College Textbook Sample 3 by Terri Wright

Earth Science & the

Environment

FOURTH EDITION

GRAHAM R.THOMPSON, PHD University of Montana

JONATHAN TURK, PHD

Australia • Brazil • Canada • Mexico • Singapore Spain • United Kingdom • United States

CHAPTER 4

• Geologic Time

567

UNIT

Brief Contents

Contents

1 Earth Systems

2 3

Minerals 22 Rocks 4 Geologic Time: A Story in the Rocks Geologic Resources 94

INTERNAL PROCESSES 6 7 8 9

UNIT

EARTH MATERIALS AND TIME 2 3 4 5

UNIT

Earth Systems

1.1 1.2 1.4

121

The Active Earth: Plate Tectonics 122 Earthquakes and the Earth’s Structure 146 Volcanoes and Plutons 169 Mountains 195

SURFACE PROCESSES

215

3.1 3.2 3.3 3.4

UNIT

17 18 19 20 21

UNIT

The Atmosphere 408 Energy Balance in the Atmosphere 428 Moisture, Clouds, and Weather 446 Climate 479 Climate Change 499

ASTRONOMY

525

22 Motions in the Heavens 526 23 Planets and Their Moons 548 24 Stars, Space, and Galaxies 574 iv

FOCUS ON: Cooling and Crystallization of Magma:

Bowen’s Experiment

407

UNIT

Rocks and the Rock Cycle 45 Igneous Rocks 47 Sedimentary Rocks 51 Metamorphic Rocks 60

THE ATMOSPHERE

FOCUS ON: Chemical Bonds

3 Rocks

15 Ocean Basins 352 16 Oceans and Coastlines 376

What Is a Mineral? 24 Elements,Atoms, and the Chemical Composition of Minerals 25 Crystals:The Crystalline Nature of Minerals 28 Physical Properties of Minerals 30 Environmentally Hazardous Rocks and Minerals 38

THE OCEANS 351

EARTH MATERIALS AND TIME 2.1 2.2 2.3 2.4 2.5

and Wetlands 245 12 Water Resources 275 13 Glaciers and Ice Ages 307 14 Deserts and Wind 342

FOCUS ON: Hypothesis,Theory, and Law

2 Minerals

10 Weathering, Soil, and Erosion 216 11 Fresh Water: Streams, Lakes, Ground Water,

UNIT

Flowers Bloom on Earth,Venus Boils, and Mars Freezes The Earth’s Four Spheres 4 Time in Earth Science 38

UNIT

THE OCEANS

351

15 Ocean Basins

353

15.1 The Origin of the Earth’s Oceans 2 15.2 The Earth’s Oceans 11 15.3 Studying the Sea Floor 11

V FOCUS ON: The Ocean Floor 26 v

UNIT

Steep-sided volcanic cones along the Chilean-Argentinean border add texture to this "study in blue." Of approximately 1800 volcanoes scattered across this region, 28 are active.

EARTH MATERIALS AND TIME 2 3 4 5

Minerals Rocks Geologic Time: A Story in the Rock Geologic Resouces

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Geologic Time: A Story in the Rocks

CHAPTER

Aerial view of the San Andreas Fault, California, USA

4.1 4.2 4.3 4.4 4.5 4.6

Earth Rocks, Earth History, and Mass Extinctionslogic Time: A Story in the Rocks Geologic Time Relative Geologic Time Unconformities and Correlation Absolute Geologic Time The Geologic Column and Time Scale

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J for Liberal Arts Mathematics. Throughout this chapter, the ThomsonNOW logo indicates an opportunity for online self-study, which: • Assesses your knowledge of important concepts and provides a personalized study plan • Links you to interactive tutorials and videos to help you study • Helps to test your knowledge of material and prepare for exams Visit ThomsonNOW at www.thomsonedu.com/1pass to access these resources.

ames Hutton, Some of the earliest geologists, realized that sandstone forms as sand slowly accumulates and is compressed into solid rock. He also saw that sand and other sediment accumulate layer by layer, so that the deepest layers of sedimentary rocks are the oldest and the layers become younger toward the Earth’s surface. Early geologists also found ancient shells, bones, and other fossils in sandstone and in other kinds of sedimentary rocks. They realized that if the rocks formed with younger layers above older ones, and if fossils were preserved as the rock formed, then fossils in deep layers must be older than the fossils in the upper layers. Therefore, scientists can trace the history of life on Earth by studying fossils embedded in successively younger rock layers. This history is now summarized in a geologic column and time scale, shown later in this chapter. ■

4.1 Earth Rocks, Earth History, and Mass Extinctions In many locations, nineteenth-century geologists found thick sequences of rock layers with abundant fossils that we now know represent millions, or hundreds of millions, of years of Earth history. But younger layers above the fossil-rich rocks contain few fossils or none at all. Looking even higher in the sequence of rock layers, they found abundant fossils again, but of organisms very different from those in the older rocks. Even more surprising, fossils of many of the most abundant animals and plants in lower layers are never seen again in younger rocks. Thus, those organisms simply disappeared from the face of the Earth forever. This fossil record suggests that sudden, catastrophic events had abruptly decimated life on Earth, and that new life forms emerged following these mass extinctions. However, scientists did not immediately accept the idea that catastrophic events had caused sudden mass extinctions. Instead, they suggested that the rock record might not be complete. Maybe some rocks had been destroyed by erosion, or perhaps sedimentary rocks had not formed for a long period of time. According to this reasoning, the part of the record that was lost could have contained evidence for the gradual decline of the species that became extinct, and for the slow emergence of new species. Following conventional wisdom of the time, they concluded that the emergence of new ones occurred slowly, as a result of gradually changing conditions. But if pieces of the rock record were missing in some locations, they must exist in other places on Earth. Geologists searched for rocks to fill in the gaps and provide evidence for gradual extinctions, but they did not

