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F O C U S O N L E A R N I N G

To assist you in learning the concepts in the Introduc- tion, you will find it helpful to focus on the following questions:

1. What are the sciences that collectively make up Earth science?

2. What are the four “spheres” that comprise Earth’s natural environment?

3. What are the principal divisions of the solid Earth?

4. Why should Earth be thought of as a system?

5. What are the sources of energy that power the Earth system?

6. What are some of Earth’s important environmental issues?

7. How is a scientific hypothesis different from a scientific theory?

Introduction to Earth Science

British Columbia’s Mount Robson is the highest point in the Canadian Rockies. Glaciers are still sculpting this landscape. All of Earth’s major spheres—geosphere, hydrosphere, atmosphere, and biosphere—are represented in this image. (Photo by Art Wolfe)

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2 Introduction to Earth Science

T he spectacular eruption of a volcano, the magnificent scenery of a rocky coast, and the destruction created by a hurricane are all subjects for the Earth scientist. The

study of Earth science deals with many fascinating and prac- tical questions about our environment. What forces produce mountains? Why is our daily weather so variable? Is climate really changing? How old is Earth and how is it related to the other planets in the solar system? What causes ocean tides? What was the Ice Age like? Will there be another? Can a successful well be located at this site?

The subject of this text is Earth science. To understand Earth is not an easy task, because our planet is not a static and unchanging mass. Rather, it is a dynamic body with many interacting parts and a long and complex history.

What Is Earth Science?

Earth science is the name for all the sciences that collective- ly seek to understand Earth and its neighbors in space. It in- cludes geology, oceanography, meteorology, and astronomy.

In this book, Units 1 through 4 focus on the science of geology, a word that literally means “study of Earth.” Geol- ogy is traditionally divided into two broad areas—physical and historical.

Physical geology examines the materials comprising Earth and seeks to understand the many processes that oper- ate beneath and upon its surface. Earth is a dynamic, ever- changing planet. Internal forces create earthquakes, build mountains, and produce volcanic structures. At the surface,

external processes break rock apart and sculpture a broad array of landforms. The erosional effects of water, wind, and ice result in a great diversity of landscapes. Because rocks and minerals form in response to Earth’s internal and exter- nal processes, the study of Earth materials is basic to an un- derstanding of our planet.

In contrast to physical geology, the aim of historical ge- ology is to understand the origin of Earth and the develop- ment of the planet through its 4.5-billion-year history. It strives to establish an orderly chronological arrangement of the multitude of physical and biological changes that have occurred in the geologic past (Figure I.1). The study of phys- ical geology logically precedes the study of Earth’s history because we must first understand how Earth works before we attempt to unravel its past.

Unit 5, The Global Ocean, is devoted to oceanography. Oceanography is actually not a separate and distinct science. Rather, it involves the application of all sciences in a compre- hensive and interrelated study of the oceans in all their as- pects and relationships. Oceanography integrates chemistry, physics, geology, and biology. It includes the study of the composition and movements of seawater, as well as coastal processes, seafloor topography, and marine life.

Unit 6, Earth’s Dynamic Atmosphere, examines the mix- ture of gases that is held to the planet by gravity and thins rapidly with altitude. Acted on by the combined effects of Earth’s motions and energy from the Sun, the formless and in- visible atmosphere reacts by producing an infinite variety of weather, which in turn creates the basic pattern of global cli- mates. Meteorology is the study of the atmosphere and the

Figure I.1 Paleontologists study ancient life. Here scientists are excavating the remains of Albertasaurus, a large carnivore similar to Tyrannosaurus rex, which lived during the late Cretaceous Period. The excavation site is near Red Deer River, Alberta, Canada. The aim of historical geology is to understand the development of Earth and its life through time. Fossils are essential tools in that quest. (Photo by Richard T. Nowitz/Science Source/Photo Researchers, Inc.)

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Earth’s Spheres 3

processes that produce weather and climate. Like oceanogra- phy, meteorology involves the application of other sciences in an integrated study of the thin layer of air that surrounds Earth.

Unit 7, Earth’s Place in the Universe, demonstrates that an understanding of Earth requires that we relate our planet to the larger universe. Because Earth is related to all of the other objects in space, the science of astronomy—the study of the universe—is very useful in probing the origins of our own environment. As we are so closely acquainted with the plan- et on which we live, it is easy to forget that Earth is just a tiny object in a vast universe. Indeed, Earth is subject to the same physical laws that govern the many other objects populating the great expanses of space. Thus, to understand explana- tions of our planet’s origin, it is useful to learn something about the other members of our solar system. Moreover, it is helpful to view the solar system as a part of the great assem- blage of stars that comprise our galaxy, which in turn is but one of many galaxies.

To understand Earth science is challenging because our planet is a dynamic body with many interacting parts and a complex history. Throughout its long existence, Earth has been changing. In fact, it is changing as you read this page and will continue to do so into the foreseeable future. Some- times the changes are rapid and violent, as when severe

storms, landslides, or volcanic eruptions occur. Just as often, change takes place so gradually that it goes unnoticed during a lifetime. Scales of size and space also vary greatly among the phenomena studied in Earth science.

Earth science is often perceived as science that is done in the out-of-doors, and rightly so. A great deal of what Earth scientists study is based on observations and experiments per- formed in the field. But Earth science is also conducted in the laboratory, where, for example, the study of various Earth ma- terials provides insights into many basic processes, and the creation of complex computer models allows for the simula- tion of our planet’s complicated climate system. Frequently, Earth scientists require an understanding and application of knowledge and principles from physics, chemistry, and biol- ogy. Geology, oceanography, meteorology, and astronomy are sciences that seek to expand our knowledge of the natural world and our place in it.

Earth’s Spheres

A view such as the one in Figure I.2A provided the Apollo 8 as- tronauts as well as the rest of humanity with a unique perspec- tive of our home. Seen from space, Earth is breathtaking in its beauty and startling in its solitude. Such an image reminds us

A. B.

Figure I.2 A. View that greeted the Apollo 8 astronauts as their spacecraft emerged from behind the Moon. B. Africa and Arabia are prominent in this classic image of Earth taken from Apollo 17. The tan cloud-free zones over the land coincide with major desert regions. The band of clouds across central Africa is associated with a much wetter climate that sustains tropical rain forests. The dark blue of the oceans and the swirling cloud patterns remind us of the importance of the oceans and the atmosphere. Antarctica, a continent covered by glacial ice, is visible at the South Pole. (Both photos courtesy of NASA/Science Source/Photo Researchers, Inc.)

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Figure I.3 The shoreline is one obvious example of an interface—a common boundary where different parts of a system interact. In this scene, ocean waves (hydrosphere) that were created by the force of moving air (atmosphere) break against California’s rocky Big Sur shore (geosphere). The force of the water can be powerful, and the erosional work that is accomplished can be great. (Photo by Carr Clifton)

Did You Know? The volume of ocean water is so large that if Earth’s solid mass

were perfectly smooth (level) and spherical, the oceans would

cover Earth’s entire surface to a uniform depth of more than

2000 m (1.2 mi)!

Did You Know? In 1492, when Columbus set sail, many Europeans thought that

Earth was flat and that Columbus would sail off the edge. How-

ever, more than 2000 years earlier, ancient Greeks realized that

Earth was spherical because it always cast a curved shadow on

the Moon during a lunar eclipse. In fact, Erastosthenes (276–194

B.C.) calculated Earth’s circumference and obtained a value

close to the modern measurement of 40,075 km (24,902 mi).

that our home is, after all, a planet—small, self-contained, and in some ways even fragile.

As we look closely at our planet from space, it becomes clear that Earth is much more than rock and soil. In fact, the most conspicuous features in Figure I.2A are not continents, but swirling clouds suspended above the surface and the vast global ocean. These features emphasize the importance of air and water to our planet.

The closer view of Earth from space, shown in Figure I.2B, helps us appreciate why the physical environment is tra- ditionally divided into three major parts: the water portion of our planet, the hydrosphere; Earth’s gaseous envelope, the atmosphere; and, of course, the solid Earth, or geosphere.

It should be emphasized that our environment is high- ly integrated and not dominated by water, air, or rock alone. Rather, it is characterized by continuous interactions as air

comes in contact with rock, rock with water, and water with air. Moreover, the biosphere, the totality of life forms on our planet, extends into each of the three physical realms and is an equally integral part of the planet. Thus, Earth can be thought of as consisting of four major spheres: the hydro- sphere, atmosphere, geosphere, and biosphere.

The interactions among Earth’s four spheres are incal- culable. Figure I.3 provides an easy-to-visualize example. The shoreline is an obvious meeting place for rock, water, and air. In this scene, ocean waves that were created by the drag of air moving across the water are breaking against the rocky shore. The force of the water can be powerful, and the erosional work that is accomplished can be great.

Hydrosphere

Earth is sometimes called the blue planet. Water, more than anything else, makes Earth unique. The hydrosphere is a dy- namic mass of water that is continually on the move, evapo-

4 Introduction to Earth Science

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Earth’s Spheres 5

Figure I.4 This jet is flying high in the atmosphere at an altitude of more than 9000 meters (30,000 feet). More than two-thirds of the atmosphere is below this height. To someone on the ground, the atmosphere seems to extend for a great distance. However, when compared to the thickness (radius) of the solid Earth, the atmosphere is a very shallow layer. (Photo by Warren Faidley/Weatherstock)

Geosphere

Lying beneath the atmosphere and the ocean is the solid Earth or geosphere. The geosphere extends from the surface to the center of the planet, a depth of nearly 6400 kilometers (4000 miles), making it by far the largest of Earth’s spheres. Much of our study of the solid Earth focuses on the more accessible sur- face and near-surface features, but it is worth noting that many of these features are linked to the dynamic behavior of Earth’s interior. Figure I.5 provides a diagrammatic view of Earth’s in- terior. As you can see, it is not uniform but is divided into var- ious layers. Based on compositional differences, three principal layers are identified: a dense inner sphere called the core; the less dense mantle; and the crust, which is the light and very thin outer skin of Earth. The crust is not a layer of uniform thickness. It is thinnest beneath the oceans and thickest where continents exist. Although the crust may seem insignificant when compared with the other layers of the geosphere, which are much thicker, it was created by the same general process- es that formed Earth’s present structure. Thus, the crust is im- portant in understanding the history and nature of our planet.

The solid Earth is also divided based on how materials behave when various forces and stresses are applied. The term lithosphere refers to the rigid outer layer that includes the crust and uppermost mantle. Beneath the rigid rocks that compose the lithosphere, the rocks of the asthenosphere are weak and are able to slowly flow in response to the uneven distribution of heat deep within Earth.

The two principal divisions of Earth’s surface are the continents and the ocean basins. The most obvious difference between these two diverse provinces is their relative levels. The average elevation of the continents above sea level is about 840 meters (2750 feet), whereas the average depth of the oceans is about 3800 meters (12,500 feet). Thus, the con- tinents stand on the average 4640 meters (about 4.6 kilome- ters or nearly 3 miles) above the level of the ocean floor.

Did You Know? During the twentieth century, the global average surface tem-

perature increased by about 0.6°C (1°F). By the end of the

twenty-first century, globally averaged surface temperature is

projected to increase by an additional 1.4 to 5.8°C (2.5 to 14.5°F).

rating from the oceans to the atmosphere, precipitating to the land, and flowing back to the ocean again. The global ocean is certainly the most prominent feature of the hydrosphere, blanketing nearly 71 percent of Earth’s surface to an average depth of about 3800 meters (12,500 feet). It accounts for about 97 percent of Earth’s water. However, the hydrosphere also includes the freshwater found underground and in streams, lakes, and glaciers. Moreover, water is an important compo- nent of all living things.

