they may infer that it formed as a beach deposit in ancient seas, since that is where such sandstones are forming today. This allows geologists to infer the presence of oceans and continents over half a billion years ago (Figure 3).

Figure 2. The scientific method is one that involves proposing a hypothesis to explain observations, followed by subsequent testing. Generally the testing involves making predictions and then checking to see if the predictions are correct. Often this requires making new observations and collecting additional data. Eventually, if a hypothesis passes enough tests, it evolves into an accepted theory. In reality, all of science is theory. This means it can change as new information and new tests become available.

1-3

Figure 3. The three views show Montara Beach in California. The top view shows typical beach sand; the middle view shows beach sand and some sandstone. The bottom view shows a geologists examining the sandstone. The sandstone itself is the fossilized remains of an ancient beach. Sandstones of this sort are evidence that beaches, similar to modern day beaches, existed in the ancient past. Besides the academic interests, Geology has many practical aspects. Earth resources are fundamental to our existence (Table 1). Energy resources, including coal, oil, gas, uranium, and geothermal, are keys to our modern societies. Copper, nickel, iron and other metals have for several hundred years been the basis of industries. Nitrates, borax and other chemicals from the Earth are in heavy demand. Sand, gravel and other building materials are used world wide. Geologists play important roles in many engineering projects, including roads, buildings, and dams. They are also concerned about natural hazards such as earthquakes, volcanoes, tidal waves or landslides. Increasingly over the last several decades, geologists have become more and more concerned with environmental problems. Water supplies, water pollution, waste disposal, and many other problems have become more acute as population and urban areas expand. Table 1-1. Earth’s geological resources are the basis for modern society. Group

Examples

metallic and semimetallic elements

gold, silver, copper, iron, manganese, nickel, aluminum

nonmetallic elements

potassium, sodium, phosphorous, sulfur

1-4 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, and processed metals and minerals from the Earth. Herodotus, in 500 B.C., studied the flooding of the Nile River, and Aristotle described fossils in 350 B.C. Theophrastus wrote the first mineralogy book Concerning Stones about 300 B.C. However, geology, like most sciences, did not 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 or alchemists. Great advances soon followed, ultimately leading to the industrial revolution. Although we could debate which events in the development of modern geological science were most important, several key ones stand out. In the late 1700's, James Hutton espoused his principle of uniformitariansim, sometimes summarized as “the present is the key to the past.” It stated 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 John Playfair, Charles Lyell and others several decades later. Playfair, for example, was the first to propose that rivers cut their own valleys over very long times, and the first to describe the way glaciers can move boulders and polish the Earth. He published several important books including, in 1802, Illustrations of the Huttonian Theory of the Earth. Lyell further promoted the ideas of Hutton. He studied geological formations in Europe, North America and England and concluded that the Earth must be millions of years old. Lyell tentatively accepted the theory of Darwinian evolution and applied some of Darwin’s principles to fossils in his The Geological Evidence of the Antiquity of Man, published in 1863. He is, however, probably best known for his Principles of Geology (1830). The early British geologists were on the right track, and the discoveries spawned by Hutton were significant. Scientists studied erosion and other processes taking place around them and estimated how long it took for different kinds of geological features to form. They concluded that the Earth must be much older than estimates based on the Bible, which placed the age of the Earth at several thousand years. The true magnitude of geological time eluded them however. A major breakthrough came at the end of the 19th century when Marie and Pierre Curie, Wilhelm Roentgen, and others discovered and applied radioactivity in their scientific quests. Soon, scientists were using radioactive age dating to estimate the age of rocks and minerals. Over just a few decades, the estimated age of the Earth went from 10's or 100's of million to billions of years, and geologists found that explaining the evolution of the Earth was much easier. They concluded that the Earth is always changing and evolving, but the processes were much slower than originally envisioned.