CHAPTER 4

• Geologic Time

find them. Eventually, the evidence for near instantaneous extinctions became compelling. The most dramatic extinction occurred 248 million years ago, at the end of the Permian period. At that time, 90 percent of all species in the oceans suddenly died out. On land, about two thirds of reptile and amphibian species and 30 percent of insect species vanished. One modern scientist exclaimed that this extinction event was “the closest life has come to complete extermination since its origin.”1 The death of most life forms at the end of Permian time left huge ecological voids in the biosphere. Ocean ecosystems changed as new organisms emerged in an environment free of predators and competition. One hundred and sixty million years later, another catastrophic extinction wiped out one fourth of all species, including the dinosaurs. Geologists searched for rocks to fill in the gaps and provide evidence for gradual extinctions, but they did not find them. The evidence for near instantaneous extinctions became compelling. This mass extinction occurred 65 million years ago. Small mammals survived this disaster, facing a new world free of the efficient predators that had hunted them. The number of mammal species increased rapidly after this extinction event, eventually leading to the evolution of humans. Geologists searched for rocks to fill in the gaps and provide evidence for gradual extinctions, but they did not find them. Eventually, the evidence for near instantaneous extinctions became compelling. Geologists searched for rocks to fill in the gaps and provide evidence for gradual extinctions, but they did not find them. Eventually, the evidence for near instantaneous extinctions became compelling. Other scientists have found evidence of a huge meteorite crater that formed 65 1. Douglas Erwin, The Great Paleozoic Crisis (New York: Columbia University Press, 1993), 187.

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V FIGURE 4.2 Today, most scientists agree with the hypothesis that the Cretaceous extinction was caused by a giant meteorite that slammed into the Earth. In this artist’s rendition, (A) a large meteorite struck just north of the Yucatan peninsula. (B) The impact pulverized both the meteorite and rocks of the Earth’s crust, creating a thick, dark cloud that blocked out the sunlight, causing the Earth’s surface to freeze. Each of these hypotheses involves large-scale systems interactions among the biosphere, the geosphere, the atmosphere, and the hydrosphere. In the first hypothesis described below, a fifth, extraterrestrial, system is seen as the trigger that initiated radical changes in all four Earth systems.

Extraterrestrial Impacts In 1977, the father-and-son team of Walter and Luis Alvarez were studying rocks that formed at the time the dinosaurs and so many other life forms became extinct. In one layer, they found abundant dinosaur fossils. Just above it, they found very few fossils of any kind and no dinosaur fossils. Between these two rock layers, they found a thin, sooty, clay layer. They brought samples of the clay to the laboratory and found that it contained high concentrations of the element iridium. This discovery was surprising because iridium is rare in Earth rocks. Where did it come from? Although rare in the Earth’s crust, iridium is more abundant in meteorites. Alvarez and Alvarez suggested that 65 million years ago, a meteorite 10 kilometers in diameter hit the Earth with energy equal to 10,000 times that of today’s entire global nuclear arsenal. The collision vaporized both the meteorite and the Earth’s crust at the point of impact, forming a plume of hot dust and gas that ignited fires around the planet (Figure 4.2). Soot from the global wildfires and iridium- rich meteorite dust rose into the upper atmosphere, circling the globe. The thick,

dark cloud blocked out the Sun and halted photosynthesis for as much as a year. Surface waters froze, and many plants and animals died. This event, called the terminal Cretaceous extinction, killed off the dinosaurs and many other life forms. Other scientists have found evidence of a huge meteorite crater that formed 65 million years ago in the Caribbean Sea north of the Yucatan Peninsula. Most scientists now agree with the meteorite impact hypothesis for the terminal Cretaceous extinction (V Figure 4.2 B). Some scientists have suggested that an extraterrestrial impact also caused the Permian extinction, but the evidence is less compelling.2

Volcanic Eruptions A volcanic eruption ejects gas and fine volcanic ash into the atmosphere. The ash acts as an umbrella that reflects sunlight back into space and thus causes climatic cooling. One volcanic gas, sulfur dioxide, forms small particles called aerosols that also reflect sunlight and cool the Earth. Carbon dioxide is another gas emitted by volcanoes. It is a greenhouse gas that can cause warming of the Earth’s atmosphere. Thus, volcanoes erupt materials capable of causing both atmospheric cooling and warming. In many eruptions, the cooling overwhelms the 2. Douglas Erwin, The Great Paleozoic Crisis (New York: Columbia University Press, 1993), 187.

4.3 Relative Geologic Time

tionships between volcanic eruptions and climatic cooling and warming are discussed further in Chapters 8 and 21. Scientists have noted that some mass extinctions coincided with unusually high rates of volcanic activity. For example, massive flood basalts erupted onto the Earth’s surface in Siberia 248 million years ago—at the same time that the Permian extinction occurred.

shallow-dwelling marine organisms. Later, new organisms evolved when the carbon dioxide levels fell. The cool air cooled the surface of the sea. When water cools, it becomes denser. Eventually a threshold was reached at which polar surface water became denser than the deep water. At this point the cool, dense surface water sank.

• The sinking surface waters forced the carbon diox-

Supercontinents and Earth’s Carbon Dioxide Budget In the modern oceans, cold polar seawater sinks to the sea floor and flows as a deep ocean current toward the equator. This current transports oxygen to the deep ocean basins and forces the deep water to rise near the equator, where it is warmed. The current thus mixes both heat and gases throughout the oceans and the atmosphere. You can read more about ocean currents in Chapter 16.

