ch02 Internal Structure of Earth and Plate Tectonics Written with

KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 36 T W O PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 37 Internal Structure of Earth and Plate Tectonics Written with the assistance of Tanya Atwater Learning Objectives The surface of Earth would be much different— relatively smooth, with monotonous topography —if not for the active tectonic processes within Earth that produce earthquakes, volcanoes, mountain chains, continents, and ocean basins.1 In this chapter we focus directly on the interior of Earth, with the following learning objectives: 쐍 Understand the basic internal structure and processes of Earth 쐍 Know the basic ideas behind and evidence for the theory of plate tectonics 쐍 Understand the mechanisms of plate tectonics 쐍 Understand the relationship of plate tectonics to environmental geology The San Andreas fault in southern California is the major boundary between the Pacific and North American plates. Here in the Indio Hills, the fault is delineated by lines of native palm trees. (Edward A. Keller) 37 PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 38 Chapter 2 2/2/07 7:31 PM Page 38 Internal Structure of Earth and Plate Tectonics CASE HISTORY Two Cities on a Plate Boundary California straddles the boundary between two tectonic plates, which are discussed in detail in this chapter. That boundary between the North American and Pacific plates is the notorious San Andreas Fault (Figure 2.1). A fault is a fracture along which one side has moved relative to the other, and the San Andreas Fault is a huge fracture zone, hundreds of kilometers long. Two major cities, Los Angeles to the south and San Francisco to the north, are located on opposite sides of this fault. San Francisco was nearly destroyed by a major earthquake in 1906, which led to the identification of the fault. Many of the moderate to large earthquakes in the Los Angeles area are on faults related to the San Andreas fault system. Most of the beautiful mountain topography in coastal California near both Los Angeles and San Francisco is a direct result of processes related to movement on the San Andreas Fault. However, this beautiful topography comes at a high cost to society. Since 1906, earthquakes on the San Andreas fault system or on nearby faults, undoubtedly influenced by the plate boundary, have cost hundreds of lives and many billions of dollars in property damage. Construction of buildings, bridges, and other structures in California is more expensive than elsewhere because they must be designed to withstand ground shaking caused by earthquakes. Older structures have to be retrofitted, or have changes made to their structure, to withstand the shaking, and many people purchase earthquake insurance in an attempt to protect themselves from the “big one.” Los Angeles is on the Pacific plate and is slowly moving toward San Francisco, which is on the North American plate. Figure 2.1 San Andreas Fault Map showing the San Andreas Fault and topography in California. Arrows show relative motion on either side of the fault. (R. E. Wallace/National Earth- quake Information Center. U.S.G.S.) S REA ND N A LT SA FAU SAN ANDREAS FAULT SAN ANDREAS FAULT SAN ANDREAS FAULT PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 39 Internal Structure of Earth In about 20 million years the cities will be side by side. If people are present, they might be arguing over which is a suburb of the other. Of course, there will still be a plate boundary between the Pacific and North American plates 20 million years from now, because large plates have long geologic lives, on the order of 100 million years. However, the boundary 39 may not be the San Andreas Fault. The plate boundary will probably have moved eastward, and the topography of what is now California may be somewhat different. In fact, some recent earthquake activity in California, such as the large 1992 Landers earthquake, east of the San Andreas fault, may be the beginning of a shift in the plate boundary. 2.1 Internal Structure of Earth You may be familiar with the situation comedy Third Rock from the Sun, a phrase that refers to our planet Earth. Far from being a barren rock, Earth is a complex dynamic planet that in some ways resembles a chocolate-covered cherry. That is, Earth has a rigid outer shell, a solid center, and a thick layer of liquid that moves around as a result of dynamic internal processes. The internal processes are incredibly important in affecting the surface of Earth. They are responsible for the largest landforms on the surface: continents and ocean basins. The configuration of the continents and ocean basins in part controls the oceans’ currents and the distribution of heat carried by seawater in a global system that affects climate, weather, and the distribution of plant and animal life on Earth. Finally, Earth’s internal processes are also responsible for regional landforms including mountain chains, chains of active volcanoes, and large areas of elevated topography, such as the Tibetan Plateau and the Rocky Mountains. The high topography that includes mountains and plateaus significantly affects both global circulation patterns of air in the lower atmosphere and climate, thereby directly influencing all life on Earth. Thus, our understanding of the internal processes of Earth is of much more than simply academic interest. These processes are at the heart of producing the multitude of environments shared by all living things on Earth. The Earth Is Layered and Dynamic. Earth (Figure 2.2a) has a radius of about 6,300 km (4,000 mi) (Figure 2.2b). Information regarding the internal layers of the Earth is shown in Figure 2.2b. We can consider the internal structure of Earth in two fundamental ways: 쐍 by composition and density (heavy or light). 