Although these latter sources constitute just a tiny frac- tion of the total, they are much more important than their meager percentage indicates. In addition to providing the freshwater that is so vital to life on the continents, streams, glaciers, and groundwater are responsible for sculpturing and creating many of our planet’s varied landforms.

Atmosphere

Earth is surrounded by a life-giving gaseous envelope called the atmosphere. When we watch a high-flying jet plane cross the sky, it seems that the atmosphere extends upward for a great distance (Figure I.4). However, when compared to the thickness (radius) of the solid Earth, (about 6400 kilometers or 4000 miles), the atmosphere is a very shallow layer. One- half lies below an altitude of 5.6 kilometers (3.5 miles), and 90 percent occurs within just 16 kilometers (10 miles) of Earth’s surface. This thin blanket of air is nevertheless an integral part of the planet. It not only provides the air that we breathe, but it also acts to protect us from the dangerous radiation emitted by the Sun. The energy exchanges that continually occur between the atmosphere and Earth’s surface and be- tween the atmosphere and space produce the effects we call weather and climate.

If, like the Moon, Earth had no atmosphere, our plan- et would be lifeless because many of the processes and in- teractions that make Earth’s surface such a dynamic place could not operate. Without weathering and erosion, the face of our planet might more closely resemble the surface of the Moon, which has not changed appreciably in nearly 3 billion years.

Did You Know? We have never sampled the mantle or core directly. The struc-

ture of Earth’s interior is determined by analyzing seismic

waves from earthquakes. As these waves of energy penetrate

Earth’s interior, they change speed and are bent and reflected

as they move through zones having different properties. Moni-

toring stations around the world detect and record this energy.

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6 Introduction to Earth Science

Outer core (liquid)

Inner core

(solid)

Continental crust

Oceanic crust

Lithosphere (solid and brittle

100 km thick)

Asthenosphere (solid, but mobile)

660 km

Lithosphere

Core (iron + nickel)

Atmosphere (gas)

Hydrosphere (liquid)

410 km

Upper mantle

Lower mantle (solid)

Mantle (high density rock)

Crust (low density rock 7–70 km thick)

U p p er

m an

tle (s

ol id

)

Layering by Physical Properties

Layering by Chemical Composition

6371 km

28 90

k m

2890 km

6 6 0 k

m 5 1 50

k m

Figure I.5 The right side of the globe shows that Earth’s interior is divided into three different layers based on compositional differences—the crust, mantle, and core. The left side of the globe shows the five main layers of Earth’s interior based on physical properties and mechanical strength—the lithosphere, asthenosphere, lower mantle, outer core, and inner core. The block diagram shows an enlarged view of the upper portion of Earth’s interior.

Biosphere

The fourth “sphere,” the biosphere, includes all life on Earth. Ocean life is concentrated in the sunlit waters of the sea. Most life on land is also concentrated near the surface, with tree roots and burrowing animals reaching a few meters under- ground and flying insects and birds reaching a kilometer or so above the surface. A surprising variety of life forms are also adapted to extreme environments. For example, on the deep ocean floor where pressures are extreme and no light penetrates, there are places where vents spew hot, mineral- rich fluids that support communities of exotic life forms. On land, some bacteria thrive in rocks as deep as 4 kilometers (2.5 miles) and in boiling hot springs. Moreover, air currents can carry microorganisms many kilometers into the atmos- phere. But even when we consider these extremes, life still must be thought of as being confined to a narrow band very near Earth’s surface.

Plants and animals depend on the physical environment for the basics of life. However, organisms do more than just respond to this environment. Through countless interactions, life forms help maintain and alter their physical environment. Without life, the makeup and nature of the geosphere, hy- drosphere, and atmosphere would be very different.

Earth as a System

Anyone who studies Earth soon learns that our planet is a dynamic body with many separate but interacting parts or spheres. The hydrosphere, atmosphere, biosphere, and geosphere and all of their components can be studied sepa- rately. However, the parts are not isolated. Each is related in some way to the others to produce a complex and continuous- ly interacting whole that we call the Earth system.

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A simple example of the interactions among different parts of the Earth system occurs every winter as moisture evaporates from the Pacific Ocean and subsequently falls as rain in the hills of southern California, triggering destructive landslides. The processes that move water from the hydro- sphere to the atmosphere and then to the geosphere have a profound impact on the plants and animals (including hu- mans) that inhabit the affected regions.

Scientists have recognized that in order to more fully understand our planet, they must learn how its individual components (land, water, air, and life forms) are interconnect- ed. This endeavor, called Earth system science, aims to study Earth as a system composed of numerous interacting parts, or subsystems. Using an interdisciplinary approach, those who practice Earth system science attempt to achieve the level of understanding necessary to comprehend and solve many of our global environmental problems.

A system is a group of interacting, or interdependent, parts that form a complex whole. Most of us hear and use the term frequently. We may service our car ’s cooling system, make use of the city’s transportation system, and be a partic- ipant in the political system. A news report might inform us of an approaching weather system. Further, we know that Earth is just a small part of a large system known as the solar system, which, in turn, is a subsystem of an even larger sys- tem called the Milky Way Galaxy.

The parts of the Earth system are linked so that a change in one part can produce changes in any or all of the other parts. For example, when a volcano erupts, lava from Earth’s interior may flow out at the surface and block a nearby val- ley. This new obstruction influences the region’s drainage system by creating a lake or causing streams to change course. The large quantities of volcanic ash and gases that can be emitted during an eruption might be blown high into the at- mosphere and influence the amount of solar energy that can reach Earth’s surface. The result could be a drop in air tem- peratures over the entire hemisphere.

Where the surface is covered by lava flows or a thick layer of volcanic ash, existing soils are buried. This causes the soil-forming processes to begin anew to transform the new surface material into soil (Figure I.6). The soil that even- tually forms will reflect the interactions among many parts of the Earth system—the volcanic parent material, the type and rate of weathering, and the impact of biological activity. Of course, there would also be significant changes in the bio- sphere. Some organisms and their habitats would be eliminat- ed by the lava and ash, whereas new settings for life, such as the lake, would be created. The potential climate change could also impact sensitive life forms.

The Earth system is characterized by processes that vary on spatial scales from fractions of millimeters to thousands of kilometers. Time scales for Earth’s processes range from mil- liseconds to billions of years. As we learn about Earth, it be- comes increasingly clear that despite significant separations in distance or time, many processes are connected and that a change in one component can influence the entire system.

The Earth system is powered by energy from two sources. The Sun drives external processes that occur in the atmosphere, hydrosphere, and at Earth’s surface. Weather and climate, ocean circulation, and erosional processes are driven by energy from the Sun. Earth’s interior is the second source of energy. Heat remaining from when our planet formed and heat that is continuously generated by radioac- tive decay power the internal processes that produce volca- noes, earthquakes, and mountains.

Humans are part of the Earth system, a system in which the living and nonliving components are entwined and in- terconnected. Therefore, our actions produce changes in all of the other parts. When we burn gasoline and coal, build sea- walls along the shoreline, dispose of our wastes, and clear the land, we cause other parts of the system to respond, often in unforeseen ways. Throughout this book, you will learn about many of Earth’s subsystems: the hydrologic system, the tectonic (mountain-building) system, and the climate

Figure I.6 When Mount St. Helens erupted in May 1980, the area shown here was buried by a volcanic mudflow. After just a few years, plants were reestablished and new soil was forming. (Photo by Jack W. Dykinga Associates)

Earth as a System 7

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8 Introduction to Earth Science

Atom

Mineral

Rock

Mountain

Planet

Solar system

Galaxy

Figure I.7 Earth science involves investigations of phenomena that range in size from atoms to galaxies and beyond.

system, to name a few. Remember that these components and we humans are all part of the complex interacting whole we call the Earth system.

Scales of Space and Time in Earth Science

When we study Earth, we must contend with a broad array of space and time scales (Figure I.7). Some phenomena are relatively easy for us to imagine, such as the size and dura-

tion of an afternoon thunderstorm or the dimensions of a sand dune. Other phenomena are so vast or so small that they are difficult to imagine. The number of stars and distances in our galaxy (and beyond!) or the internal arrangement of atoms in a mineral crystal are examples of such phenomena.

Some of the events we study occur in fractions of a sec- ond. Lightning is an example. Other processes extend over spans of tens or hundreds of millions of years. The lofty Hi- malaya Mountains began forming about 45 million years ago, and they continue to develop today.

The concept of geologic time is new to many nonscien- tists. People are accustomed to dealing with increments of time that are measured in hours, days, weeks, and years. Our history books often examine events over spans of centuries, but even a century is difficult to appreciate fully. For most of us, someone or something that is 90 years old is very old, and a 1000-year-old artifact is ancient.

By contrast, those who study Earth science must rou- tinely deal with vast time periods—millions or billions (thou- sands of millions) of years. When viewed in the context of Earth’s 4.5-billion-year history, an event that occurred 100 million years ago may be characterized as “recent” by a ge- ologist, and a rock sample that has been dated at 10 million years may be called “young.”

An appreciation for the magnitude of geologic time is important in the study of our planet because many process- es are so gradual that vast spans of time are needed before sig- nificant changes occur.

How long is 4.5 billion years? If you were to begin counting at the rate of one number per second and contin- ued 24 hours a day, seven days a week and never stopped, it would take about two lifetimes (150 years) to reach 4.5 billion!

Over the past 200 years or so, Earth scientists have de- veloped the geologic time scale of Earth history. It subdi- vides the 4.5-billion-year history of Earth into many different units and provides a meaningful time frame within which the events of the geologic past are arranged (Figure I.8). The principles used to develop the geologic time scale are exam- ined at some length in Chapter 8.

Resources and Environmental Issues

Environment refers to everything that surrounds and influ- ences an organism. Some of these things are biological and so- cial, but others are nonliving. The factors in this latter category are collectively called our physical environment. The physical environment encompasses water, air, soil, and rock, as well as conditions such as temperature, humidity, and sunlight. The phenomena and processes studied by the Earth sciences are basic to an understanding of the physical environment. In this sense, most of Earth science may be char- acterized as environmental science.

However, when the term environmental is applied to Earth science today, it usually means relationships between people and the physical environment. Application of the Earth sciences is necessary to understand and solve problems that arise from these interactions.

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Resources and Environmental Issues 9

Eon

Era Period Epoch

Phanerozoic

P ro

te ro

zo ic

A rc

h e a n

C e n o

zo ic

M e so

zo ic

P a le

o zo

Quaternary

Tertiary

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

145.5 m.y.

199.6 m.y.

251 m.y.

299 m.y.

318 m.y.

359 m.y.

416 m.y.

444 m.y.

488 m.y.

542 m.y.

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

1.8 m.y.

65.5 m.y.

P re

c a m

b ri a n

N e o

g e n e

P a le

o g

e n e

542 m.y.

2500 m.y.

4500 m.y.

Figure I.8 The geologic time scale divides the vast 4.5-billion-year history of Earth into eons, eras, periods, and epochs. We presently live in the Holocene epoch of the Quaternary period. This period is part of the Cenozoic era, which is the latest era of the Phanerozoic eon. There is much more about geologic time in Chapter 8.

We can dramatically influence natural processes. For example, river flooding is natural, but the magnitude and fre- quency of flooding can be changed significantly by human activities such as clearing forests, building cities, and con- structing dams. Unfortunately, natural systems do not always adjust to artificial changes in ways we can anticipate. Thus, an alteration to the environment that was intended to bene- fit society may have the opposite effect.