Supercontinents Ocean currents are partially controlled by the positions of the continents. Recall from Chapter 1 that the continents slowly drift across the globe. Several times in Earth history, all the continents have joined together to form one giant supercontinent. One such supercontinent assembled during Permian time (V Figure 4.3). Computer calculations indicate that the assembly of all continents into a single land mass, and the consequent development of a single global ocean prevented mixing between the surface water and the ocean depths.3

• •

Each of these three hypotheses involves large scale interactions among the atmosphere, the hydrosphere, the biosphere, and the geosphere, and the first also includes an extraterrestrial factor. The third hypothesis, if correct, is a classic example of both Earth systems feedback mechanisms and a threshold effect. The feedback process occurred as tectonic plate movement altered the shapes of the ocean basins. Ocean currents changed drastically when the basins changed. The alteration in currents, in turn, changed atmospheric composition, and unleashed a catastrophic event that killed almost living organisms on the planet Earth.

4.2

SURFACE MARINE ORGANISMS Absorb atmospheric carbon dioxide and use it to produce organic tissue. These organisms then die and settle to the sea floor. This process removes carbon dioxide from the atmosphere, converts the carbon to organic tissue, and transports it to the sea floor. Organisms living near the deep sea floor then consume the fallen litter and release carbon dioxide into the deep ocean water. Much of this gas dissolves in seawater and is held there by the pressure of overlying water. Modern ocean currents mix deep and shallow seawater, returning the carbon dioxide to the atmosphere. However, because there was little vertical mixing in the Permian oceans, carbon accumulated in ever-increasing concentration in the deep oceans (Figure 4.3 B). This process continued to remove carbon dioxide from the atmosphere and stored it in the deep oceans during late Permian time. But carbon dioxide is a greenhouse gas that absorbs heat and warms the atmosphere. As carbon dioxide was removed from the air and stored in deep ocean water, the atmosphere cooled. Some scientists have suggested that continental ice sheets formed as a result of the global cooling. Sea level fell as water evaporated from the oceans and accumulated on continental glaciers, exposing continental shelves and stressing populations of

CHAPTER 4

• Geologic Time

ide-rich deep water to rise to the sea surface, where it rapidly released massive amounts of carbon dioxide into the atmosphere (Figure 4.3 C). According to this hypothesis, the carbon dioxide asphyxiated life both in the seas and on the continents, killing most life on Earth. Later, new organisms evolved when the carbon dioxide levels fell.

Geologic Time While most of us think of time in terms of days or years, Earth scientists commonly refer to events that happened millions or billions of years ago. In Chapter 1 you learned that the Earth is approximately 4.6 billion years old. Yet humans and our human-like ancestors have existed for only 5 to 7 million years, and recorded history is only a few thousand years old. How do scientists measure the ages of rocks and events that occurred millions or billions of years ago, long before recorded history or even human existence? Scientists measure geologic time in two different ways. Relative age measurement refers only to the order in which events occurred. Determination of relative age is based on a simple principle: In order for an event to affect a rock, the rock must exist first. Thus, the rock must be older than the event. This principle seems obvious, yet it is the basis of much geologic work. As you learned in Chapter 3, sediment normally accumulates in horizontal layers. However, the sedimentary rocks in Figure 4.4 are folded and tilted. We deduce that the folding and tilting occurred after the sediment accumulated.

567

Table 2.1 The Eight Most Abundant Chemical Elements in the Earth’s Crust*

Element

Symbol

Weight Percent

Atom Percent

Volume Percent†

Oxygen Silicon Aluminum Iron

O Si Al Fe

46.60 27.72 8.13 5.00

62.55 21.22 6.47 1.92

93.8 0.9 0.5 0.4

Side head over more than one column

Calcium Sodium Potassium Magnesium Magnesium

Ca Na K Mg Mg

3.63 2.83 2.59 2.09 2.09

1.94 2.64 1.42 1.84 1.84

1.0 1.3 1.8 0.3 0.3

Total

98.59

100.00

*Abundances are given in percentages by weight, by numbers of atoms, and by volume. † These numbers will vary somewhat as a function of the ionic radii chosen for the calculations. SOURCE: From Principles of Geochemistry by Brian Mason and Carleton B. Moore. © 1982 by John Wiley & Sons, Inc.

While most of us think of time in terms of days or years, Earth scientists commonly refer to events that happened millions or billions of years ago. In Chapter 1 you learned that the Earth is approximately 4.6 billion years old. Yet humans and our human-like ancestors have existed for only 5 to 7 million years, and recorded history is only a few thousand years old. How do scientists measure the ages of rocks and events that occurred millions or billions of years ago, long before recorded history or even human existence? Scientists measure geologic time in two different ways. Relative age measurement refers only to the order in which events occurred. Determination of relative age is based on a simple principle: In order for an event to affect a rock, the rock must exist first. We deduce that the folding and tilting occurred after the sediment accumulated. The order in which rocks and geologic features formed can usually be interpreted by such observation and logic. The order in which rocks and geologic features formed can usually be interpreted by such logic. Absolute age is age in years. Dinosaurs became extinct 65 million years ago. The terminal Permian extinction occurred 248 million years ago. The Teton Range in Wyoming began rising 6 million years ago. Absolute age tells us both the order in which events occurred and the amount of time that has passed since they occurred. The terminal Permian extinction occurred 248 million years ago. The Teton Range in Wyoming began rising 6 million years ago. Absolute age tells us both the order in which events occurred and the amount of time that has passed since they occurred. The order in which rocks and geologic features formed can usually be interpreted by such logic.