1-5 Figure 4. This view shows the relative position of continents about 130 million years ago. Since then, they have drifted apart, at rates equivalent to how fast your fingernails grow. Evidence for continental drift includes (1) the way the continents seem to fit together, (2) matching fossils, rocks, mountain chains on different sides of modern oceans, (3) the existence of a mid ocean ridge and measurable seafloor spreading that takes place there. Much of the early work by geologists was descriptive. Explaining why things had happened was more difficult. One major breakthrough, which allowed scientists to answer some of the unanswered “why” questions, came in the early 1960's. J.T. Wilson is often given credit for developing and popularizing the idea of continental drift at this time, although others had proposed the concept previously. Anyone could see that South America and Africa fit together, much like two puzzle pieces (Figure 4). Fossil and other evidence, too, suggested that the continents had once been joined and subsequently had “drifted” apart. Before 1960, however, geologists could not understand why continents would move relative to each other and, without knowing the mechanism, the theory of continental drift Earth Materials stalled. In the 1960's, however, much better seafloor mapping, better seismic networks, regolith: general term for layer of fragmental, and many other things all came together. loose material of any origin and type that Geophysicists proposed credible hypotheses overlies bedrock to explain continental drift. Within a decade, soil: unconsolidated earth material, rich in the theory of plate tectonics was firmly organic components, that provides the natural established and it has passed all scientific medium for plant growth tests since then. Today it forms the sediment: solid fragmental material, either fundamental basis for much of modern organic or inorganic, that originates from geology, and has provided us with many of weathering of rocks the missing puzzle pieces. mineral: naturally occuring inorganic solids that are crystalline, that have a specific Earth Materials chemicl composition, and that have fixed Regolith, Sediment and Soil physical properties rock: naturally forming aggregates composed An unconsolidated layer of material primarily of inorganic Earth materials called regolith covers most of Earth’s land

1-6 surfaces. Regolith is mostly rock and mineral debris produced by mechanical or chemical decomposition (weathering) of preexisting rocks. We call such debris sediment. Regolith may also 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 and air. In most places, regolith forms a relatively thin layer at the Earth’s surface; it is underlain at some depth by bedrock. The thickness of the regolith depends on many factors, most important, perhaps, climate and topography. The composition of regolith also depends on these factors but, most significantly, on the type of bedrock from which it was derived. We will discuss 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 a specific chemical composition, and that have fixed physical properties.” Crystalline means that each mineral has a specific, orderly and repetitive, internal arrangement of atoms. Some crystals have flat crystal faces, but many do not; they are still considered crystals. Minerals, just like all matter, consist of atoms of specific elements. They are compounds, which among other things, means that we can describe them with a chemical formula (Table 1-2). Some minerals, such as graphite or sulfur, contain only one element. Most contain several or many elements. Having a specific chemical composition means that all samples 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 number or sodium (Na) and chlorine (Cl) atoms in a sample of halite, whatever its size. Halite is crystalline, 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 low levels. Because all halite has the same atomic arrangement and composition, all halite has the same 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 have different arrangements of carbon atoms in their atomic structure. Consequently they have different physical properties. Diamond is the hardest mineral known, graphite is one of the softest. Mineral

Formula

graphite or diamond

C

quartz

SiO2

feldspar

KAlSi3O8

1-7 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 than 3,000 minerals, but most are rare; less than 200 can be considered common. Plagioclase is the most abundant mineral in the Earth’s crust (Figure 6). It is abundant for several reasons. Most important, plagioclase contains the elements, oxygen, silicon, aluminum, calcium, sodium and potassium, six of the seven most common elements in the crust. Minerals vary widely in their compositions and properties; we will talk about them more later. Figure 5. Model showing atoms in halite. Halite has formula NaCl, meaning there is one sodium (Na) atom for every chlorine (Cl) atom. In this drawing, the large spheres represent sodium and the small ones represent chlorine. Figure 6. Plagioclase, a type of feldspar, is the most abundant mineral in Earth’s crust.