Resources

Resources are an important focus of the Earth sciences that is of great value to people. Resources include water and soil, a great variety of metallic and nonmetallic minerals, and ener- gy. Together, they form the very foundation of modern civi- lization (Figure I.9). The Earth sciences deal not only with the formation and occurrence of these vital resources but also with maintaining supplies and the environmental impact of their extraction and use.

Few people who live in highly industrialized nations realize the quantity of resources needed to maintain their present standard of living. For example, the annual per capi- ta consumption of metallic and nonmetallic mineral resources for the United States is nearly 10,000 kilograms (11 tons). This is each person’s prorated share of the materials required by industry to provide the vast array of products modern soci- ety demands. Figures for other highly industrialized coun- tries are comparable.

Resources are commonly divided into two broad cate- gories. Some are classified as renewable, which means that they can be replenished over relatively short time spans. Common examples are plants and animals for food, natural fibers for clothing, and forest products for lumber and paper. Energy from flowing water, wind, and the Sun is also con- sidered renewable.

By contrast, many other basic resources are classified as nonrenewable. Important metals such as iron, aluminum, and copper fall into this category, as do our most important fuels: oil, natural gas, and coal. Although these and other re- sources continue to form, the processes that create them are so slow that significant deposits take millions of years to ac- cumulate. In essence, Earth contains fixed quantities of these substances. When the present supplies are mined or pumped from the ground, there will be no more. Although some non- renewable resources, such as aluminum, can be used over and over again, others, such as oil, cannot be recycled.

How long will the remaining supplies of basic resources last? How long can we sustain the rising standard of living in today’s industrial countries and still provide for the growing needs of developing regions? How much environmental de- terioration are we willing to accept in pursuit of basic re- sources? Can alternatives be found? If we are to cope with an increasing demand and a growing world population, it is important that we have some understanding of our present and potential resources.

Did You Know? Each year, an average American requires huge quantities of

Earth materials. Imagine receiving your annual share in a sin-

gle delivery. A large truck would pull up to your home and un-

load 12,965 lbs. of stone, 8945 lbs. of sand and gravel, 895 lbs.

of cement, 395 lbs. of salt, 361 lbs. of phosphate, and 974 lbs. of

other nonmetals. In addition, there would be 709 lbs. of metals,

including iron, aluminum, and copper.

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10 Introduction to Earth Science

B.A.

Figure I.9 Coal represented about 22 percent of U.S. energy consumption in 2004. It is a major fuel used in power plants to generate electricity. By comparison, petroleum accounts for about 40 percent of U.S. energy use. A. Modern underground coal mining is highly mechanized and relatively safe. (Photo by Melvin Grubb/Grubb Photo Services, Inc.) B. This modern offshore drilling operation is in the North Sea. (Photo by Peter Bowater/Photo Researchers, Inc.)

A. B.

Figure I.10 Urban air pollution and soil erosion represent two of the environmental problems Earth scientists deal with. A. A severe air pollu- tion episode at Kuala Lumpur, Malaysia, on August 10, 2005. Fuel combustion by motor vehicles and power plants provides a high proportion of the pollutants. Meteorological factors determine whether pollutants remain “trapped” in the city or are dispersed. (Photo by Viviane Moos/ CORBIS) B. Gully erosion in poorly protected soil, southern Colombia. Just 1 millimeter of soil lost from a single acre of land amounts to about 5 tons. (Photo by Carl Purcell/Photo Researchers, Inc.)

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The Nature of Scientific Inquiry 11

Environmental Problems

In addition to the quest for adequate mineral and energy re- sources, the Earth sciences must also deal with a broad array of other environmental problems. Some are local, some are regional, and still others are global in extent. Serious difficul- ties face developed and developing nations alike. Urban air pollution, acid rain, ozone depletion, and global warming are just a few that pose significant threats (Figure I.10). Other problems involve the loss of fertile soils to erosion, the dispos- al of toxic wastes, and the contamination and depletion of water resources. The list continues to grow.

In addition to human-induced and human-accentuat- ed problems, people must also cope with the many natural hazards posed by the physical environment (Figure I.11). Earthquakes, landslides, volcanic eruptions, floods, and hur- ricanes are just five of the many risks. Others, such as drought, although not as spectacular, are nevertheless equal- ly important environmental concerns. In many cases, the threat of natural hazards is aggravated by increases in pop- ulation as more people crowd into places where an impend- ing danger exists or attempt to cultivate marginal lands that should not be farmed.

It is clear that as world population continues its rapid growth, pressures on the environment will increase as well. Therefore, an understanding of Earth is not only essential for the location and recovery of basic resources but also for deal- ing with the human impact on the environment and mini- mizing the effects of natural hazards. Knowledge about our planet and how it works is necessary to our survival and well- being. Earth is the only suitable habitat we have, and its re- sources are limited.

The Nature of Scientific Inquiry

As members of a modern society, we are constantly remind- ed of the benefits derived from science. But what exactly is the nature of scientific inquiry? Developing an understanding of how science is done and how scientists work is an important theme in this book. You will explore the difficulties in gath- ering data and some of the ingenious methods that have been developed to overcome these difficulties. You will also see examples of how hypotheses are formulated and tested, as well as learn about the evolution and development of some major scientific theories.

All science is based on the assumption that the natural world behaves in a consistent and predictable manner. The overall goal of science is to discover the underlying patterns in the natural world and then to use this knowledge to predict what will or will not happen, given certain facts or circumstances.

The development of new scientific knowledge involves some basic logical processes that are universally accepted. To determine what is occurring in the natural world, scien- tists collect facts through observation and measurement (Figure I.12). Because some error is inevitable, the accuracy of a particular measurement or observation is always open

B.A.

Figure I.11 Natural hazards are a part of living on Earth. A. Aerial view of earthquake destruction at Muzaffarabad, Pakistan, January 31, 2006. (Photo by Danny Kemp/AFP/Getty Images, Inc.) B. Satellite image of Hurricane Katrina in late August 2005 shortly before it devastated the Gulf Coast. (NASA)

Did You Know? It took until about the year 1800 for the world population to

reach 1 billion. Since then, the planet has added nearly 6 billion

more people.

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to question. Nevertheless, these data are essential to science and serve as the springboard for the development of scien- tific theories.

Hypothesis

Once facts have been gathered and principles have been for- mulated to describe a natural phenomenon, investigators try to explain how or why things happen in the manner ob- served. They can do this by constructing a preliminary, untested explanation, which we call a scientific hypothesis or model. (The term model, although often used synonymously with hypothesis, is a less precise term because it is sometimes used to describe a scientific theory as well.) It is best if an in- vestigator can formulate more than one hypothesis to explain a given set of observations. If an individual scientist is un- able to devise multiple hypotheses, others in the scientific community will almost always develop alternative explana- tions. A spirited debate frequently ensues. As a result, exten- sive research is conducted by proponents of opposing models, and the results are made available to the wider scientific com- munity in scientific journals.

Before a hypothesis can become an accepted part of sci- entific knowledge, it must pass objective testing and analysis. (If a hypothesis cannot be tested, it is not scientifically useful, no matter how interesting it might seem.) The verification process requires that predictions be made based on the hypoth- esis being considered and that the predictions be tested by comparing them against objective observations of nature. Put another way, hypotheses must fit observations other than those used to formulate them in the first place. Those hypotheses that fail rigorous testing are ultimately discarded. The history of science is littered with discarded hypotheses. One of the best known is the Earth-centered model of the universe—a propos- al that was supported by the apparent daily motion of the Sun, Moon, and stars around Earth. As the mathematician Jacob Bronowski so ably stated, “Science is a great many things, but in the end they all return to this: Science is the acceptance of what works and the rejection of what does not.”

Theory

When a hypothesis has survived extensive scrutiny, and when competing hypotheses have been eliminated, a hypothesis may be elevated to the status of a scientific theory. In every- day language, we may say, “That’s only a theory.” But a sci- entific theory is a well-tested and widely accepted view that scientists agree best explains certain observable facts.

Theories that are extensively documented are held with a very high degree of confidence. Theories of this stature that are comprehensive in scope have a special status. They are called paradigms because they explain a large number of in- terrelated aspects of the natural world. One well-known ex- ample is the theory of evolution, which explains the development of life on Earth. Another example is the theory of plate tectonics, which is a paradigm of the geological sci- ences that provides the framework for understanding the ori- gin of mountains, earthquakes, and volcanic activity. In addition, plate tectonics explains the evolution of the conti- nents and the ocean basins through time—a topic we will consider later in this book.

Scientific Methods

The process just described, in which scientists gather facts through observations and formulate scientific hypotheses that may eventually become theories, is termed the scientific method. Contrary to popular belief, the scientific method is not a standard recipe that scientists apply in a routine man- ner to unravel the secrets of our natural world. Rather, it is an endeavor that involves creativity and insight. Rutherford and Ahlgren put it this way: “Inventing hypotheses or theories to imagine how the world works and then figuring out how

Figure I.12 This geologist is taking a lava sample from a “skylight” in a lava tube near Kilauea Volcano, Hawaii. (Photo by G. Brad Lewis/Getty Images Inc.—Stone Allstock)

Did You Know? A scientific law is a basic principle that describes a particular

behavior of nature that is generally narrow in scope and can be

stated briefly—often as a simple mathematical equation.

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The Chapter in Review 13

they can be put to the test of reality is as creative as writing poetry, composing music, or designing skyscrapers.”*

There is no fixed path that scientists always follow that leads unerringly to scientific knowledge. Nevertheless, many scientific investigations involve the following steps: (1) the collection of scientific facts through observation and meas- urement (Figure I.13); (2) the development of one or more working hypotheses to explain these facts; (3) development of observations and experiments to test the hypothesis; and (4) the acceptance, modification, or rejection of the hypothe- sis based on extensive testing.

Other scientific discoveries represent purely theoretical ideas, which stand up to extensive examination. Some re- searchers use high-speed computers to simulate what is hap- pening in the “real” world. These models are useful when dealing with natural processes that occur on very long time scales or take place in extreme or inaccessible locations. Still other scientific advancements have been made when a total- ly unexpected happening occurred during an experiment. These serendipitous discoveries are more than pure luck, for as Louis Pasteur said, “In the field of observation, chance fa- vors only the prepared mind.”

Scientific knowledge is acquired through several av- enues, so it might be best to describe the nature of scientific inquiry as the methods of science rather than the scientific method. In addition, it should always be remembered that even the most compelling scientific theories are still simpli- fied explanations of the natural world.

Studying Earth Science

In this book, you will discover the results of centuries of sci- entific work. You will see the end product of millions of ob- servations, thousands of hypotheses, and hundreds of theories. We have distilled all of this to give you a “briefing” in Earth science.

Figure I.13 Gathering data is a basic part of the scientific method. This Automated Surface Observing System (ASOS) installation is one of nearly 900 in use for data gathering as part of the U.S. primary surface observing network.

*F. James Rutherford and Andrew Ahlgren, Science for All Americans (New York: Oxford University Press, 1990), p. 7.

But realize that our knowledge of Earth is changing daily, as thousands of scientists worldwide make satellite ob- servations, analyze drill cores from the seafloor, study earth- quake waves, develop computer models to predict climate, examine the genetic codes of organisms, and discover new facts about our planet’s long history. This new knowledge often updates hypotheses and theories. Expect to see many new discoveries and changes in scientific thinking in your lifetime.

The Chapter in Review

1. Earth science is the name for all the sciences that collec- tively seek to understand Earth and its neighbors in space. It includes geology, oceanography, meteorology, and astron- omy. Geology is traditionally divided into two broad areas— physical and historical.