Scientists measure geologic time in two different ways. Relative age measurement refers only to the order in which events occurred. Determination of relative age is based on a simple principle: In order for an event to affect a rock, the rock must exist first. Thus, the rock must be older than the event. This principle seems obvious, yet it is the basis of much geologic work. While most of us think of time in terms of days or years, Earth scientists commonly refer to events that happened millions or billions of years ago. In Chapter 1 you learned that the Earth is approximately 4.6 billion years old. Yet humans and our human-like ancestors have existed for only 5 to 7 million years, and recorded history is only a few thousand years old. How do scientists measure the ages of rocks and events that occurred millions or billions of years ago, long before recorded history or even human existence? Scientists measure geologic time in two different ways. Relative age measurement refers only to the order in which events occurred. Determination of relative age is based on a simple principle: In order for an event to affect a rock, the rock must exist first.

Absolute age measurements have become common only

4.3

Relative Geologic Time Prior to that time, geologists used field observations to determine relative ages. Even today, with sophisticated laboratory processes available, most field geologists routinely use relative age measurement. Geologists use a combination of common sense and a few simple principles to determine the order in which rocks formed over time. 1. The sinking surface waters forced the carbon dioxide-rich deep water to rise to the sea surface, where it rapidly released massive amounts of carbon dioxide into the atmosphere (Figure 4.3 C). 2. According to this hypothesis, the carbon dioxide asphyxiated life both in the seas and on the continents, killing most life on Earth. 3. Later, new organisms evolved when the carbon dioxide levels fell.The principle of superposition states that sedimentary rocks become younger from bottom to top (as long as tectonic forces have not turned them upside down or thrust an older layer over a younger one). This is because younger layers of sediment always accumulate on top of older layers. In Figure 4.6, the sedimentary layers become progressively younger in the order 1, 2, 3, 4, and 5. The principle of cross-cutting relationships is based on the obvious fact that a rock must first exist before anything can happen to it. Figure 4.7 shows a basalt dike 4.3 Relative Geologic Time

Systems Perspective The Earth and Its Four Spheres

V ACTIVE FIGURE 4.2 A disconformity is created by uplift and erosion, followed by deposition of additional layers of sediment.

INTERACTIVE QUESTION: In a normal part of the crust, what are the temperature ane pressure at a depth of 25 kilometers? What grade of metamorphic rocks would you expect to exist there?

4.3 Absolute Geologic Time How does a geologist measure the absolute age of an event that occurred before calendars and even before humans evolved to keep calendars? Think of how a calendar measures time. The Earth rotates about its axis at a constant rate, once a day. Thus, each time the Sun rises, you know that a day has passed and you check it off on your calendar. If you mark off each day as the Sun rises, you record the passage of time. To know how many days have passed since you started keeping time, you just count the checkmarks (V Figure 4.3). Absolute age measurement depends on two factors: a process that

V FIGURE 4.2 Geographic information systems store dif-

ferent information and data as individual map layers. GIS technology is widely used in geographic and environmental studies.

CHAPTER 4

• Geologic Time

occurs at a constant rate (the Earth rotates once every 24 hours) and some way to keep a cumulative record of that process (marking the calendar each time the Sun rises). Measurement of time with a calendar, a clock, an hourglass, or any other device depends on these two factors. Geologists have found a natural process that occurs at a constant rate and accumulates its own record: It is the radioactive decay of elements that are present in many rocks. Thus, many rocks have built-in calendars. We must understand radioactivity to read the calendars. Recall from Chapter 2 that an atom consists of a small, dense nucleus surrounded by a cloud of electrons. A nucleus consists of positively charged protons and neutral particles called neutrons. All atoms of any given element have the same number of protons in the nucleus. However, the number of neutrons may vary. Isotopes are atoms of the same element with different numbers of neutrons. For example, all isotopes of potassium-40 contain 19 protons and 21 neutrons. Potassium-39 has 19 protons but only 20 neutrons. Many isotopes are stable, meaning that they do not change with time. If you studied a sample of potassium39 for 10 billion years, all the atoms would remain unchanged. Other isotopes are unstable or radioactive. Given time, their nuclei spontaneously decay. Potassium-40 decays naturally to form two other isotopes, argon-40 and calcium-40 (V Figure 4.18). A radioactive isotope such as potassium-40 is known as a “parent” isotope. An isotope created by radioactivity, such as argon-40 or calcium-40, is called a “daughter” isotope. Many common elements, such as potassium, consist of a mixture of radioactive and nonradioactive isotopes. With time, the radioactive isotopes decay, but the nonradioactive ones do not. Some elements, such as uranium, consist only of radioactive isotopes. The amount of ura-

567

V FIGURE 4.3 Most of the Earth is solid rock surrounded by the hydrosphere, the biosphere, and the atmosphere.

4.3 Relative Geologic Time

FOCUS ON METALLIC BONDS In a metallic bond, the outer electrons are free; that is, they are not associated with particular atoms. The metal atoms sit in a “sea” of outerlevel electrons that are free to move from one atom to another. That arrangement allows the nuclei to pack together as closely as possible, resulting in the characteristic high density of metals and metallic minerals, such as pyrite. Because the electrons are free to move through the entire crystal, metallic minerals are excellent conductors of electricity.

Chemical Bonding Chemical bonds join atoms together. A molecule is the smallest particle of matter that can exist in a free state; it can be a single atom, or a group of atoms bonded together. Four types of chemical bonds are found in minerals: ionic, covalent, and metallic bonds, and van der Waals forces.

IONIC BONDS Cations and anions are attracted by

their opposite electronic charges, and thus bond together.This union is called an ionic bond.An ionic compound (made up of two or more ions) is neutral because the positive and negative charges balance each other.

For example, when sodium and chlorine form an ionic bond, the sodium atom loses one electron to become a cation and chlorine gains one to become an anion. When they combine, the 11 charge balances the 21 charge (V Figure 1).