쐍 by physical properties (for example, solid or liquid, weak or strong). Our discussion will explore the two ways of looking at the interior of our planet. Some of the components of the basic structure of Earth1 are 쐍 A solid inner core with a thickness of more than 1,300 km (808 mi) that is roughly the size of the moon but with a temperature about as high as the temperature of the surface of the Sun.2 The inner core is believed to be primarily metallic, composed mostly of iron (about 90 percent by weight), with minor amounts of elements such as sulfur, oxygen, and nickel. 쐍 A liquid outer core with a thickness of just over 2,000 km (1,243 mi) with a composition similar to that of the inner core. The outer core is very fluid, more similar to water than to honey. The average density of the inner and outer core is approximately 10.7 grams per cubic centimeter (0.39 pounds per cubic inch). The maximum near the center of Earth is about 13 g/cm3 (0.47 lb/in3). By comparison, the density of water is 1 g/cm3 (0.04 lb/in3) and the average density of Earth is approximately 5.5 g/cm3 (0.2 lb/in3). 쐍 The mantle, nearly 3,000 km (1,864 mi) thick, surrounds the outer core and is mostly solid, with an average density of approximately 4.5 g/cm3 (0.16 lb/in3). Rocks in the mantle are primarily iron- and magnesium-rich silicates. Interestingly, the density difference between the outer core and the PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 40 Chapter 2 2/2/07 7:31 PM Page 40 Internal Structure of Earth and Plate Tectonics Sea level Marine sediment Mohorovicic discontinuity Oceanic crust Continental crust AVERAGE DENSITY, g/cm3 0 60 Rigid km 40 e Mantl 20 80 phere Lithos Continental crust 2.8 Oceanic crust 2.9 Mantle 4.5 Core 10.7 Entire Earth Crust 5.5 Asthenosphere 0 km 1000 Mantle 2000 (a) 3000 Outer core 4000 Figure 2.2 Earth and its interior (a) Earth from space. (National Geophysical Data Center, National Oceanic and Atmospheric Administration) (b) Idealized diagram showing the internal structure of Earth and its layers extending from the center to the surface. Notice that the lithosphere includes the crust and part of the mantle, and the asthenosphere is located entirely within the mantle. Properties of the various layers have been estimated on the basis of (1) interpretation of geophysical data (primarily seismic waves from earthquakes); (2) examination of rocks thought to have risen from below by tectonic processes; and (3) meteorites, thought to be pieces of an old Earthlike planet. (From Levin, H. L. 1986. Contemporary physical geology, 2nd ed. Philadelphia: 5000 Inner core (b) 6000 Saunders) overlying mantle is greater than that between the rocks at the surface of Earth and the overlying atmosphere! In the case of the outer core and mantle, the more fluid phase of the outer core is beneath the solid phase of the mantle. This is just the opposite of the case of the rock-atmosphere relationship, where the fluid atmosphere overlies the solid lithosphere. Because it is liquid, the outer core is dynamic and greatly influences the overlying mantle and, thus, the surface of Earth. 쐍 The crust, with variable thickness, is the outer rock layer of the Earth. The boundary between the mantle and crust is known as the Mohorovicˇi´c discontinuity (also called the Moho). It separates the lighter rocks of the crust with an average density of approximately 2.8 g/cm3 (0.10 lb/in3) from the denser rocks of the mantle below. PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 41 41 How We Know about the Internal Structure of Earth Pan filled with water Cool wa water ol Hot water Co Convection cell ter Continents and Ocean Basins Have Significantly Different Properties and History. Within the uppermost portion of the mantle, near the surface of Earth our terminology becomes more complicated. For example, the cool, strong outermost layer of Earth is also called the lithosphere (lithos means “rock”). It is much stronger and more rigid than the material underlying it, the asthenosphere (asthenos means “without strength”), which is a hot and slowly flowing layer of relatively weak rock. The lithosphere averages about 100 km (62 mi) in thickness, ranging from a few kilometers (1 to 2 mi) thick beneath the crests of mid-ocean ridges to about 120 km (75 mi) beneath ocean basins and 20 to 400 km (13 to 250 mi) beneath the continents. The crust is embedded in the top of the lithosphere. Crustal rocks are less dense than the mantle rocks below, and oceanic crust is slightly denser than continental crust. Oceanic crust is also thinner: The ocean floor has a uniform crustal thickness of about 6 to 7 km (3.7 to 4.4 mi), whereas the crustal thickness of continents averages about 35 km (22 mi) and may be up to 70 km (44 mi) thick beneath mountainous regions. Thus, the average crustal thickness is less than 1 percent of the total radius of Earth and can be compared to the thin skin of a tangerine. Yet