Rocks

1-8 Returning to the glossary, we find that rocks “...are naturally forming aggregates composed primarily of inorganic Earth materials.” Most rocks contain one or more minerals but a few, such as obsidian (volcanic glass), contain no minerals 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 seeing what they are made of without a microscope is impossible. Geologists divide rocks into three categories, each of which has a fundamentally different origin: igneous rocks, sedimentary rocks and metamorphic rocks. Igneous rocks form by the cooling and solidification of magma, molten material generally originating deep within the Earth. As magma moves upwards it cools and eventually crystallizes to form a solid rock. The race between upward movement and crystallization determines the kind of rock. If the magma reaches the Earth’s surface, we get an extrusive igneous rock. If it solidifies underground, we get an intrusive igneous rock. Sometimes magmas at the surface result in volcanoes, other times they produce flat lying lava flows that cover large areas. Although we often think of volcanoes spewing lava that flows across the land, many volcanic rocks form from volcanic ash. Basalt, a fine grained dark colored rock, is the most common rock formed from lava. Ash deposits are highly variable, but often result in cinder cones associated with volcanoes. Intrusive igneous rocks form when magmas cool and crystallize before reaching the Earth’s surface. Granite is one kind of intrusive rock, but there are many others. Intrusive rocks are composed of the same minerals as extrusive rocks, but generally have larger mineral grains because they cooled more slowly. Sedimentary rocks form when preexisting rocks are weathered, producing sediment that subsequently accumulates to produce a new rock. Most sedimentary rocks form from loose or dissolved material transported by water and deposited on river, lake or ocean bottoms. The process that converts loose sediment to hard rock is lithification. Sedimentary rocks that form from 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 by weathering of preexisting rock. Pressure, recrystallization and chemical cements bind the loose grains together to produce a rock from loose sediment. Siltstone and shale are detrital rocks having smaller grains than sandstone. Sedimentary rocks formed by precipitation of dissolved material are chemical sedimentary rocks. Limestone is a chemical sedimentary rock that often forms when calcite (a mineral) precipitates from ocean water. Rock salt is another example of a chemical sedimentary rock. Metamorphic rocks form when heat, pressure or chemical reactions change the mineralogy, texture or composition of a preexisting rock. Most metamorphism occurs when rocks are buried deep in the Earth or when they are “baked” by heat given off from a magma body. Because most metamorphic rocks form at great depths in the Earth, they are typically found in mountain belts where they have been exposed by uplift and erosion. Gneiss and schist are two of the more common types of metamorphic rocks.

1-9 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 a magma that solidifies yielding an igneous rock. Geologists use the rock cycle (Figure 7) to describe the ways one kind of rock may change into another. Some parts of the cycle occur at the Earth’s surface, others require that rocks are buried to great depths within the Earth. Use of the word “cycle” is misleading. It implies that rocks continuously change from one kind to another, following some sort of repetitive path. Some rocks may indeed cycle, but most rocks that we see today have simpler histories and may have followed only one (or none) of the arrows in 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 you would notice would be Earth’s oceans. Earth is sometimes called the blue planet because blue ocean 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, but the Pacific is nearly as large as the other three together. It is also the deepest ocean, averaging around 3900 meters deep (12,800 feet), and reaching nearly 7000 meters (23,000 feet) in some places. Oceans are the major source of water in the Earth’s atmosphere; nature maintains a balance and evaporation removes from the oceans about the same amount that enters by rain or

1-10 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 until eventually you reach the zone of waves and coastal currents. Waves and currents are, in most places, energized primarily by wind. In shallow areas, waves may form breakers as they interact with the ocean bottom, but in most places they simply appear as wind driven swells. Swells appear to travel across the ocean’s surface, although in reality most of the water movement is up and down or circular. Waves may travel 1000's of miles across an ocean. Major storms and high winds in the South Pacific may result in huge (need a cool surfer term here) for surfers to enjoy in southern California. Besides waves, ocean tides can cause significant water movement in shallow areas. Tides result from the gravitational attraction of the moon and sun and by the Earth’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 and gravity move and sort the unconsolidated material. Much is carried out to deeper waters, but some may remain to form beaches. Coastal waters and continental shelves are also dynamic because they are often regions of reefs and other biological activity. Most of the worlds’ human population lives with 75 kilometers (45 miles) and, in many places, humans spend lots of time and money trying to keep coastlines they way they want them, instead of the way nature tries to shape them. Continental Terranes Although continents account for less than a third of the Earth’s surface, they have received much more attention from geologists than the oceans. Due to ease of access and the many uses we make of continental resources, we have amassed much information and know many details about continental geology. If we do a simple inventory of the continents, we find that continental surfaces are mostly covered by water, by regolith and other soft sediments, or by rocks. Most geologists do not study unconsolidated material, and are more interested in bedrock geology. Although generally originally forming as horizontal layers, in many places the bedrock has been deformed (folded, tilted, faulted, etc.), and geologists find it convenient to divide continents into two kinds of terranes: a. areas where the bedrock is composed of generally undeformed flat lying rocks b. areas where the bedrock is composed of deformed rocks Most continental crust fall into category “b.” Over geological time scales, ocean spreading and drifting, 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 or faulting. A general term for this deformation is tectonics, and if tectonic events are great enough to produce mountains we call them orogenies. Some orogenies, such as the Himalayan orogeny