2. Earth’s physical environment is traditionally divided into three major parts: the solid Earth, the geosphere; the water portion of our planet, the hydrosphere; and Earth’s gaseous envelope, the atmosphere. In addition, the biosphere, the totality of life on Earth, interacts with each of the three physical realms and is an equally integral part of Earth.

3. Although each of Earth’s four spheres can be studied separately, they are all related in a complex and continuously interacting whole that we call the Earth system. Earth system science uses an interdisciplinary approach to integrate the knowledge of several academic fields in the study of our planet and its global environmental problems.

4. The two sources of energy that power the Earth system are (1) the Sun, which drives the external processes that occur in the atmosphere, hydrosphere, and at Earth’s surface, and (2) heat from the Earth’s interior that powers the internal processes that produce volcanoes, earthquakes, and mountains.

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14 Introduction to Earth Science

5. Environment refers to everything that surrounds and influences an organism. These influences can be biological, social, or physical. When applied to Earth science today, the term environmental is usually reserved for those aspects that focus on the relationships between people and the natural environment.

6. Resources are an important environmental concern. The two broad categories of resources are (1) renewable, which means that they can be replenished over relatively short time spans, and (2) nonrenewable. As population grows, the de- mand for resources expands as well.

7. Environmental problems can be local, regional, or global. Human-induced problems include urban air pollu- tion, acid rain, ozone depletion, and global warming. Nat- ural hazards include earthquakes, landslides, floods, and

hurricanes. As world population grows, pressures on the en- vironment also increase.

8. All science is based on the assumption that the natural world behaves in a consistent and predictable manner. The process by which scientists gather facts through observation and careful measurement and formulate scientific hypothe- ses and theories is called the scientific method. To determine what is occurring in the natural world, scientists often (1) col- lect facts, (2) develop a scientific hypothesis, (3) construct ex- periments to validate the hypothesis, and (4) accept, modify, or reject the hypothesis on the basis of extensive testing. Other discoveries represent purely theoretical ideas that have stood up to extensive examination. Still other scientific advance- ments have been made when a totally unexpected happening occurred during an experiment.

Key Terms

asthenosphere (p. 5)

atmosphere (p. 5)

astronomy (p. 3)

biosphere (p. 6)

core (p. 5)

crust (p. 5)

Earth science (p. 2)

Earth system (p. 6)

geologic time scale (p. 8)

geology (p. 2)

geosphere (p. 5)

hydrosphere (p. 3)

hypothesis (p. 12)

lithosphere (p. 5)

mantle (p. 5)

meteorology (p. 3)

model (p. 12)

nonrenewable resource (p. 9)

oceanography (p. 2)

paradigm (p. 12)

physical environment (p. 8)

renewable resource (p. 9)

system (p. 7)

theory (p. 12)

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Online Study Guide 15

Questions for Review

1. Name the specific Earth science described by each of the following statements: a. This science deals with the dynamics of the oceans. b. This word literally means “the study of Earth.” c. An understanding of the atmosphere is the primary

focus of this science. d. This science helps us understand Earth’s place in the

universe. 2. List and briefly define the four “spheres” that consti-

tute our environment.

3. Name the three principal divisions of the solid Earth based on compositional differences. Which one is thinnest?

4. The oceans cover nearly _____ percent of Earth’s sur- face and contain about _____ percent of the planet’s total water supply.

5. What are the two sources of energy for the Earth system?

6. Contrast renewable and nonrenewable resources. Give one or more examples of each.

Online Study Guide

The Foundations of Earth Science Web site uses the resources and flexibility of the Internet to aid in your study of the top- ics in this chapter. Written and developed by Earth science instructors, this site will help improve your understanding of Earth science. Visit http://www.prenhall.com/lutgens and click on the cover of Foundations of Earth Science 5e to find:

• Online review quizzes. • Critical thinking exercises. • Links to chapter-specific Web resources. • Internet-wide key-term searches.

http://www.prenhall.com/lutgens

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EARTH MATERIALSIU N I T

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F O C U S O N L E A R N I N G

To assist you in learning the important concepts in this chapter, you will find it helpful to focus on the following questions:

1. What are minerals, and how are they different from rocks?

2. What are the smallest particles of matter? How do atoms bond together?

3. How do isotopes of the same element vary from one another, and why are some isotopes radioactive?

4. What are some of the physical and chemical properties of minerals? How can these properties be used to distinguish one mineral from another?

5. What are the eight elements that make up most of Earth’s continental crust?

6. What is the most abundant mineral group? What do all minerals within this group have in common?

7. When is the term ore used with reference to a mineral? What are the common ores of iron and lead?

Minerals: Building Blocks of Rocks

1 C H A P T E R

Topaz crystal with albite. (Photo by Jeff Scovil)

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E arth’s crust and oceans are the source of a wide variety of useful and essential minerals. Most people are famil- iar with the common uses of many basic metals, includ-

ing aluminum in beverage cans, copper in electrical wiring, and gold and silver in jewelry. But some people are not aware that the lead in pencils contains the greasy-feeling mineral graphite and that bath powders and many cosmetics contain the mineral talc. Moreover, many do not know that drill bits impregnated with diamonds are used by dentists to drill through tooth enamel, or that the common mineral quartz is the source of silicon for computer chips. In fact, practically every manufactured product contains materials obtained from minerals. As the mineral requirements of modern soci- ety grow, the need to locate additional supplies of useful min- erals also grows, becoming more challenging as well.

In addition to the economic uses of rocks and minerals, all of the processes studied by geologists are in some way de- pendent on the properties of these basic Earth materials. Events such as volcanic eruptions, mountain building, weath- ering and erosion, and even earthquakes involve rocks and minerals. Consequently, a basic knowledge of Earth materials is essential to the understanding of all geologic phenomena.

Minerals: The Building Blocks of Rocks

Earth Materials � Minerals

We begin our discussion of Earth materials with an overview of mineralogy because minerals are the building blocks of rocks (Figure 1.1). In addition, minerals have been employed by humans for both useful and decorative purposes for thousands of years.

(mineral = mineral, ology = the study of ),

18 Chapter 1 Minerals: Building Blocks of Rocks

Figure 1.1 Collection of well-developed quartz crystals found near Hot Springs, Arkansas. (Photo by Jeff Scovil)

Did You Know? Archaeological finds show that more than 2000 years ago, the

Romans used lead for pipes to transport water within their

buildings. In fact, Roman smelting of lead and copper ores be-

tween 500 B.C. and A.D. 300 caused a small but significant rise

in atmospheric pollution, as recorded in Greenland ice cores.

EE A

T H SC I E N CE

The first minerals mined were flint and chert, which people fashioned into weapons and cutting tools. As early as 3700 B.C., Egyptians began mining gold, silver, and copper; and by 2200 B.C., humans discovered how to combine copper with tin to make bronze, a tough, hard alloy. The Bronze Age began its decline when the ability to extract iron from minerals such as hematite was discovered. By about 800 B.C., iron-working technology had advanced to the point that weapons and many everyday objects were made of iron rather than cop- per, bronze, or wood. During the Middle Ages, mining of a variety of minerals was common throughout Europe, and the impetus for the formal study of minerals was in place.

The term mineral is used in several different ways. For example, those concerned with health and fitness extol the benefits of vitamins and minerals. The mining industry typ- ically uses the word when referring to anything taken out of the ground, such as coal, iron ore, or sand and gravel. The guessing game known as Twenty Questions usually begins with the question, Is it animal, vegetable, or mineral? What cri- teria do geologists use to determine whether something is a mineral?

Geologists define a mineral as any naturally occurring inorganic solid that possesses an orderly crystalline structure and a well-defined chemical composition. Thus, those Earth mater- ials that are classified as minerals exhibit the following characteristics:

1. Naturally occurring—This tells us that a mineral forms by natural, geologic processes. Consequently, synthetic diamonds and rubies, as well as a variety of other use- ful materials produced by chemists are not considered minerals.

2. Solid—In order to be considered a mineral, substances must be solids at temperatures normally experienced at Earth’s surface. Thus, ice (frozen water) is considered a mineral, whereas liquid water is not.

3. Orderly crystalline structure—Minerals are crystalline substances, which means their atoms are arranged in an orderly, repetitive manner as shown in Figure 1.2. This orderly packing of atoms is reflected in the regu- larly shaped objects we call crystals (Figure 1.2D). Some naturally occurring solids, such as volcanic glass (ob- sidian), lack a repetitive atomic structure and are re- ferred to as amorphous (without form) and are not considered minerals.

4. Well-defined chemical composition—Most minerals are chemical compounds made up of two or more

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Elements: Building Blocks of Minerals 19

A. Sodium and chlorine ions.

B. Basic building block of the mineral halite.

C. Collection of basic building blocks (crystal).D. Intergrown crystals of the mineral halite.

Na+

Cl–

Figure 1.2 Illustration of the orderly arrangement of sodium and chloride atoms (ions) in the mineral halite. The arrangement of atoms into basic building blocks having a cubic shape results in regularly shaped cubic crystals. (Photo by M. Claye/Jacana Scientific Control/Photo Researchers, Inc.)

elements. A few, such as gold and silver, consist of only a single element. The common mineral quartz consists of two oxygen (O) atoms for every silicon (Si) atom, giv- ing it a chemical composition expressed by the formula

Thus, no matter what the environment is, when- ever atoms of oxygen and silicon join together in the ratio of 2 to 1, the product is always quartz. However, it is not uncommon for elements of similar size and elec- trical charge to substitute freely for one another. As a result the chemical composition of a mineral may vary somewhat from sample to sample.

5. Generally inorganic—Inorganic crystalline solids, as exemplified by ordinary table salt (halite), that are found naturally in the ground are considered minerals. Organic compounds, on the other hand, are generally not. Sugar, a crystalline solid like salt, comes from sugar cane or sugar beets and is a common example of such an organic compound. However, many marine animals secrete inorganic compounds, such as calcium carbon- ate (calcite), in the form of shells and coral reefs. These materials are considered minerals by most geologists.

Rocks, on the other hand, are more loosely defined. Sim- ply, a rock is any solid mass of mineral or mineral-like mat- ter that occurs naturally as part of our planet. A few rocks are composed almost entirely of one mineral. A common exam- ple is the sedimentary rock limestone, which consists of im- pure masses of the mineral calcite. However, most rocks, like the common rock granite (shown in Figure 1.3), occur as ag- gregates of several kinds of minerals. The term aggregate im- plies that the minerals are joined in such a way that the properties of each mineral are retained. Note that you can

SiO2.

easily identify the mineral constituents of the sample of gran- ite shown in Figure 1.3.

A few rocks are composed of nonmineral matter. These include the volcanic rocks obsidian and pumice, which are non- crystalline glassy substances, and coal, which consists of solid organic debris.

Although this chapter deals primarily with the nature of minerals, keep in mind that most rocks are simply aggre- gates of minerals. Because the properties of rocks are deter- mined largely by the chemical composition and crystalline structure of the minerals contained within them, we will first consider these Earth materials. Then, in Chapter 2, we will take a closer look at Earth’s major rock groups.

Elements: Building Blocks of Minerals

You are probably familiar with the names of many elements, including copper, gold, oxygen, and carbon. An element is a substance that cannot be broken down into simpler sub- stances by chemical or physical means. There are about 90 different elements found in nature, and, consequently, about 90 different kinds of atoms. In addition, scientists have suc- ceeded in creating about 23 synthetic elements.

The elements can be organized into rows so that those with similar properties are in the same column. This arrange- ment, called the periodic table, is shown in Figure 1.4. Notice that symbols are used to provide a shorthand way of represent- ing an element. Each element is also known by its atomic num- ber, which is shown above each symbol on the periodic table.