it is this layer that is of particular interest to us because we live at the surface of the continental crust. In addition to differences in density and thickness, continental and oceanic crust have very different geologic histories. Oceanic crust of the present ocean basins is less than approximately 200 million years old, whereas continental crust may be several billion years old. Three thousand kilometers (1,865 mi) below us, at the core-mantle boundary, processes may be occurring that significantly affect our planet at the surface. It has been speculated that gigantic cycles of convection occur within Earth’s mantle, rising from as deep as the core-mantle boundary up to the surface and then falling back again. The concept of convection is illustrated by heating a pan of hot water on a stove (Figure 2.3). Heating the water at the bottom of the pan causes the water to become less dense and more unstable, so it rises to the top. The rising water displaces denser, cooler water, which moves laterally and sinks to the bottom of the pan. It is suggested that Earth layers contain convection cells and operate in a similar fashion. A complete cycle in the mantle may take as long as 500 million years.1 Mantle convection is fueled at the core-mantle boundary both by heat supplied from the molten outer core of Earth and by radioactive decay of elements (such as uranium) scattered throughout the mantle. Let us now examine some of the observations and evidence that reveal the internal structure of Earth. Gas stove Figure 2.3 Convection Idealized diagram showing the concept of convection. As the pan of water is heated, the less dense hot water rises from the bottom to displace the denser cooler water at the top, which then sinks down to the bottom. This process of mass transport is called convection, and each circle of rising and falling water is a convection cell. 2.2 How We Know about the Internal Structure of Earth What We Have Learned about Earth from Earthquakes. Our knowledge concerning the structure of Earth’s interior arises primarily from our study of seismology. Seismology is the study of earthquakes and the passage of seismic waves through Earth.3 When a large earthquake occurs, seismic energy is released and seismic waves move both through Earth and along its surface. The properties of these waves are discussed in detail in Chapter 6 with earthquake hazards. Some waves move through solid and liquid materials while others move through solid, but not liquid materials. The rates at which seismic waves propagate are on the order of a few kilometers per second (1 or 2 miles per second). Their actual velocity varies with the properties of the materials through which the waves are propagating (moving). When the seismic waves encounter a boundary, such as the mantle-core boundary, some of them are reflected back. Others cross the boundary and are refracted (change the direction of propagation). Still others fail to propagate through the liquid outer core. Thousands of seismographs (instruments PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 42 Chapter 2 2/2/07 7:31 PM Page 42 Internal Structure of Earth and Plate Tectonics that record seismic waves) are stationed around the world. When an earthquake occurs, the reflected and refracted waves are recorded when they emerge at the surface. Study of these waves has been a powerful tool for deducing the layering of the interior of Earth and the properties of the materials found there. In summary, the boundaries that delineate the internal structure of Earth are determined by studying seismic waves generated by earthquakes and recorded on seismographs around Earth. As seismology has become more sophisticated, we have learned more and more about the internal structure of Earth and are finding that the structure can be quite variable and complex. For example, we have been able to recognize 쐍 where magma, which is molten rock material beneath Earth’s surface, is generated in the asthenosphere 쐍 the existence of slabs of lithosphere that have apparently sunk deep into the mantle 쐍 the extreme variability of lithospheric thickness, reflecting its age and history 2.3 Plate Tectonics The term tectonics refers to the large-scale geologic processes that deform Earth’s lithosphere, producing landforms such as ocean basins, continents, and mountains. Tectonic processes are driven by forces within the Earth. These processes are part of the tectonic system, an important subsystem of the Earth system. Movement of the Lithospheric Plates What Is Plate Tectonics? The lithosphere is broken into large pieces called lithospheric plates that move relative to one another (Figure 2.4a).4 Processes associated with the creation, movement, and destruction of these plates are collectively known as plate tectonics. Locations of Earthquakes and Volcanoes Define Plate Boundaries. A lithospheric plate may include both a continent and part of an ocean basin or an ocean region alone. Some plates are very large and some are relatively small, though they are significant on a regional scale. For example, the Juan de Fuca plate off the Pacific Northwest coast of the United States, which is relatively small, is responsible for many of the earthquakes in northern California. The boundaries between lithospheric plates are geologically active areas. Most earthquakes and many volcanoes are associated with these