COVALENT BONDS A covalent bond develops when two or more atoms share their electrons to produce the effect of filled outer electron shells. For example, carbon needs four electrons to fill its outermost shell. It can achieve this by forming four covalent bonds with four adjacent carbon atoms. It “gains” four electrons by shar-

V FIGURE 2 Carbon atoms in diamond form a tetrahedral network similar to that of quartz. ing one with another carbon atom at each of the four bonds. Diamond consists of a three-dimensional network of carbon atoms bonded into a network of tetrahedra, similar to the silicate framework structure of quartz (V Figure 2). The strength and homogeneity of the bonds throughout the crystal make diamond the hardest of all minerals.

Electrical Forces

V FIGURE 1 When sodium and chlorine atoms combine, sodium loses one electron, becoming a cation, Na1.

Chlorine acquires the electron to become an anion, Cl2.

How does a geologist measure the absolute age of an event that occurred before calendars and even before humans evolved to keep calendars? Think of how a calendar measures time. The Earth rotates about its axis at a constant rate, once a day. Thus, each time the Sun rises, you know that a day has passed and you check it off on your calendar. If you mark off each day as the Sun rises, you record the passage of time. To know how many days have passed since you started keeping time, you just count the checkmarks (V Figure 4.3). Absolute age measurement depends on two factors: a process that occurs at a constant rate (the Earth rotates once every 24 hours) and some way to keep a cumulative record of that

CHAPTER 4

• Geologic Time

process (marking the calendar each time the Sun rises). Measurement of time with a calendar, a clock, an hourglass, or any other device depends on these two factors.

Measurement of Time Geologists have found a natural process that occurs at a constant rate and accumulates its own record: It is the radioactive decay of elements that are present in many rocks. Thus, many rocks have built-in calendars. We must understand radioactivity to read the calendars. Recall from Chapter 2 that an atom consists of a small, dense nucleus surrounded by a cloud of electrons.

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In most minerals, the bonds between atoms are partly covalent and partly ionic. The combined characteristics of the different bond types determine the physical properties of those minerals. The strength and homogeneity of the bonds throughout the crystal make diamond the hardest of all minerals.

VAN DER WAALS FORCES Weak electrical forces called van der Waals forces also bond molecules together.These weak bonds result from an uneven distribution of electrons around individual molecules, so that one portion of a molecule may have a greater density of negative charge while another portion has a partial positive charge. These weak bonds result from an uneven distribution of electrons. These weak bonds result from an uneven distribution of electrons around individual molecules, so that one portion of a molecule may have a greater density of negative charge while another portion has a partial positive charge. FOCUS QUESTION Why do some minerals, such as native gold, silver, and graphite, conduct electricity, whereas others, such as quartz and feldspar, do not? Discuss relationships among other physical properties of minerals and the types of chemical bonds found in these minerals.

CLICK Earth Science Interactive to work through an activity on the definition of Atoms and Crystals.

process (marking the calendar each time the Sun rises). Measurement of time with a calendar, a clock, an hourglass, or any other device depends on these two factors. Geologists have found a natural process that occurs at a constant rate and accumulates its own record: It is the radioactive decay of elements that are present in many rocks. Thus, many rocks have built-in calendars. We must understand radioactivity to read the calendars. Recall from Chapter 2 that an atom consists of a small, dense nucleus surrounded by a cloud of electrons. A nucleus consists of positively charged protons and neutral particles called neutrons. All atoms of any given element have the same number of protons in the nucleus. 4.3 Relative Geologic Time

4.3

•

The Geologic Column and Time Scale

According to this hypothesis, the carbon dioxide asphyxiated life both in the seas and on the continents, killing most life on Earth.

•

According to this hypothesis, the carbon dioxide asphyxiated life both in the seas and on the continents, killing most life on Earth.

•

According to this hypothesis, the carbon dioxide asphyxiated life both in the seas and on the continents, killing most life on Earth.

4.6 The Geologic Column and Time Scale

As mentioned earlier, no single locality exists on Earth where a complete sequence of rocks formed continuously throughout geologic time. However, geologists have correlated rocks from many different localities around the world to create the geologic column and geologic time scale, which is a nearly complete composite record of geologic time (Figure 4.20). The worldwide geologic column is frequently revised as geologic mapping continues. Just as a year is subdivided into months, months into weeks, and weeks into days, geologic time is split into smaller intervals. The units are named, just as months and days are. The largest time units are eons, which are divided into eras. Eras are subdivided, in turn, into periods, which are further subdivided into epochs. The geologic column and time scale were originally constructed on the basis of relative age determinations. When geologists developed radiometric dating, they added absolute ages to the column and time scale. Geologists commonly use the time scale to date rocks in the field. Imagine that you are studying sedimentary rocks in the walls of Grand Canyon. If you find an index fossil or a key bed that has already been radiometrically dated by other scientists, you know the age of the rock you are studying, and you do not need to send the sample to a laboratory for radiometric dating.

The Three Gorges Dam—China

When geologists developed radiometric dating, they added absolute ages to the column and time scale. Imagine that you are studying sedimentary rocks in the walls of Grand Canyon. If you find an index fossil or a key bed that has already been radiometrically dated by other scientists, you know the age of the rock you are studying, and you do not need to send the sample to a laboratory for radiometric dating. If you find an index fossil or a key bed that has already been radiometrically dated by other scientists, you know the age of the rock you are studying.

According to this hypothesis, the carbon dioxide asphyxiated life both in the seas and on the continents, killing most life on Earth.

•

According to this hypothesis, the carbon dioxide asphyxiated life both in the seas and on the continents, killing most life on Earth.