Some elements, such as copper (number 29) and gold (number 79), exist in nature with single atoms as the basic

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20 Chapter 1 Minerals: Building Blocks of Rocks

Did You Know? Although wood pulp is the main ingredient for making news-

print, many higher grades of paper contain large quantities

of clay minerals. In fact, each page of this textbook consists of

about 25 percent clay (the mineral kaolinite). If rolled into a

ball, this much clay would be roughly the size of a tennis ball.

Granite (Rock)

Quartz (Mineral)

Hornblende (Mineral)

Feldspar (Mineral)

Figure 1.3 Rocks are aggregates of one or more minerals. (Top and left photos courtesy E. J. Tarbuck; center and right photos by GeoScience Resources/ American Geological Institute)

unit. Thus, native copper and gold are minerals made entire- ly from one element. However, many elements are quite re- active and join together with atoms of one or more other elements to form chemical compounds. As a result, most minerals are chemical compounds consisting of two or more different elements.

Atoms

To understand how elements combine to form compounds, we must first consider the atom. The atom

is the smallest particle of matter that retains the essential characteristics of an element. It is this extremely small parti- cle that does the combining.

Protons, Neutrons, and Electrons The central region of an atom is called the nucleus. The nucleus contains protons and neutrons. Protons are very dense particles with positive elec-

cut) (a = not, tomos =

trical charges. Neutrons have the same mass as a proton but lack an electrical charge.

Orbiting the nucleus are electrons, which have negative electrical charges. For convenience, we sometimes diagram atoms to show the electrons orbiting the nucleus, like the or- derly orbiting of the planets around the Sun (Figure 1.5A). However, electrons move in a less predictable way than do planets. As a result, they create a sphere-shaped negative zone around the nucleus. A more realistic picture of the po- sitions of electrons can be obtained by envisioning a cloud of negatively charged electrons surrounding the nucleus (Figure 1.5B).

Studies of electron configurations predict that individ- ual electrons move within regions around the nucleus called principal shells, or energy levels. Furthermore, each of these shells can hold a specific number of electrons, with the out- ermost principal shell containing the valence electrons. Va- lence electrons are important because, as you will see later, they are involved in chemical bonding.

Atomic Number Some atoms have only a single proton in their nuclei, while others contain more than 100 protons. The number of protons in the nucleus of an atom is called the atomic number. For example, all atoms with six protons are carbon atoms; hence, the atomic number of carbon is 6. Like- wise, every atom with eight protons is an oxygen atom.

Atoms in their natural state also have the same num- ber of electrons as protons, so the atomic number also equals

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Why Atoms Bond 21

2 He 4.003

Helium

Nonmetals

Metals

Transition metals

Noble gases

Lanthanide series

Actinide seriesI A II A III A IV A V A VI A VII A

VIII A

III B IV B V B VI B VII B I B II B

Atomic number

Symbol of element

Atomic weight

Name of element

2 He 4.003

Helium

1 H

1.0080 Hydrogen

87 Fr

(223) Francium

88 Ra

226.05 Radium

89 TO 103

55 Cs

132.91 Cesium

56 Ba

137.34 Barium

57 TO 71

72 Hf

178.49 Hafnium

73 Ta

180.95 Tantalum

74 W

183.85 Tungsten

75 Re 186.2

Rhenium

76 Os 190.2

Osmium

77 Ir

192.2 Iridium

78 Pt

195.09 Platinum

79 Au

197.0 Gold

80 Hg

200.59 Mercury

81 Tl

204.37 Thallium

82 Pb

207.19 Lead

83 Bi

208.98 Bismuth

84 Po (210)

Polonium

85 At

(210) Astatine

86 Rn (222)

Radon

37 Rb 85.47

Rubidium

38 Sr

87.62 Strontium

39 Y

88.91 Yttrium

40 Zr

91.22 Zirconium

41 Nb 92.91

Niobium

42 Mo 95.94

Molybdenum

43 Tc (99)

Technetium

44 Ru 101.1

Ruthenium

45 Rh

102.90 Rhodium

46 Pd 106.4

Palladium

47 Ag

107.87 Silver

48 Cd

112.40 Cadmium

49 In

114.82 Indium

50 Sn

118.69 Tin

51 Sb

121.75 Antimony

52 Te

127.60 Tellurium

53 I

126.90 Iodine

54 Xe

131.30 Xenon

19 K

39.102 Potassium

20 Ca 40.08

Calcium

21 Sc

44.96 Scandium

22 Ti

47.90 Titanium

23 V

50.94 Vanadium

24 Cr

52.00 Chromium

25 Mn 54.94

Manganese

26 Fe

55.85 Iron

27 Co 58.93 Cobalt

28 Ni

58.71 Nickel

29 Cu 63.54

Copper

30 Zn

65.37 Zinc

31 Ga 69.72

Gallium

32 Ge 72.59

Germanium

33 As

74.92 Arsenic

34 Se

78.96 Selenium

35 Br

79.909 Bromine

36 Kr

83.80 Krypton

11 Na

22.990 Sodium

12 Mg 24.31

Magnesium

13 Al

26.98 Aluminum

14 Si

28.09 Silicon

15 P

30.974 Phosphorus

16 S

32.064 Sulfur

17 Cl

35.453 Chlorine

18 Ar

39.948 Argon

3 Li

6.939 Lithium

4 Be 9.012

Beryllium

5 B

10.81 Boron

6 C

12.011 Carbon

7 N

14.007 Nitrogen

8 O

15.9994 Oxygen

9 F

18.998 Fluorine

10 Ne

20.183 Neon

57 La

138.91 Lanthanum

58 Ce

140.12 Cerium

59 Pr

140.91 Praseodymium

60 Nd

144.24 Neodymium

61 Pm (147)

Promethium

62 Sm

150.35 Samarium

63 Eu

151.96 Europium

64 Gd

157.25 Gadolinium

65 Tb

158.92 Terbium

66 Dy

162.50 Dysprosium

67 Ho

164.93 Holmium

68 Er

167.26 Erbium

69 Tm

168.93 Thullium

70 Yb

173.04 Ytterbium

71 Lu

174.97 Lutetium

89 Ac (227)

Actinium

90 Th

232.04 Thorium

91 Pa (231)

Protactinium

92 U

238.03 Uranium

93 Np (237)

Neptunium

94 Pu (242)

Plutonium

95 Am (243)

Americium

96 Cm (247)

Curium

97 Bk (249)

Berkelium

98 Cf

(251) Californium

99 Es (254)

Einsteinium

100 Fm (253)

Fermium

101 Md (256)

Mendelevium

102 No (254)

Nobelium

103 Lw (257)

Lawrencium

VIII B

Tendency to lose outermost

electrons to uncover full

outer shell

Tendency to lose electrons

Tendency to fill outer shell by

sharing electrons

Tendency to gain

electrons to make full outer shell

Noble gases (inert)

Figure 1.4 Periodic table of the elements.

the number of electrons surrounding the nucleus. Therefore, carbon has six electrons to match its six protons, and oxygen has eight electrons to match its eight protons. Neutrons have no charge, so the positive charge of the protons is exactly bal- anced by the negative charge of the electrons. Consequently, atoms in the natural state are neutral electrically and have no overall electrical charge.

Why Atoms Bond

Why do elements join together to form chemical compounds? It has been learned from experimentation that the forces hold- ing the atoms together are electrical. Further, it is known that chemical bonding results in a change in the electron config- uration of the bonded atoms. As we noted earlier, it is the va- lence electrons (outer-shell electrons) that are generally involved in chemical bonding. Figure 1.6 shows a shorthand way of representing the number of electrons in the outer prin- cipal shell (valence electrons). Notice that the elements in group I have one valence electron, those in group II have two, and so forth up to group VIII, which has eight valence electrons.

Other than the first shell, which can hold a maximum of two electrons, a stable configuration occurs when the valence shell

contains eight electrons. Only the noble gases, such as neon and argon, have a complete outermost principal shell. Hence, the noble gases are the least chemically reactive and are desig- nated as “inert.”

When an atom’s outermost shell does not contain the maximum number of electrons (8), the atom is likely to chem- ically bond with one or more other atoms. A chemical bond is the sharing or transfer of electrons to attain a stable electron configuration among the bonding atoms. If the electrons are transferred, the bond is an ionic bond. If the electrons are shared, the bond is called a covalent bond. In either case, the bonding atoms get stable electron configurations, which usu- ally consist of eight electrons in the outer shell.

Did You Know? The purity of gold is expressed by the number of karats. Twenty-

four karats is pure gold. Gold less than 24 karats is an alloy

(mixture) of gold and another metal, usually copper or silver.

For example, 14-karat gold contains 14 parts gold (by weight)

mixed with 10 parts of other metals.

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22 Chapter 1 Minerals: Building Blocks of Rocks

Protons (+ charge)

Neutrons (no charge)

Nucleus

Electrons (– charge)

First energy level (shell)

Second energy level (shell)

Nucleus

Electron cloud

Figure 1.5 Two models of the atom. A. This very simplified view of the atom has a central nucleus, consisting of protons and neutrons, encircled by high-speed electrons. B. Another model of the atom showing spherically shaped electron clouds (energy-level shells). Note that these models are not drawn to scale. Electrons are minuscule in size compared to protons and neutrons, and the relative space between the nucleus and electron shells is much greater than illustrated.

Ionic Bonds: Electrons Transferred

Perhaps the easiest type of bond to visualize is an ionic bond. In ionic bonding, one or more valence electrons are transferred from one atom to another. Simply, one atom gives up its va- lence electrons, and the other uses them to complete its outer shell. A common example of ionic bonding is sodium (Na) and chlorine (Cl) joining to produce sodium chloride (com- mon table salt). This is shown in Figure 1.7A. Notice that sodi- um gives up its single valence electron to chlorine. As a result,

sodium achieves a stable configuration having eight electrons in its outermost shell. By acquiring the electron that sodium loses, chlorine—which has seven valence electrons—gains the eighth electron needed to complete its outermost shell. Thus, through the transfer of a single electron, both the sodium and chlorine atoms have acquired a stable electron configuration.

Once electron transfer takes place, atoms are no longer electrically neutral. By giving up one electron, a neutral sodi- um atom becomes positively charged (11 protons/10 electrons). Similarly, by acquiring one electron, the neutral chlorine atom becomes negatively charged (17 protons/18 electrons). Atoms such as these, which have an electrical charge because of the unequal numbers of electrons and protons, are called ions.

We know that ions with like charges repel, and those with unlike charges attract. Thus, an ionic bond is the attrac- tion of oppositely charged ions to one another, producing an electrically neutral compound. Figure 1.7B illustrates the arrangement of sodium and chlorine ions in ordinary table salt. Notice that salt consists of alternating sodium and chlo- rine ions, positioned in such a manner that each positive ion is attracted to and surrounded on all sides by negative ions, and vice versa. This arrangement maximizes the attraction between ions with unlike charges while minimizing the repul- sion between ions with like charges. Thus, ionic compounds consist of an orderly arrangement of oppositely charged ions as- sembled in a definite ratio that provides overall electrical neutrality.

The properties of a chemical compound are dramatically different from the properties of the elements comprising it. For example, sodium, a soft, silvery metal, is extremely reac- tive and poisonous. If you were to consume even a small amount of elemental sodium, you would need immediate medical attention. Chlorine, a green poisonous gas, is so toxic it was used as a chemical weapon during World War I. To- gether, however, these elements produce sodium chloride, a

Electron Dot Diagrams for Some Representative Elements

VIII

VII

III

Figure 1.6 Dot diagrams for some representative elements. Each dot represents a valence electron found in the outermost principal shell.