boundaries. In fact, plate boundaries are defined by the areas in which concentrated seismic activity occurs (Figure 2.4b). Over geologic time, plates are formed and destroyed, cycling materials from the interior of Earth to the surface and back again at these boundaries (Figure 2.5). The continuous recycling of tectonic processes is collectively called the tectonic cycle. Seafloor Spreading Is the Mechanism for Plate Tectonics. As the lithospheric plates move over the asthenosphere, they carry the continents embedded within them.5 The idea that continents move is not new; it was first suggested by German scientist Alfred Wegener in 1915. The evidence he presented for continental drift was based on the congruity of the shape of continents, particularly those across the Atlantic Ocean, and on the similarity in fossils found in South America and Africa. Wegener’s hypothesis was not taken seriously because there was no known mechanism that could explain the movement of continents around Earth. The explanation came in the late 1960s, when seafloor spreading was discovered. In seafloor regions called mid-oceanic ridges, or spreading centers, new crust is PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 43 43 Plate Tectonics 80° NORTH AMERICAN PLATE JUAN DE FUCA PLATE 40° 63 PHILIPPINE PLATE CARIBBEAN PLATE 92 ATLANTIC OCEAN COCOS PLATE PACIFIC PLATE 40° 23 San Andreas Fault 20° 0° 35 EURASIAN PLATE 19 AFRICAN PLATE 32 CAROLINE PLATE 0° FIJI PLATE Equator PACIFIC OCEAN INDIAN OCEAN 141 NAZCA PLATE 40° SOUTH AMERICAN PLATE PACIFIC PACIFIC PLATE OCEAN 49 72 33 62 INDO-AUSTRALIAN PLATE 91 0° Transform fault Convergent 25 (a) 160° 0 Uncertain plate boundary Divergent (spreading ridge offset by transform faults) 80° 40° 120° 80° ANTARCTIC PLATE 0 120° 1,500 1,500 60° 3,000 Miles 3,000 Kilometers Direction of plate motion (relative motion rates in mm/yr) 40° 0° 40° 80° 120° 160° 80° 40° 40° ATLANTIC OCEAN PACIFIC 20° OCEAN 20° PACIFIC OCEAN 0° Equator 0° INDIAN OCEAN 40° Volcanoes 60° 0 0 (b) 1,500 1,500 3,000 Miles 60° Earthquakes 3,000 Kilometers Figure 2.4 Earth’s plates (a) Map showing the major tectonic plates, plate boundaries, and direction of plate movement. (Modified from Christopherson, R. W. 1994. Geosystems, 2nd ed. Englewood Cliffs, NJ: Macmillan) (b) Volcanoes and earthquakes: Map showing location of volcanoes and earthquakes. Notice the correspondence between this map and the plate boundaries. (Modified after Hamblin, W. K. 1992. Earth’s dynamic systems, 6th ed. New York: Macmillan) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 44 Chapter 2 2/2/07 7:31 PM Page 44 Internal Structure of Earth and Plate Tectonics Figure 2.5 Model of plate tectonics Diagram of the model of plate tectonics. New oceanic lithosphere is being produced at the spreading ridge (divergent plate boundary). Elsewhere, oceanic lithosphere returns to the interior of Earth at a convergent plate boundary (subduction zone). (Modified from Lutgens, F., and Tarbuck, E. 1992. Essentials of geology. New York: Macmillan) Divergent boundary Convergent boundary Transform fault Transform fault Transform fault Lithosphere Hot rock rising Oceanic spreading ridge Cool rock sinking Subduction zone Asthenosphere continuously added to the edges of lithospheric plates (Figure 2.5, left). As oceanic lithosphere is added along some plate edges (spreading centers), it is destroyed along other plate edges, for example, at subduction zones (areas where one plate sinks beneath another and is destroyed) (Figure 2.5, right). Thus continents do not move through oceanic crust; rather they are carried along with it by the movement of the plates. Also, because the rate of production of new lithosphere at spreading centers is balanced by consumption at subduction zones, the size of Earth remains constant, neither growing nor shrinking. Sinking Plates Generate Earthquakes. The concept of a lithospheric plate sinking into the upper mantle is shown in diagrammatic form in Figure 2.5. When the wet, cold oceanic crust comes into contact with the hot asthenosphere, magma is generated. The magma rises back to the surface, producing volcanoes, such as those that ring the Pacific Ocean basin, over subduction zones. The path of the descending plate (or slab, as it sometimes is called) into the upper mantle is clearly marked by earthquakes. As the oceanic plate subducts, earthquakes are produced both between it and the overriding plate and within the interior of the subducting plate. The earthquakes occur because the sinking lithospheric plate is relatively cooler and stronger than the surrounding asthenosphere; this difference causes rocks to break and seismic energy to be released.6 The paths of descending plates at subduction zones may vary from a shallow dip to nearly vertical, as traced by the earthquakes in the slabs. These dipping planes of earthquakes are called Wadati-Benioff zones (Figure 2.6). The very PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 45 Plate Tectonics 45 Volcanism re he p s e ho er Lit sph o en ts h SL Trench Depth (km) 100 250 Upper mantle Figure 2.6 Subduction zone Idealized diagram of a subduction zone showing the Wadati-Benioff zone, which is an array of earthquake foci from shallow to deep that delineate the subduction zone and the descending lithospheric plate. A Magma Wadati-Benioff zone 400 Earthquake focal depth 600 Shallow (<75 km) Intermediate (75–325 km) Deep (>325 km) existence of Wadati-Benioff zones is strong evidence that subduction of rigid “breakable” lithosphere is occurring.6 Plate Tectonics Is a Unifying Theory. The theory of plate tectonics is to geology what Darwin’s origin of species is to biology: a unifying concept that explains an enormous variety of phenomena. Biologists now have an understanding of evolutionary change. In geology, we are still seeking the exact mechanism that drives plate tectonics, but we think it is most likely convection within Earth’s mantle. As rocks are heated deep in Earth, they become less dense and rise. Hot materials, including magma, leak out, and are added to the surfaces of plates at spreading centers. As the rocks move laterally, they cool, eventually becoming dense enough to sink back into the mantle at subduction zones. This circulation is known as convection, which was introduced in Section 2.1. Figure 2.7 illustrates the cycles of convection that may drive plate tectonics. Oceanic ridge So ut h A m er ic a Ocean floor fr A Asthe nosp her e a ic Lithosphere Trench Mantle Core Figure 2.7 Plate movement Model of plate movement and mantle.The outer layer (or lithosphere) is approximately 100 km (approximately 62 mi) thick and is stronger and more rigid than the deeper asthenosphere, which is a hot and slowly flowing layer of relatively low-strength rock.The oceanic ridge is a spreading center where plates pull apart, drawing hot, buoyant material into the gap. After these plates cool and become dense, they descend at oceanic trenches (subduction zones), completing the convection system.This process of spreading produces ocean basins, and mountain ranges often form where plates converge at subduction zones. A schematic diagram of Earth’s layers is shown in Figure 2.2b. (Grand, S. P. 1994. Mantle shear structure beneath the Americas and surrounding oceans. Journal of Geophysical Research 99:11591–621. Modified after Hamblin, W. K. 1992. Earth’s dynamic systems, 6th ed. New York: Macmillan) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 46 Chapter 2 TABLE 2.1 2/2/07 7:31 PM Page 46 Internal Structure of Earth and Plate Tectonics Types of Plate Boundaries: Dynamics, Results, and Examples Plate Boundary Plates Involved Dynamics Results Example Divergent Usually oceanic Spreading. The two plates move away from one another and molten rock rises up to fill the gap. Mid-ocean ridge forms and new material is added to each plate. African and North American plate boundary (Figure 2.4a) Mid-Atlantic Ridge Convergent Convergent Ocean-continent Ocean-ocean Oceanic plate sinks beneath continental plate. Mountain ranges and a subduction zone are formed with a deep trench. Earthquakes and volcanic activity are found here. Nazca and South American plate boundary (Figure 2.4a) Older, denser, oceanic plate sinks beneath the younger, less dense oceanic plate. A subduction zone is formed with a deep trench. Earthquakes and volcanic activity are found here. Fiji plate (Figure 2.4a) Andes Mountains Peru-Chile Trench Fiji Islands Convergent Continent-continent Neither plate is dense enough to sink into the asthenosphere; compression results. A large, high mountain chain is formed, and earthquakes are common. Indo-Australian and Eurasian plate boundary (on land) (Figure 2.4a) Transform Ocean-ocean or continent-continent The plates slide past one another. Earthquakes common. May result in some topography. North American and Pacific plate boundary (Figure 2.10) Himalaya Mountains San Andreas fault Types of Plate Boundaries There are three basic types of plate boundaries: divergent, convergent, and transform, shown in Figures 2.4 and 2.5 and Table 2.1. These boundaries are not narrow cracks as shown on maps and diagrams but are zones that range from a few to hundreds of kilometers across. Plate boundary zones are narrower in ocean crust and broader in continental crust. Divergent boundaries occur where new lithosphere is being produced and neighboring parts of plates are moving away from each other. Typically this process occurs at mid-ocean ridges, and the process is called seafloor spreading (Figure 2.5). Mid-ocean ridges form when hot material from the mantle rises up to form a broad ridge typically with a central rift valley. It is called a rift valley, or rift, because the plates moving apart are pulling the crust apart and splitting, or rifting, it. Molten volcanic rock that is erupted along this rift valley cools and forms new plate material. The system of mid-oceanic ridges along divergent plate boundaries forms linear submarine mountain chains that are found in virtually every ocean basin on Earth. Convergent boundaries occur where plates collide. If one of the converging plates is oceanic and the other is continental, an oceanic-continental plate collision results. The higher-density oceanic plate descends, or subducts, into the mantle beneath the leading edge of the continental plate, producing a subduction zone (Figure 2.5). The convergence or collision of a continent with an ocean plate can result in compression. Compression is a type of stress, or force per unit area. When an oceanic-continental plate collision occurs, compression is exerted on the lithosphere, resulting in shortening of the surface of Earth, like pushing a table cloth to produce folds. Shortening can cause folding, as in the table cloth example, and