CHAPTER 4

• Geologic Time

Early in this century, a geologist named N. L. Bowen discovered that when magma cools and crystallizes to form an igneous rock, all the minerals do not crystallize at the same time and temperature. Those that crystallize first, at the highest temperature, contain the lowest percentage of silica. Minerals with higher silica contents crystallize later, at lower temperatures. Bowen conducted his experiments by placing finely ground rock samples into gold tubes. He used gold because it is chemically inert and does not affect chemical reactions. Bowen welded the tubes closed to form a

tight seal and placed them into a strong, hollow steel cylinder with a screw cap, called a bomb. He then heated the bomb until the powdered rock melted, creating an artificial magma. Finally, he cooled the mixture slightly and let it sit long enough, often for a few months, for minerals to begin crystallizing from the melt. At that point, his bombs contained a mixture of crystals and the melt. He then recorded the temperature of each bomb and plunged it into a cold liquid. The rapid cooling preserved the crystals and solidified the melt as glass. By extracting the sample from the bomb and examining it

Sedimentary rocks of the Phanerozoic Eon contain abundant fossils (phaneros is Greek for “evident”). Four changes occurred at the beginning of Phanerozoic time that greatly improved the fossil record: 1. The number of species with shells and skeletons dramatically increased. 2. The total number of individual organisms preserved as fossils increased greatly. 3. The total number of species preserved as fossils increased greatly. Humans have evolved and lived wholly in the Cenozoic era.

Table 2.1 The Eight Most Abundant Chemical Elements in the Earth’s Crust*

According to this hypothesis, the carbon dioxide asphyxiated life both in the seas and on the continents, killing most life on Earth.

•

Cooling and Crystallization of Magma: Bowen’s Experiment

The Phanerozoic Eon

When geologists developed radiometric dating, they added absolute ages to the column and time scale. Geologists commonly use the time scale to date rocks in the field. Imagine that you are studying sedimentary rocks in the walls of Grand Canyon. If you find an index fossil or a key bed that has already been radiometrically dated by other scientists, you know the age of the rock you are studying, and you do not need to send the sample to a laboratory for radiometric dating. The geologic column and time scale were originally constructed on the basis of relative age determinations.

•

FOCUS ON

Element

Symbol

Weight Percent

Atom Percent

Volume Percent†

Oxygen Silicon Aluminum Iron

O Si Al Fe

46.60 27.72 8.13 5.00

62.55 21.22 6.47 1.92

93.8 0.9 0.5 0.4

Side head over more than one column

Calcium Sodium Potassium Magnesium Magnesium

567

Ca Na K Mg Mg

3.63 2.83 2.59 2.09 2.09

1.94 2.64 1.42 1.84 1.84

1.0 1.3 1.8 0.3 0.3

Total

98.59

100.00

V FIGURE 1 Bowen’s reaction series shows the order in which minerals crystallize from cooling magma and react with the magma as it cools to form minerals lower on the diagram.

Measurement of time with a calendar, a clock, an hourglass, or any other device depends on these two factors.

EARTH SYSTEMS INTERACTIONS

vide evidence for gradual extinctions, but they did not find them. Eventually, the evidence for near instantaneous extinctions became compelling. The most dramatic extinction occurred 248 million years ago, at the end of the Permian period. At that time, 90 percent of all species in the oceans suddenly died out. On land, about two thirds of reptile and amphibian species and 30 percent of insect species vanished. One modern scientist exclaimed that this extinction event was “the closest life has come to complete extermination since its origin.”1 The death of most life forms at the end of Permian time left huge ecological voids in the biosphere. Ocean ecosystems changed as new organisms emerged in an environment free of predators and competition. One hundred and sixty million years later, another catastrophic extinction wiped out one fourth of all species, including the dinosaurs. Geologists searched for rocks to fill in the gaps and provide evidence for gradual extinctions, but they did not find them. The evidence for near instantaneous extinctions became compelling. This mass extinction occurred 65 million years ago. Small mammals survived this disaster, facing a new world free of the efficient predators that had hunted them. The number of mammal species increased rapidly after this extinction event, eventually leading to the evolution of humans. Geologists searched for rocks to fill in the gaps and provide evidence for gradual extinctions, but they did not find them. Eventually, the evidence for near instantaneous extinctions became compelling. Geologists searched for rocks to fill in the gaps and provide evidence for gradual extinctions, but they did not find them. Eventually, the evidence for near instantaneous extinctions became compelling. Other scientists have found evidence of a huge meteorite crater that formed 65

ALTHOUGH MINERALS MAKE UP the solid Earth – the geosphere – the elements that compose minerals cycle continuously among all four of the major Earth systems. For example, rain and soil moisture decompose rocks, slowly dissolving minerals and transferring their ions from the geosphere to the hydrosphere. Streams then carry the dissolved ions to the sea, where marine animals such as clams and oysters extract some of the ions from sea water to form their shells, incorporating the materials into the biosphere. Waves may whip the sea surface into foamy breakers, and wind carries some of the dissolved elements in the sea foam into the atmosphere. Rain then returns the ions to the land, where they may crystallize to form soil minerals, or back to the sea where they may precipitate as evaporate minerals. Humans mine and refine some minerals for fertilizers critical to modern agriculture, transferring the materials from the geosphere to the biosphere and the hydrosphere. In these ways, even the minerals of the seemingly permanent geosphere make their ways through the four major Earth realms. Exchanges of materials among the geosphere, the hydrosphere, the biosphere, and the atmosphere supply necessary nutrients to soil, plants, and animals, including humans, and provide necessary resources for life on Earth. ■ CHAPTER 4

• Geologic Time

EARTH SYSTEMS

Earth Systems Interactions

INTERACTIONS

Box colors are 50% 08 Medium Gold

Volcanoes warm the atmosphere, which affects life.

Biosphere

Geosphere

Some of the rocks form from the remains of plants and animals. Plants weather rock.