+Na Cl Cl –

Na+

Sodium ion

Chlorine ion

Figure 1.7 Chemical bonding of sodium chloride (table salt). A. Through the transfer of one electron in the outer shell of a sodium atom to the outer shell of a chlorine atom, the sodium becomes a positive ion and chlorine a negative ion. B. Diagram illustrating the arrangement of sodium and chlorine ions in table salt.

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Properties of Minerals 23

ClCl Cl Cl

Dot structures of chlorine atoms

Shared pair of electrons

Figure 1.8 Dot diagrams used to illustrate the sharing of a pair of electrons between two chlorine atoms to form a chlorine molecule. Notice that by sharing a pair of electrons, both chlorine atoms achieve a full outer shell (8 electrons).

Did You Know? One of the world’s heaviest cut and polished gemstones is the

22,892.5-carat golden-yellow topaz. Currently housed in the

Smithsonian Institution, this roughly 10-pound gem is about

the size of an automobile headlight and could hardly be used

as a piece of jewelry, except perhaps by an elephant.

harmless flavor enhancer that we call table salt. When ele- ments combine to form compounds, their properties change dramatically.

Covalent Bonds: Electrons Shared

Not all atoms combine by transferring electrons to form ions. Some atoms share electrons. For example, the gaseous ele- ments oxygen hydrogen and chlorine exist as stable molecules consisting of two atoms bonded together, without a complete transfer of electrons.

Figure 1.8 illustrates the sharing of a pair of electrons between two chlorine atoms to form a molecule of chlorine gas By overlapping their outer shells, these chlorine atoms share a pair of electrons. Thus, each chlorine atom has acquired, through cooperative action, the needed eight elec- trons to complete its outer shell. The bond produced by the sharing of electrons is called a covalent bond.

A common analogy may help you visualize a covalent bond. Imagine two people at opposite ends of a dimly lit room, each reading under a separate lamp. By moving the lamps to the center of the room, they are able to combine their light sources so each can see better. Just as the overlapping light beams meld, the shared electrons that provide the “elec- trical glue” in covalent bonds are indistinguishable from each other. The most common mineral group, the silicates, con- tains the element silicon, which readily forms covalent bonds with oxygen.

Isotopes and Radioactive Decay

Subatomic particles are so incredibly small that a special unit, called an atomic mass unit, was devised to express their mass. A proton or a neutron has a mass just slightly more than one atomic mass unit, whereas an electron is only about one two- thousandth of an atomic mass unit. Thus, although electrons play an active role in chemical reactions, they do not con- tribute significantly to the mass of an atom.

The mass number of an atom is simply the total of its neutrons and protons. Atoms of the same element always have the same number of protons. But the number of neu- trons for atoms of the same element can vary. Atoms with the same number of protons but different numbers of neutrons are isotopes of that element. Isotopes of the same element are labeled by placing the mass number after the element’s name or symbol.

(Cl2).

(Cl2)(H2),(O2),

For example, carbon has three well-known isotopes. One has a mass number of 12 (carbon-12), another has a mass number of 13 (carbon-13), and the third, carbon-14, has a mass number of 14. Since all atoms of the same element have the same number of protons, and carbon has six, carbon-12 also has six neutrons to give it a mass number of 12. Likewise, car- bon-14 has six protons plus eight neutrons to give it a mass number of 14.

In chemical behavior, all isotopes of the same element are nearly identical. To distinguish among them is like try- ing to differentiate identical twins, with one being slightly heavier. Because isotopes of an element react the same chem- ically, different isotopes can become parts of the same miner- al. For example, when the mineral calcite forms from calcium, carbon, and oxygen, some of its carbon atoms are carbon-12 and some are carbon-14.

The nuclei of most atoms are stable. However, many el- ements do have isotopes in which the nuclei are unstable. “Unstable” means that the isotopes disintegrate through a process called radioactive decay. Radioactive decay occurs when the forces that bind the nucleus are not strong enough.

During radioactive decay, unstable atoms radiate ener- gy and emit particles. Some of this energy powers the move- ments of Earth’s crust and upper mantle. The rates at which unstable atoms decay are measurable. Therefore, certain ra- dioactive atoms can be used to determine the ages of fossils, rocks, and minerals. A discussion of radioactive decay and its applications in dating past geologic events is found in Chapter 8.

Properties of Minerals

Earth Materials � Minerals

Each mineral has a definite crystalline structure and chemi- cal composition that give it a unique set of physical and chem- ical properties shared by all samples of that mineral. For example, all specimens of halite have the same hardness and the same density and break in a similar manner. Because the internal structure and chemical composition of a mineral are difficult to determine without the aid of sophisticated tests and equipment, the more easily recognized physical proper- ties are frequently used in identification.

The diagnostic physical properties of minerals are those that can be determined by observation or by performing a

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24 Chapter 1 Minerals: Building Blocks of Rocks

Did You Know? The name crystal is derived from the Greek (krystallos, meaning

“ice”) and was applied to quartz crystals. The ancient Greeks

thought quartz was water that had crystallized at high pres-

sures deep inside Earth.

5 cm

Figure 1.9 The freshly broken sample of galena (right) displays a metallic luster, while the sample on the left is tarnished and has a submetallic luster. (Photo courtesy of E. J. Tarbuck)

simple test. The primary physical properties that are com- monly used to identify hand samples are luster, color, streak, crystal shape (habit), tenacity, hardness, cleavage, fracture, and density or specific gravity. Secondary (or “special”) prop- erties that are exhibited by a limited number of minerals in- clude magnetism, taste, feel, smell, double refraction, and chemical reaction to hydrochloric acid.

Optical Properties

Of the many optical properties of minerals, four—luster, the ability to transmit light, color, and streak—are most frequent- ly used for mineral identification.

Luster The appearance or quality of light reflected from the surface of a mineral is known as luster. Minerals that have the appearance of metals, regardless of color, are said to have a metallic luster (Figure 1.9). Some metallic minerals, such as native copper and galena, develop a dull coating or tarnish when exposed to the atmosphere. Because they are not as shiny as samples with freshly broken surfaces, these samples are often said to exhibit a submetallic luster.

Most minerals have a nonmetallic luster and are de- scribed using various adjectives such as vitreous or glassy. Other nonmetallic minerals are described as having a dull or earthy luster (a dull appearance like soil), or a pearly luster (such as a pearl, or the inside of a clamshell). Still others ex- hibit a silky luster (like satin cloth), or a greasy luster (as though coated in oil).

The Ability to Transmit Light Another optical property used in the identification of minerals is the ability to transmit light. When no light is transmitted, the mineral is described as opaque; when light but not an image is transmitted through

a mineral it is said to be translucent. When light and an image are visible through the sample, the mineral is described as transparent.

Color Although color is generally the most conspicuous characteristic of any mineral, it is considered a diagnostic property of only a few minerals. Slight impurities in the com- mon mineral quartz, for example, give it a variety of tints in- cluding pink, purple, yellow, white, gray, and even black. Other minerals, such as tourmaline, also exhibit a variety of hues, with multiple colors sometimes occurring in the same sample. Thus, the use of color as a means of identification is often ambiguous or even misleading.

Streak Although the color of a sample is not always help- ful in identification, streak—the color of the powdered min- eral—is often diagnostic. The streak is obtained by rubbing the mineral across a piece of unglazed porcelain, termed a streak plate, and observing the color of the mark it leaves (Figure 1.10). Although the color of a mineral may vary from sample to sample, the streak usually does not.

Streak can also help distinguish minerals with metallic luster from those having nonmetallic luster. Metallic minerals generally have a dense, dark streak, whereas minerals with nonmetallic luster have a streak that is typically light colored.

Figure 1.10 Although the color of a mineral is not always helpful in identification, the streak, which is the color of the powdered mineral, can be very useful. (Photo by Dennis Tasa)

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Properties of Minerals 25

Figure 1.11 Although most minerals exhibit only one common crystal shape, some such as pyrite have two or more characteristic habits. (Photo by Dennis Tasa)

A. Bladed B. Prismatic

C. Banded D. Botryoidal

Figure 1.12 Some common crystal habits. A. Bladed. Elongated crystals that are flattened in one direction. B. Prismatic. Elongated crystals with faces that are parallel to a common direction. C. Banded. Minerals that have stripes or bands of color or texture. D. Botryoidal. Groups of intergrown crystals resembling a bunch of grapes.

It should be noted that not all minerals produce a streak when using a streak plate. For example, if the mineral is hard- er than the streak plate, no streak is observed.

Crystal Shape or Habit

Mineralogists use the term habit to refer to the common or characteristic shape of a crystal or aggregate of crystals. A few minerals exhibit somewhat regular polygons that are helpful in their identification. While most minerals have only one common habit, a few such as pyrite have two or more characteristic crystal shapes (Figure 1.11).

By contrast, some minerals rarely develop perfect geo- metric forms. Many of these do, however, develop a charac- teristic shape that is useful for identification. Some minerals tend to grow equally in all three dimensions, whereas others tend to be elongated in one direction, or flattened if growth in one dimension is suppressed. Commonly used terms to describe these and other crystal habits include equant (equidi- mensional), bladed, fibrous, tabular, prismatic, platy, blocky, and botryoidal (Figure 1.12).

Mineral Strength

How easily minerals break or deform under stress relates to the type and strength of the chemical bonds that hold the crystals together. Mineralogists use terms including tenacity, hardness, cleavage, and fracture to describe mineral strength and how minerals break when stress is applied.

Tenacity The term tenacity describes a mineral’s toughness, or its resistance to breaking or deforming. Minerals that are ionically bonded, such as fluorite and halite, tend to be brittle and shatter into small pieces when struck. By contrast, min-

erals with metallic bonds, such as native copper, are malleable, or easily hammered into different shapes. Minerals, including gypsum and talc, that can be cut into thin shavings are de- scribed as sectile. Still others, notably the micas, are elastic and will bend and snap back to their original shape after the stress is released.

Hardness One of the most useful diagnostic properties is hardness, a measure of the resistance of a mineral to abra- sion or scratching. This property is determined by rubbing a mineral of unknown hardness against one of known hard- ness, or vice versa. A numerical value of hardness can by ob- tained by using the Mohs scale of hardness, which consists of 10 minerals arranged in order from 1 (softest) to 10 (hard- est), as shown in Figure 1.13A. It should be noted that the Mohs scale is a relative ranking, and it does not imply that mineral number 2, gypsum, is twice as hard as mineral 1, talc. In fact, gypsum is only slightly harder than talc as shown in Figure 1.13B.

In the laboratory, other common objects can be used to determine the hardness of a mineral. These include a human fingernail, which has a hardness of about 2.5, a copper penny 3.5, and a piece of glass 5.5. The mineral gypsum, which has a hardness of 2, can be easily scratched with a fingernail. On the other hand, the mineral calcite, which has a hardness of 3, will scratch a fingernail but will not scratch glass. Quartz, one of the hardest common minerals, will easily scratch glass. Diamonds, hardest of all, scratch anything.

Cleavage Some minerals have atomic structures that are not the same in every direction and chemical bonds that vary in strength. As a result, these minerals tend to break, so that

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26 Chapter 1 Minerals: Building Blocks of Rocks

the broken fragments are bounded by more or less flat, planer surfaces, a property called cleavage Cleav- age can be recognized by rotating a sample and looking for smooth, even surfaces that reflect light like a mirror. Note, however, that cleavage surfaces can occur in small, flat seg- ments arranged in stair-step fashion.