faulting, or displacement of rocks along fractures to thicken the lithosphere (Figure 2.8a). This process of deformation produces major mountain chains and volcanoes such as the Andes in South America and the Cascade Mountains in the Pacific Northwest of the United States (see A Closer Look: The Wonder of Mountains). If two oceanic lithospheric plates collide (oceanic-to-oceanic plate collision), one plate subducts beneath the other, and a subduction zone and arcshaped chain of volcanoes known as an island arc are formed (Figure 2.8b) as, for example, the Aleutian Islands of the North Pacific. A submarine trench, relatively PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 47 47 Plate Tectonics Continental plate Volcanoes Mountains Mountains Trench Island arc Volcanic island chain Trench Oceanic plate Subduction zone Oceanic plate Oceanic plate (a) Continental plate Suture zone Subduction zone Shortening, thickening uplift Magma Continental plate Magma (c) (b) Shortening, thickening uplift Figure 2.8 Convergent plate boundaries Idealized diagram illustrating characteristics of convergent plate boundaries: (a) continental-oceanic plate collision, (b) oceanic-oceanic plate collision, and (c) continental-continental plate collision. narrow, usually several thousand km long and several km deep depression on the ocean, is often formed as the result of the convergence of two colliding plates with subduction of one. A trench is often located seaward of a subduction zone associated with an oceanic-continental plate or oceanic-oceanic plate collision. Submarine trenches are sites of some of the deepest oceanic waters on Earth. For example, the Marianas trench at the center edge of the Philippine plate is 11 km (7 mi) deep. Other major trenches include the Aleutian trench south of Alaska and the Peru-Chile trench west of South America. If the leading edges of both plates contain relatively light, buoyant continental crust, subduction into the mantle of one of the plates is difficult. In this case a continent-to-continent plate collision occurs, in which the edges of the plates collide, causing shortening and lithospheric thickening due to folding and faulting. (Figure 2.8c). Where the two plates join is known as a suture zone. Continent-to-continent collision has produced some of the highest mountain systems on Earth, such as the Alpine and Himalayan mountain belts (Figure 2.9). Many older mountain belts were formed in a similar way; for example, the Appalachians formed during an ancient continent-tocontinent plate collision 250 to 350 million years ago. Figure 2.9 Mountains in Italy Mountain peaks (the Dolomites) in southern Italy are part of the Alpine mountain system formed from the collision between Africa and Europe. (Edward A. Keller) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 48 Chapter 2 2/2/07 7:31 PM Page 48 Internal Structure of Earth and Plate Tectonics A CLOSER LOOK The Wonder of Mountains Mountains have long fascinated people with their awesome presence. We are now discovering a fascinating story concerning their origin. The story removes some of the mystery as to how mountains form, but it has not removed the wonder. The new realization that mountains are systems (see Chapter 1) resulting from the interaction between tectonic activity (that leads to crustal thickening), the climate of the mountain, and Earth surface processes (particularly erosion) has greatly expanded our knowledge of how mountains develop.7,8 Specifically, we have learned the following: 쐍 Tectonic processes at convergent plate boundaries lead to crustal thickening and initial development of mountains. The mean (or average) elevation that a mountain range attains is a function of the uplift rate, which varies from less than 1 mm to about 10 mm per year (0.04 to 0.4 in. per year). The greater the rate of uplift, the higher the point to which the mean elevation of a mountain range is likely to rise during its evolution. 쐍 As a mountain range develops and gains in elevation, it begins to modify the local and regional climate by blocking storm paths and producing a “rain shadow” in which the mountain slopes on the rain-shadow side receive much less rainfall than does the other side of the mountain. As a result, rates of runoff and erosion on the side of the rain shadow are less than for the other side. Nevertheless, the rate of erosion increases as the elevation of the mountain range increases, and eventually the rate of erosion matches the rate of uplift. When the two match, the mountain reaches its maximum mean elevation, which is a dynamic balance between the uplift and erosion. At this point, no amount of additional uplift will increase the mean elevation of the mountains above the dynamic maximum. However, if the uplift rate increases, then a higher equilibrium mean elevation of the range may be reached. Furthermore, when the uplift ceases or there is a reduction in the rate of uplift, the mean elevation of the mountain range will decrease.7 Strangely, the elevations of individual peaks may still increase! 