Ions from weathered rocks turn ocean salty.

Water weathers rock to form sediment. Water erodes sediment. Streams transport sediment. Water in rock affects metamosphism.

Volcanic eruptions emit carbon dioxide.

Air weathers rocks.

Atmosphere

Hydrosphere

4.3 Relative Geologic Time

EARTH SYSTEMS

Earth Systems Interactions

INTERACTIONS

Box colors are 50% 08 Medium Gold

RAIN AND GROUND WATER of the hydrosphere combine with atmospheric gases and chemicals secreted by plants to form weak acids that continually attack and decompose rocks and minerals at the Earth’s surface. These processes, in turn, create soil that supports plant and animal life on land. Erosion and transport of soils by flowing water, landslides, glaciers, and wind leads to the formation of sedimentary rocks. Thus, the weathering, soil-forming, and erosion cycle directly involves interactions among all four of the major Earth systems. ■

EARTH SYSTEMS INTERACTIONS

Volcanoes warm the atmosphere, which affects life.

Biosphere

Geosphere

SUMMARY

Some of the rocks form from the remains of plants and animals. Plants weather rock.

Ions from weathered rocks turn ocean salty.

Water weathers rock to form sediment. Water erodes sediment. Streams transport sediment. Water in rock affects metamosphism.

Volcanic eruptions emit carbon dioxide.

Air weathers rocks.

t least five catastrophic mass extinctions have occurred in geologic history. The greatest, 248 million years ago, annihilated 90 percent of all marine species, two thirds of reptile and amphibian species, and 30 percent of insect species. Determinations of relative time are based on geologic relationships among rocks and the evolution of life forms through time. The criteria for relative dating are summarized in a few simple principles: the principle of original horizontality, the principle of superposition,the principle of cross-cutting relationships, and the principle of faunal succession. Layers of sedimentary rock are conformable if they were deposited without major interruptions. An unconformity represents a major interruption of deposition and a significant time gap between formation of successive layers of rock. In a disconformity, layers of sedimentary rock on either side of the unconformity are parallel. An angular unconformity forms when lower layers of rock are tilted and partially eroded prior to deposition of the upper beds. In a nonconformity, sedimentary layers lie on top of an erosion surface developed on igneous or metamorphic rocks. Fossils are used to date rocks according to the principle of faunal succession. Correlation is the demonstration of equivalency of rocks that are geographically separated. Index fossils and key beds are important tools in time correlation, the demonstration that sedimentary rocks from different geographic localities formed at the same time. Worldwide correlation of rocks of all ages has resulted in the geologic column, a composite record of rocks formed throughout the history of the Earth. Absolute time is measured by radiometric dating, which relies on the fact that radioactive parent isotopes decay to form daughter isotopes at a fixed, known rate as expressed by the half-life of the isotope. The cumulative effects of the radioactive decay process can be determined because the daughter isotopes accumulate in rocks and minerals. The major units of the geologic time scale are eons, eras, periods, and epochs. The Phanerozoic Eon is finely and accurately subdivided because sedimentary rocks deposited at this time are often well preserved and they contain abundant well-preserved fossils. In contrast, Precambrian rocks and time are only coarsely subdivided because fossils are scarce and poorly preserved and the rocks are often altered.

Key Terms

Atmosphere

CHAPTER 4

Hydrosphere

• Geologic Time

mass extinction 12 relative age 12 absolute age 13 principle of original horizontality principle of superposition 15

principle of cross-cutting relationships 16 evolution 17 fossil 18 principle of faunal succession 18

conformable 19 unconformity 20 disconformity 21 angular unconformity nonconformity 22 SUMMARY

SUMMARY

Common Igneous Rocks

Key Terms

mass extinction 12 relative age 12 absolute age 13 principle of original horizontality 14 principle of superposition 15 principle of cross-cutting relationships 16 evolution 17 fossil 18 principle of faunal succession 18 conformable 19 unconformity 20 disconformity 21 angular unconformity 22 nonconformity 22

SUMMARY

correlation 23 index fossil 24 key bed 24 isotope 25 half-life 28 radiometric dating 29 geologic column 29 geologic time scale 30 eon 31 era 32 period 32 epoch 33 Hadean Eon 33 Archean Eon 34 Proterozoic Eon 35 Phanerozoic Eon 36 Precambrian time 37

Paleozoic era 38 Mesozoic era 38 era 32 period 32 epoch 33 Hadean Eon 33 Archean Eon 34 Proterozoic Eon 35 Phanerozoic Eon 36 Precambrian time 37 Paleozoic era 38 Mesozoic era 38 era 32 period 32 epoch 33 Hadean Eon 33 Archean Eon 34

Common Metamorphic Rocks

Felsic

Intermediate

Mafic

Ultramafic

Unfoliated

Foliated

obsidian granite rhyolite

andesite diorite

basalt gabbro

peridotite

marble quartzite

slate phyllite schist gneiss migmatite

Common Sedimentary Rocks Clastic

Bioclastic

Organic

Chemical

conglomerate sandstone siltstone shale

limestone dolomite (dolostone) coquina chalk

chert coal

evaporite

For Review 1. Describe the two ways of measuring geologic time. How do they differ? 2. Give an example of how the principle of original horizontality might be used to determine the order of events affecting a sequence of folded sedimentary rocks. 3. How does the principle of superposition allow us to determine the relative ages of a sequence of unfolded sedimentary rocks? 4. Explain the principle of cross-cutting relationships and how it can be used to determine age relationships among sedimentary rocks. 5. Explain a conformable relationship in sedimentary rocks. 6. What geologic events are recorded by an angular unconformity? A disconformity? A nonconformity? What can be inferred about the timing of each set of events? 7. Discuss the principle of faunal succession and the use of index fossils in time correlation. 8. What are the two different types of correlation of rock units? How do they differ? 9. What tools or principles are most commonly used in correlation? 10. What is radioactivity?