The simplest type of cleavage is exhibited by the micas (Figure 1.14). Because the micas have much weaker bonds in one direction than in the others, they cleave to form thin, flat sheets. Some minerals have excellent cleavage in several di- rections, whereas others exhibit fair or poor cleavage, and still others have no cleavage at all. When minerals break even- ly in more than one direction, cleavage is described by the number of cleavage planes and the angle(s) at which they meet (Figure 1.15).

Do not confuse cleavage with crystal shape. When a mineral exhibits cleavage, it will break into pieces that have the same geometry as one another. By contrast, the smooth-

(kleiben = carve).

sided quartz crystals shown in Figure 1.1 (p. 18) do not have cleavage. If broken, they fracture into shapes that do not re- semble one another or the original crystals.

Fracture Minerals that have structures that are equally, or nearly equally, strong in all directions fracture to form irreg- ular surfaces. Those, such as quartz, that break into smooth curved surfaces resembling broken glass exhibit a conchoidal fracture (Figure 1.16). Others break into splinters or fibers, but most minerals display an irregular fracture.

Density and Specific Gravity

Density is an important property of matter defined as mass per unit volume usually expressed as grams per cubic cen- timeter. Mineralogists often use a related measure called specific gravity to describe the density of minerals. Specific gravity is a unitless number representing the ratio of a min- eral’s weight to the weight of an equal volume of water.

Most common rock-forming minerals have a specific gravity between 2 and 3. For example, quartz has a specific gravity of 2.65. By contrast, some metallic minerals such as pyrite, native copper, and magnetite are more than twice as dense as quartz. Galena, which is an ore of lead, has a specif- ic gravity of roughly 7.5, whereas the specific gravity of 24-karat gold is approximately 20.

INDEX MINERALS COMMON OBJECTS

Diamond 10

Corundum 9

Topaz 8

Quartz 7

Orthoclase 6

Apatite 5

Fluorite 4

Calcite 3

Gypsum 2

Talc 1

Glass & knife blade (5.5)

Copper penny (3.5)

Fingernail (2.5)

Streak plate (6.5)

Wire nail (4.5)

Diamond

C o

ru n d

u m

To p

a z

Q u a rt

O rt

h o

c la

A p

a ti te

F lu

o ri te

C a lc

it e

G yp

su m

Ta lc

1 2 3 4 5 6 7 8 9 10

Mohs Scale

A b

so lu

te H

a rd

n e ss

V a lu

e s

A. Mohs scale (Relative hardness)

B. Comparison of Mohs scale and an absolute scale

Figure 1.13 Hardness scales. A. Mohs scale of hardness, with the hardness of some common objects. B. Relationship between Mohs relative hardness scale and an absolute hardness scale.

Knife blade

Figure 1.14 The thin sheets shown here were produced by splitting a mica (muscovite) crystal to its perfect cleavage. (Photo by Breck P. Kent)

Did You Know? Clay minerals have been used as an additive to thicken milk-

shakes in fast-food restaurants.

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Properties of Minerals 27

Number of Cleavage Directions

Shape Sketch

2 at 90˚

2 not at 90˚

3 at 90˚

3 not at 90˚

Flat sheets

Elongated form with rectangle

cross section (prism)

Cube

Rhombohedron

Octahedron

Elongated form with parallelogram

cross section (prism)

Directions of Cleavage

Sample

Muscovite

Feldspar

Hornblende

Halite

Calcite

Fluorite

Figure 1.15 Common cleavage directions exhibited by minerals. (Photos by E. J. Tarbuck and Dennis Tasa)

With a little practice, you can estimate the specific grav- ity of a mineral by hefting it in your hand. Ask yourself, does this mineral feel about as “heavy” as similar sized rocks you have handled? If the answer is “yes,” the specific gravity of the sample will likely be between 2.5 and 3.

Other Properties of Minerals

In addition to the properties already discussed, some miner- als can be recognized by other distinctive properties. For ex- ample, halite is ordinary salt, so it can be quickly identified through taste. Talc and graphite both have distinctive feels: Talc feels soapy, and graphite feels greasy. Further, the streak of many sulfur-bearing minerals smells like rotten eggs. A

few minerals, such as magnetite, have a high iron content and can be picked up with a magnet, while some varieties (lodestone) are natural magnets and will pick up small iron- based objects such as pins and paper clips (see Figure 1.23A, p. 32).

Moreover, some minerals exhibit special optical proper- ties. For example, when a transparent piece of calcite is placed over printed material, the letters appear twice. This optical property is known as double refraction (Figure 1.17).

One very simple chemical test involves placing a drop of dilute hydrochloric acid from a dropper bottle onto a fresh- ly broken mineral surface. Certain minerals, called carbon- ates, will effervesce (fizz) as carbon dioxide gas is released (Figure 1.18). This test is especially useful in identifying the common carbonate mineral calcite.

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28 Chapter 1 Minerals: Building Blocks of Rocks

Did You Know? The names of precious gems often differ from the names of

parent minerals. For example, sapphire is one of two gems that

are varieties of the same mineral, corundum. Tiny amounts of

the elements titanium and iron in corundum produce the most

prized blue sapphires. When the mineral corundum contains

chromium, it exhibits a brilliant red color and the gem is

called ruby. Figure 1.17 Double refraction illustrated by the mineral calcite. (Photo by Chip Clark)

Figure 1.18 Calcite reacting with a weak acid. (Photo by Chip Clark)

Did You Know? The mineral pyrite is commonly known as “fool’s gold,” be-

cause its golden-yellow color can lead to its being mistaken for

gold. The name pyrite is derived from the Greek pyros (“fire”),

because it gives off sparks when struck sharply.

Mineral Groups

Earth Materials � Minerals

Nearly 4000 minerals have been named, and several new ones are identified each year. Fortunately, for students who are be- ginning to study minerals, no more than a few dozen are

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Figure 1.16 Conchoidal fracture. The smooth, curved surfaces result when minerals break in a glasslike manner. (Photo by E. J. Tarbuck)

abundant! Collectively, these few make up most of the rocks of Earth’s crust and, as such, are often referred to as the rock- forming minerals. It is also interesting to note that only eight el- ements make up the bulk of these minerals and represent more than 98 percent (by weight) of the continental crust (Figure 1.19). These elements, in order of abundance, are oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg).

As shown in Figure 1.19, silicon and oxygen are by far the most common elements in Earth’s crust. Furthermore, these two elements readily combine to form the framework for the most common mineral group, the silicates, which ac- count for more than 90 percent of Earth’s crust.

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Mineral Groups 29

Oxygen (O) 46.6%

Silicon (Si) 27.7%

Aluminum (Al) 8.1%

Iron (Fe) 5.0% Calcium (Ca)

3.6% Sodium (Na)

2.8%

Other elements

1.5%

Potassium (K) 2.6%

Magnesium (Mg) 2.1%

Figure 1.19 Relative abundance of the eight most common elements in the continental crust.

Oxygen

Silicon

Figure 1.20 Two representations of the silicon-oxygen tetrahedron. A. The four large spheres represent oxygen atoms, and the blue sphere represents a silicon atom. The spheres are drawn in proportion to the radii of the atoms. B. A model of the tetrahedron using rods to depict the bonds that connect the atoms.

Because other mineral groups are far less abundant in Earth’s crust than the silicates, they are often grouped togeth- er under the heading nonsilicates. Although not as common as the silicates, some nonsilicate minerals are very important economically. They provide us with the iron and aluminum to build our automobiles, gypsum for plaster and drywall to construct our homes, and copper for wire to carry electricity and to connect us to the Internet. Some common nonsilicate mineral groups include the carbonates, sulfates, and halides. In addition to their economic importance, these mineral groups include members that are major constituents in sedi- ments and sedimentary rocks.

We will first discuss the most common mineral group, the silicates, and then consider some of the prominent non- silicate mineral groups.

Silicate Minerals

Each of the silicate minerals contains oxygen and silicon atoms. Except for a few silicate minerals, such as quartz, most silicate minerals also contain one or more additional elements that join together to produce an electrically neutral com- pound. These elements give rise to the great variety of silicate minerals and their varied properties.

All silicates have the same fundamental building blocks, the silicon-oxygen tetrahedron. This structure consists of four oxygen atoms surrounding a much smaller silicon atom, as shown in Figure 1.20. Thus, a typical hand-size silicate mineral specimen contains millions of these silicon-oxygen tetrahedra, joined together in a variety of ways.

In some minerals, the tetrahedra are joined into chains, sheets, or three-dimensional networks by sharing oxygen atoms (Figure 1.21). These larger silicate structures are then connected to one another by other elements. The primary el- ements that join silicate structures are iron (Fe), magnesium (Mg), potassium (K), sodium (Na), and calcium (Ca).

Major groups of silicate minerals and common exam- ples are given in Figure 1.21. The feldspars are by far the most plentiful group, comprising more than 50 percent of Earth’s

crust. Quartz, the second most abundant mineral in the con- tinental crust, is the only common mineral made completely of silicon and oxygen.

Notice in Figure 1.21 that each mineral group has a par- ticular silicate structure. A relationship exists between this in- ternal structure of a mineral and the cleavage it exhibits. Because the silicon-oxygen bonds are strong, silicate miner- als tend to cleave between the silicon-oxygen structures rather than across them. For example, the micas have a sheet struc- ture and thus tend to cleave into flat plates (see Figure 1.14, p. 26). Quartz, which has equally strong silicon-oxygen bonds in all directions, has no cleavage but fractures instead.

How do silicate minerals form? Most crystallize from molten rock as it cools. This cooling can occur at or near Earth’s surface (low temperature and pressure) or at great depths (high temperature and pressure). The environment dur- ing crystallization and the chemical composition of the molten rock mainly determine which minerals are produced. For ex- ample, the silicate mineral olivine crystallizes at high tem- peratures (about 1200°C [2200°F]), whereas quartz crystallizes at much lower temperatures (about 700°C [1300°F]).

In addition, some silicate minerals form at Earth’s sur- face from the weathered products of other silicate minerals. Still other silicate minerals are formed under the extreme

Did You Know? Most “scoopable” kitty litters sold on the market today contain a

naturally occurring material called bentonite. Bentonite is largely

composed of highly absorbent clay minerals that swell and

clump up in the presence of moisture. This allows kitty waste

to be isolated and scooped out, leaving behind clean litter.

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30 Chapter 1 Minerals: Building Blocks of Rocks

Mineral/Formula Cleavage Silicate Structure

Olivine group (Mg, Fe)2SiO4

None

Independent tetrahedron

Pyroxene group (Augite)

(Mg,Fe)SiO3

Two planes at right angles

Single chains

Amphibole group (Hornblende)

Ca2 (Fe,Mg)5Si8O22(OH)2

Two planes at 60° and 120°

M ic

a s

Biotite K(Mg,Fe)3AlSi3O10(OH)2

One plane

Sheets

Double chains

Muscovite KAl2(AlSi3O10)(OH)2

F e ld

sp a rs

Potassium feldspar (Orthoclase)

KAlSi3O8

Plagioclase feldspar (Ca,Na)AlSi3O8

Two planes at 90°

Three-dimensional networks

Quartz SiO2

None

Example

Olivine

Augite

Hornblend

Biotite

Muscovite

Potassium feldspar

Quartz

Figure 1.21 Common silicate minerals. Note that the complexity of the silicate structure increases down the chart.