쐍 Despite erosion, the elevation of a mountain peak in a range may actually increase. This statement seems counterintuitive until we examine in detail some of the physical processes resulting from erosion. The uplift that results from the erosion is known as isostatic uplift. Isostasy is the principle whereby thicker, more buoyant crust stands topographically higher than crust that is thinner and denser. The principle governing how erosion can result in uplift is illustrated in Figure 2.A. The fictitious Admiral Frost has been marooned on an iceberg and is uncomfortable being far above the surface of the water. He attempts to remove the ice that is above the water line. Were it not for isostatic (buoyant) uplift, he would have reached his goal to be close to the water line. Unfortunately for Admiral Frost, this is not the way the world works; continuous isostatic uplift of the block as ice is removed always keeps one-tenth of the iceberg above the water. So, after removing the ice above the water line, he still stands almost as much above the water line as before.7 쐍 Mountains, of course, are not icebergs, but the rocks of which they are composed are less dense than the rocks of the mantle beneath. Thus, they tend to “float” on top of the denser mantle. Also, in mountains, erosion is not uniform but is generally confined to valley walls and bottoms. Thus, as erosion continues and the mass of the mountain range is reduced, isostatic compensation occurs and the entire mountain range rises in response. As a result of the erosion, the maximum elevation of mountain peaks actually may increase, while the mean elevation of the entire mountain block decreases. As a general rule, as the equivalent of 1 km (0.6 mi) of erosion across the entire mountain block occurs, the mean elevation of mountains will rise approximately five-sixths of a kilometer (one-half mile). In summary, research concerning the origin of mountains suggests that they result in part from tectonic processes that cause the uplift, but they also are intimately related to climatic and erosional processes that contribute to the mountain building process. Erosion occurs during and after tectonic uplift, and isostatic compensation to that erosion occurs for millions of years. This is one reason it is difficult to remove mountain systems from the landscape. For example, mountain systems such as the Appalachian Mountains in the southeastern United States were originally produced by tectonic uplift several hundred million years ago when Europe collided with North America. There has been sufficient erosion of the original Appalachian Mountains to have removed them as topographic features many times over were it not for continued isostatic uplift in response to the erosion. Transform boundaries, or transform faults, occur where the edges of two plates slide past one another, as shown in Figure 2.5. If you examine Figures 2.4a and 2.5, you will see that a spreading zone is not a single, continuous rift but a series of rifts that are offset from one another along connecting transform faults. Although the most common locations for transform plate boundaries are within oceanic crust, some occur within continents. A well-known continental transform boundary is the San Andreas Fault in California, where the rim of the Pacific plate is sliding horizontally past the rim of the North American plate (see Figures 2.4a and 2.10). Locations where three plates border one another are known as triple junctions. Figure 2.10 shows several such junctions: Two examples are the meeting point of the Juan de Fuca, North American, and Pacific plates on the West Coast of North America (this is known as the Mendocino triple junction) and the junction of the spreading ridges associated with and Nazca plates west of South America. P R Ethe L I M Pacific, I N A R Y P RCocos, OOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 49 Plate Tectonics 49 10,000 kg 90,000 kg (a) (b) 9000 kg 81,000 kg 90,000 kg?? (c) (d) Figure 2.A Isostasy Idealized diagram or cartoon showing the principle of isostatic uplift. Admiral Frost is left adrift on an iceberg and is uncomfortable being so far above the surface of the water (a). He decides to remove the 10,000 kg (22,046 lb) of ice that is above the water line on the iceberg on which he is standing (b). Were it not for isostatic (buoyant) uplift, Admiral Frost would reach his goal (c). However, in a world with isostasy, uplift results from removal of the ice, and there is always one-tenth of the iceberg above the water (d). What would have happened if Admiral Frost had elected to remove 10,000 kg of ice from only one-half of the area of ice exposed above the sea? Answer: The maximum elevation of the iceberg above the water would have actually increased. Similarly, as mountains erode, isostatic adjustments also occur, and the maximum elevation of mountain peaks may actually increase as a result of the erosion alone! (From Keller, E. A., and Pinter, N. 1996. Active tectonics. Upper Saddle River, NJ: Prentice Hall) Rates of Plate Motion Plate Motion Is a Fast Geologic Process. The directions in which plates move are shown on Figure 2.4a. In general, plates move a few centimeters per year, about as fast as some people’s fingernails or hair grows. The Pacific plate moves past the North American plate along the San Andreas Fault about 3.5 cm per year (1.4 in. per year), so that features such as rock units or streams are gradually displaced over time where they cross the fault (Figure 2.11). During the past 5 million years, there has been about 175 km (about 110 miles) of displacement, a distance equivalent to driving two hours at 55 mph on a highway along the San Andreas Fault. Although the