1-11 are ongoing today. The Rocky Mountains were formed during the Laramide orogeny which occurred about 50 million years ago. The Appalachian Mountains formed during several orogenies 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 have relatively young rocks at their margins compared with their centers. Some continents, such as South America, have orogenies taking place at one or more of their margins today. Most orogenies 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, are not. 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 of mountain building that occurred over several hundred million years. The Appalachians contain many varied rocks. Deformed sedimentary rocks dominate in some places, metamorphic or igneous rocks in others. Volcanism was locally important. Much faulting occurred in some parts of the Appalachians, in other Figure 8. Major mountain belts and shields of North places the rocks are folded, and in America. others most of the mountain building was caused by gentler uplift. The Cordillera is a chain of mountains that extends up the west coast of South America, through Central America and the United States, and up the west coast of Canada to Alaska. At the latitude of San Francisco, the Cordillera is 1600 kilometers wide; it is much narrower in other places. In South and Central America, most Cordillera mountains are volcanic. There, and in the Cascade Range of Washington and Oregon, active volcanism continues today. Not all Cordillera mountains are volcanic, however. The Laramide Orogeny created the uplifted Rocky Mountains of Colorado. The Rockys form the eastern part of the North American Cordillera. To the west of the Rockys, mountains of the Basin and Range Province in Nevada and

1-12 western Utah are fault-block mountains. Faulting produced uplifted blocks of the crust separated by downdropped valleys. The Sierra Nevada Mountains in California were formed from large bodies of intrusive igneous rock called batholiths. In western Canada, mountain building occurred when “microcontinents” accreted onto a growing North America. Formation of the Appalachian and Rocky Mountains of eastern and western North America 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 of eroded flat rocks, sometimes called basement rocks, covered by sediments. The centers of cratons 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 long ago were eroded smooth. In contrast with shields, the deformed rocks at present day continental margins are relatively young and mountains may still stand high. Much of what we know about the evolution of the early Earth is based on studies of shield geology. The Canadian Shield of North America, which extends into the United States in the New York, Michigan, Wisconsin and Minnesota, contains evidence of mountain building events that took place up to 3.2 billion years ago. All major continents contain shields and rocks similar to those found in the Canadian Shield.

Figure 9. Cratons and Orogens in North America. This figure shows the deep basement rocks that would be exposed if all the mountain belts, sedimentary rocks and sediment were stripped away from North America. The oldest cratons and orogens are in the center of the continent. Together they make up the Canadian Shield. See Figure 8.

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

1-13 primarily due to climate variations. In some places they include rich soils, and in other places they are not. The bedrock beneath is also variable, but geologists interpret most of it as indicating that shallow seas covered the central portions of North America at various times in the past. Figure 10. Landforms of the continental United States.

Figure 10 shows the landforms of the United States. The Appalachian and Cordilleran Orogens show well – they include many mountain belts. Between the two, the North American Platform is relatively flat. Most National Parks in the United States are in the arid west, especially in the mountainous land of the Cordillera. This is primarily an accident of history – by the time the United States got around to designating parks, the eastern half of the country and the western coast were all ready highly developed. Only the arid lands of the west, and some of the high mountains remained undeveloped. 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

1-14 Cave and Isle Royale, all others are in the western part of the continental United States, in Alaska, or in Hawaii.