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Mineral Groups 31

Table 1.1 Common nonsilicate mineral groups

Mineral Group Name Chemical Formula Economic Use

Oxides Hematite Ore of iron, pigment Magnetite Ore of iron Corundum Gemstone, abrasive Ice Solid form of water

Sulfides Galena PbS Ore of lead Sphalerite ZnS Ore of zinc Pyrite Sulfuric acid production Chalcopyrite Ore of copper Cinnabar HgS Ore of mercury

Sulfates Gypsum Plaster Anhydrite Plaster Barite Drilling mud

Native elements Gold Au Trade, jewelry Copper Cu Electrical conductor Diamond C Gemstone, abrasive Sulfur S Sulfa drugs, chemicals Graphite C Pencil lead, dry lubricant Silver Ag Jewelry, photography Platinum Pt Catalyst

Halides Halite NaCl Common salt Fluorite Used in steelmaking Sylvite KCl Fertilizer

Carbonates Calcite Portland cement, lime Dolomite Portland cement, limeCaMg(CO3)2

CaCO3

CaF2

BaSO4

CaSO4

CaSO4 # 2H2O

CuFeS2

FeS2

H2O Al2O3

Fe3O4

Fe2O3

pressures associated with mountain building. Each silicate mineral, therefore, has a structure and a chemical composition that indicate the conditions under which it formed. Thus, by care- fully examining the mineral makeup of rocks, geologists can often determine the circumstances under which the rocks formed.

Important Nonsilicate Minerals

Although nonsilicates make up only about 8 percent of Earth’s crust, some minerals, such as gypsum, calcite, and halite, are major constituents in sedimentary rocks. Further- more, many others are important economically. Table 1.1 lists some of the nonsilicate mineral classes and a few examples of each. Some of the most common nonsilicate minerals belong to one of three classes of minerals—the carbonates the sulfates and the halides

The carbonate minerals are much simpler structurally than the silicates. This mineral group is composed of the car- bonate ion and one or more kinds of positive ions. The most common carbonate mineral is calcite, (calci- um carbonate). This mineral is the major constituent in two well-known rocks: limestone and marble. Limestone has many uses, including as road aggregate, as building stone, and as the main ingredient in portland cement. Marble is used decoratively.

CaCO3 (CO3

2-)

(Cl1-, F1-, B1-).(SO4 2-),

(CO3 2-),

Two other nonsilicate minerals frequently found in sed- imentary rocks are halite and gypsum. Both minerals are com- monly found in thick layers that are the last vestiges of ancient seas that have long since evaporated (Figure 1.22). Like limestone, both are important nonmetallic resources. Halite is the mineral name for common table salt (NaCl). Gypsum which is calcium sulfate with water bound into the structure, is the mineral of which plaster and other similar building materials are composed.

Most nonsilicate mineral classes contain members that are prized for their economic value. This includes the oxides, whose members hematite and magnetite are important ores of iron (Figure 1.23). Also significant are the sulfides, which are basically compounds of sulfur (S) and one or more metals.

(CaSO4 # 2H2O),

Did You Know? Gypsum, a white-to-transparent mineral, was first used as a

building material in Anatolia (present-day Turkey) around

6000 B.C. It is also found on the interiors of the great pyramids

in Egypt, which were erected in about 3700 B.C. Today, an aver-

age new American home contains more than 7 metric tons of

gypsum in the form of 6000 square feet of wallboard.

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32 Chapter 1 Minerals: Building Blocks of Rocks

Examples of important sulfide minerals include galena (lead), sphalerite (zinc), and chalcopyrite (copper). In addition, native elements, including gold, silver, and carbon (diamonds), plus a host of other nonsilicate minerals—fluorite (flux in making steel), corundum (gemstone, abrasive), and uraninite (a ura- nium source)—are important economically.

Mineral Resources

Mineral resources are Earth’s storehouse of useful minerals that can be recovered for use. Resources include already iden- tified deposits from which minerals can be extracted prof-

itably, called reserves, as well as known deposits that are not yet recoverable under present economic conditions or tech- nology. Deposits inferred to exist, but not yet discovered, are also considered mineral resources.

The term ore denotes useful metallic minerals that can be mined at a profit. In common usage, the term is also ap- plied to some nonmetallic minerals, such as fluorite and sul- fur. However, materials used for such purposes as building stone, road paving, abrasives, ceramics, and fertilizers are not usually called ores; rather, they are classified as industrial rocks and minerals.

Recall that more than 98 percent of Earth’s crust is com- posed of only eight elements. Except for oxygen and silicon, all other elements make up a relatively small fraction of com- mon crustal rocks (see Figure 1.19, p. 29). Indeed, the natural concentrations of most elements in crustal rocks are exceed- ingly small. A rock containing the average crustal percentage of a valuable element such as gold has no economic value, because the cost of extracting it greatly exceeds the value of the gold that could be recovered.

To have economic value, an element must be concen- trated above the level of its average crustal abundance. For example, copper makes up about 0.0135 percent of the crust. For a deposit to be considered an ore of copper, it must con- tain a concentration that is about 100 times this amount. Aluminum, on the other hand, represents 8.13 percent of the crust and can be extracted profitably when it is found in concentrations only about four times its average crustal per- centage.

It is important to realize that economic changes affect whether or not a deposit is profitable to extract. If demand for a metal increases and prices rise sufficiently, the status of a previously unprofitable deposit changes and it becomes an ore. The status of unprofitable deposits may also change if a technological advance allows the material to be extracted at a lower cost than before.

Conversely, changing economic factors can turn a once profitable ore deposit into an unprofitable deposit that can

Figure 1.22 Thick bed of halite (salt) at an underground mine in Grand Saline, Texas. (Photo by Tom Bochsler)

A. B.

Figure 1.23 Two important ores of iron. A. Magnetite and B. Hematite. (Photos by E. J. Tarbuck)

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Figure 1.24 Aerial view of Bingham Canyon copper mine near Salt Lake City, Utah. This huge open-pit mine is about 4 kilometers (2.5 miles) across and 900 meters (nearly 3000 feet) deep. Although the amount of copper in the rock is less than 1 percent, the huge volumes of material removed and processed each day (about 200,000 tons) yield, significant quantities of metal. (Photo by Michael Collier)

The Chapter in Review 33

no longer be called an ore. This situation was illustrated recently at the copper-mining operation located at Bingham Canyon, Utah, one of the largest open-pit mines on Earth (Figure 1.24). Mining was halted there in 1985 because out- moded equipment had driven the cost of extracting the cop-

per beyond the current selling price. The owners responded by replacing an antiquated 1000-car railroad with conveyor belts and pipelines for transporting the ore and waste. These devices achieved a cost reduction of nearly 30 percent and returned this mining operation to profitability.

The Chapter in Review

1. A mineral is a naturally occurring inorganic solid that possesses an orderly crystalline structure and a definite chem- ical composition. Most rocks are aggregates composed of two or more minerals.

2. The building blocks of minerals are elements. An atom is the smallest particle of matter that still retains the character- istics of an element. Each atom has a nucleus, which contains protons and neutrons. Orbiting the nucleus of an atom are electrons. The number of protons in an atom’s nucleus deter- mines its atomic number and the name of the element. Atoms bond together to form a compound by either gaining, losing, or sharing electrons with another atom.

3. Isotopes are variants of the same element but with a dif- ferent mass number (the total number of neutrons plus pro- tons found in an atom’s nucleus). Some isotopes are unstable

and disintegrate naturally through a process called radioactive decay.

4. The properties of minerals include crystal shape (habit), luster, color, streak, hardness, cleavage, fracture, and density or specific gravity. In addition, a number of special physical and chemical properties (taste, smell, elasticity, feel, magnetism, dou- ble refraction, and chemical reaction to hydrochloric acid) are use- ful in identifying certain minerals. Each mineral has a unique set of properties that can be used for identification.

5. Of the nearly 4000 minerals, no more than a few dozen make up most of the rocks of Earth’s crust and, as such, are classified as rock-forming minerals. Eight elements (oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium) make up the bulk of these minerals and represent more than 98 percent (by weight) of Earth’s continental crust.

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34 Chapter 1 Minerals: Building Blocks of Rocks

Key Terms

atom (p. 20)

atomic number (p. 20)

chemical compound (p. 20)

cleavage (p. 26)

color (p. 24)

covalent bond (p. 23)

density (p. 26)

electron (p. 20)

element (p. 19)

energy levels (p. 20)

fracture (p. 26)

habit (p. 25)

hardness (p. 25)

ionic bond (p. 22)

ions (p. 22)

isotope (p. 23)

luster (p. 24)

mass number (p. 23)

mineral (p. 18)

mineralogy (p. 18)

mineral resource (p. 32)

Mohs scale (p. 25)

neutron (p. 20)

nonsilicate (p. 29)

nucleus (p. 20)

ore (p. 32)

periodic table (p. 19)

principal shell (p. 20)

proton (p. 20)

radioactive decay (p. 23)

reserve (p. 32)

rock (p. 19)

silicates (p. 28)

silicon-oxygen tetrahedron (p. 29)

specific gravity (p. 26)

streak (p. 24)

tenacity (p. 25)

valence electrons (p. 20)

Questions for Review

1. Briefly describe the five characteristics an Earth material should have in order to be considered a mineral.

2. Define the term rock. 3. List the three main particles of an atom and explain how

they differ from one another. 4. If the number of electrons in an atom is 35 and its mass

number is 80, calculate the following: a. the number of protons b. the atomic number c. the number of neutrons

5. What occurs in an atom to produce an ion? 6. What is an isotope? 7. Although all minerals have an orderly internal arrange-

ment of atoms (crystalline structure), most mineral sam- ples do not visibly demonstrate their crystal form. Why?

8. Why might it be difficult to identify a mineral by its color?

9. If you found a glassy-appearing mineral while rock hunting and had hopes that it was a diamond, what simple test might help you make a determination?

10. Table 1.1 (p. 31) lists a use for corundum as an abrasive. Explain why it makes a good abrasive in terms of the Mohs hardness scale.

11. Gold has a specific gravity of almost 20. If a five-gallon pail of water weighs about 40 pounds, how much would a five-gallon pail of gold weigh?

12. What are the two most common elements in Earth’s crust?

13. What is the term used to describe the basic building block of all silicate minerals?

14. What are the two most common silicate minerals in Earth’s crust?

15. List three nonsilicate minerals that are commonly found in rocks.

16. Contrast a mineral resource and a mineral reserve. 17. What might cause a mineral deposit that had not been

considered an ore to become reclassified as an ore?

6. The most common mineral group is the silicates. All sil- icate minerals have the silicon-oxygen tetrahedron as their fun- damental building block. In some silicate minerals, the tetrahedra are joined in chains; in others, the tetrahedra are arranged into sheets or three-dimensional networks. Each sil- icate mineral has a structure and a chemical composition that indicates the conditions under which it formed. The nonsilicate mineral groups include the oxides (e.g., magnetite, mined for iron), sulfides (e.g., sphalerite, mined for zinc), sulfates (e.g., gypsum, used in plaster and frequently found in

sedimentary rocks) native elements (e.g., graphite, a dry lubri- cant), halides (e.g., halite, common salt, and frequently found in sedimentary rocks), and carbonates (e.g., calcite, used in portland cement and a major constituent in two well-known rocks: limestone and marble).

7. The term ore is used to denote useful metallic minerals, such as hematite (mined for iron) and galena (mined for lead), that can be mined at a profit, as well as some nonmetallic min- erals, such as fluorite and sulfur, that contain useful substances.

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GEODe: Earth Science 35

Online Study Guide

• Online review quizzes. • Critical thinking exercises. • Links to chapter-specific Web resources. • Internet-wide key-term searches.

http://www.prenhall.com/lutgens

GEODe: Earth Science

GEODe: Earth Science makes studying more effective by re- inforcing key concepts using animation, video, narration, in-

teractive exercises, and practice quizzes. A copy is included with every copy of Foundations of Earth Science 5e.

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