11. What is a stable isotope? An unstable isotope? 12. What is the relationship between parent and daughter isotopes? 13. What is meant by the half-life of a radioactive isotope? How is the half-life used in radiometric age dating? 14. Why are some radioactive isotopes useful for measuring relatively young ages, whereas others are useful for measuring older ages? 15. What geologic event is actually measured by a radiometric age determination of an igneous rock or mineral? 16. Why is the Phanerozoic Eon separated into so many subdivisions in contrast to much longer Precambrian time, which has few subdivisions? 17. List major events that occurred during the Hadean, Archean, and Proterozoic Eons. 18. List four changes that occurred at the beginning of Phanerozoic time that greatly improved the fossil record. 19. Briefly discuss the three major eras of the Phanerozoic Eon. 20. Describe three hypotheses that explain mass extinctions.

For Discussion 1. Suppose that you landed on the Moon and were able to travel in a vehicle that could carry you over the lunar surface to see a wide variety of rocks, but that

you had no laboratory equipment to work with. What principles and tools could you use to determine relative ages of Moon rocks? SUMMARY

2. Imagine that one species lived between 544 and 505 million years ago and another lived between 520 and 248 million years ago. What can you say about the age of a rock that contains fossils of both species? 3. Imagine that someone handed you a sample of sedimentary rock containing abundant fossils. What could you tell about its age if you did not use radiometric dating and you did not know where it was collected from? What additional information could you determine if you studied the outcrop that it came from? 4. What geologic events are represented by a potassiumargon age from flakes of biotite in (a) granite, (b) biotite schist, and (c) sandstone? 5. Suppose you were using the potassium-argon (K/Ar) method to measure the age of biotite in granite. What would be the effect on the age measurement if the biotite had been slowly leaking small amounts of

CHAPTER 4

• SUMMARY

argon since it crystallized? 6. On rare occasions, layered sedimentary rocks are overturned so that the oldest rocks are on the top and the youngest are buried most deeply. Discuss two ways of determining whether such an event had occurred. 7. Devise two metaphors for the length of geologic time in addition to the metaphor used in Chapter 1. Locate some of the most important time boundaries in your analogy. 8. Discuss how sedimentary rocks could record major changes in temperature or rainfall in an area. 9. Suppose that you landed on the Moon and were able to travel in a vehicle that could carry you over the lunar surface to see a wide variety of rocks, but that you had no laboratory equipment to work with. What principles and tools could you use to determine relative ages of Moon rocks?

APPENDIX

A Identifying Common Minerals

At least five catastrophic mass extinctions have occurred in geologic history. The greatest, 248 million years ago, annihilated 90 percent of all marine species, two thirds of reptile and amphibian species, and 30 percent of insect species. Determinations of relative time are based on geologic relationships among rocks and the evolution of life forms through time. The criteria for relative dating are summarized in a few simple principles: the principle of original horizontality, the principle of superposition,the principle of cross-cutting relationships, and the principle of faunal succession. Layers of sedimentary rock are conformable if they were deposited without major interruptions. An unconformity represents a major interruption of deposition and a significant time gap between formation of successive layers of rock. In a disconformity, layers of sedimentary rock on either side of the unconformity are parallel. An angular unconformity forms when lower layers of

rock are tilted and partially eroded prior to deposition of the upper beds. In a nonconformity, sedimentary layers lie on top of an erosion surface developed on igneous or metamorphic rocks. Fossils are used to date rocks according to the principle of faunal succession. Correlation is the demonstration of equivalency of rocks that are geographically separated. Index fossils and key beds are important tools in time correlation, the demonstration that sedimentary rocks from different geographic localities formed at the same time. Worldwide correlation of rocks of all ages has resulted in the geologic column, a composite record of rocks formed throughout the history of the Earth. Absolute time is measured by radiometric dating, which relies on the fact that radioactive parent isotopes decay to form daughter isotopes at a fixed, known rate as expressed by the half-life of the isotope. The cumulative effects of the radioactive decay process can be determined because the daughter isotopes accumulate in rocks

APPENDIX A

Background screen to clear figure elements by 12pts x 4 sides. Note that land mass will require color adjustment to show up against background screen (I just scanned figure from the book).

Note that land mass will require color adjustment to show up against background screen (I just scanned figure from the book).

Desert

Desert Latitude: Variable

Latitude: Variable

Average temperature difference: Variable (winter to summer)

Average annual precipitation: Less than 25 cm/yr

Climograph colors

100% 02 Blue box band header 70% 09 Medium Blue box screen 25% 01 Gold figure background screeen

Lima, Peru 12°S

BWh 77°W

Background screen to clear figure elements by 12pts x 4 sides.

Note that land mass will require color adjustment to show up against background screen (I just scanned figure from the book). V FIGURE 4.3 Climograpah from one representativestation.

Precip.: 4cm (1.6 in) Av. temp.: 20°C (68°F) Range: 9°C (15.5°F)

Desert

Desert Latitude: Variable

Latitude: Variable

Average temperature difference: Variable (winter to summer)

Average annual precipitation: Less than 25 cm/yr

Lima, Peru 12°S

BWh 77°W

V FIGURE 4.3 Climograpah from one representativestation.

CHAPTER 4

• Geologic Time

567

BWh 77°W

Precip.: 4cm (1.6 in) Av. temp.: 20°C (68°F) Range: 9°C (15.5°F)

Lima, Peru 12°S

V FIGURE 4.3 Climograpah from one representativestation.

4.3 Relative Geologic Time