central portions of the plates move along at a steady slow rate, plates interact at their boundaries, where collision or subduction or both occur, and movement may not be smooth or steady. The plates often get stuck together. Movement is analogous to sliding one rough wood Pboard Movement R E L I M I Nover A R Y P another. ROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 50 2/2/07 Chapter 2 7:31 PM Page 50 Internal Structure of Earth and Plate Tectonics AFRICAN PLATE ATLANTIC OCEAN 63 NORTH AMERICAN PLATE 53 PACIFIC OCEAN San Andreas Fault JUAN DE FUCA PLATE PACIFIC PLATE Subduction zone Spreading center Transform fault 63 35 CARIBBEAN PLATE Triple Junction Plate motion (rate in mm/yr) Direction of relative displacement on transform fault (rate in mm/yr) 35 SOUTH AMERICAN PLATE COCOS PLATE 127 58 Triple Junction 91 NAZCA PLATE Figure 2.10 North American plate boundary Detail of boundary between the North American and Pacific plates. (Courtesy of Tanya Atwater) Figure 2.11 The San Andreas Fault The fault is visible from the lower left to upper right diagonally across the photograph, as if a gigantic plow had been dragged across the landscape. (James Balog/Getty Images Inc.) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 51 A Detailed Look at Seafloor Spreading 51 occurs when the splinters of the boards break off and the boards move quickly by one another. When rough edges along the plate move quickly, an earthquake is produced. Along the San Andreas Fault, which is a transform plate boundary, the displacement is horizontal and can amount to several meters during a great earthquake. During an earthquake in 1857 on the San Andreas Fault a horse corral across the fault was reportedly changed from a circle to an “S” shape. Fortunately, such an event generally occurs at any given location only once every 100 years or so. Over long time periods, rapid displacement from periodic earthquakes and more continuous slow “creeping” displacements add together to produce the rate of several centimeters of movement per year along the San Andreas Fault. 2.4 A Detailed Look at Seafloor Spreading When Alfred Wegener proposed the idea of continental drift in 1915, he had no solid evidence of a mechanism that could move continents. The global extent of mid-oceanic ridges was discovered in the 1950s, and in 1962 geologist Harry H. Hess published a paper suggesting that continental drift was the result of the process of seafloor spreading along those ridges. The fundamentals of seafloor spreading are shown in Figure 2.5. New oceanic lithosphere is produced at the spreading ridge (divergent plate boundary). The lithospheric plate then moves laterally, carrying along the embedded continents in the tops of moving plates. These ideas produced a new major paradigm that greatly changed our ideas about how Earth works.3,6,9 The validity of seafloor spreading was established from three sources: (1) identification and mapping of oceanic ridges, (2) dating of volcanic rocks on the floor of the ocean, and (3) understanding and mapping of the paleomagnetic history of ocean basins. Paleomagnetism We introduce and discuss Earth’s magnetic field and paleomagnetic history in some detail in order to understand how seafloor spreading and plate tectonics were discovered. Earth has had a magnetic field for at least the past 3 billion years2 (Figure 2.12a). The field can be represented by a dipole magnetic field with lines of magnetic force extending from the South Pole to the North Pole. A dipole magnetic field is one that has equal and opposite charges at either end. Convection occurs in the iron-rich, fluid, hot outer core of Earth because of compositional Magnetic axis N Magnetic axis S Magnetic equator S N Axis of rotation (a) Normal polarity Axis of rotation (b) Reversed polarity Figure 2.12 Magnetic reversal Idealized diagram showing the magnetic field of Earth under (a) normal polarity and (b) reversed polarity. (From Kennett, J. 1982. Marine geology. Englewood Cliffs, NJ: Prentice Hall) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 52 Chapter 2 2/2/07 7:31 PM Page 52 Internal Structure of Earth and Plate Tectonics changes and heat at the inner-outer core boundary. As more buoyant material in the outer core rises, it starts the convection (Figures 2.3 and 2.7). The convection in the outer core, along with the rotation of Earth that causes rotation of the outer core, initiates a flow of electric current in the core. This flow of current within the core produces and sustains Earth’s magnetic field.2,3 Earth’s magnetic field is sufficient to permanently magnetize some surface rocks. For example, volcanic rock that erupts and cools at mid-oceanic ridges becomes magnetized at the time it passes through a critical temperature. At that critical temperature, known as the Curie point, iron-bearing minerals (such as magnetite) in the volcanic rock orient themselves parallel to the magnetic field. This is a permanent magnetization known as thermoremnant magnetization.3 The term paleomagnetism refers to the study of the magnetism of rocks at the time their magnetic signature formed. It is used to determine the magnetic history of Earth. The magnetic field, based on the size and conductivity of Earth’s core, must be continuously generated or it would decay away in about 20,000 years. It would decay because the temperature of the core is too high to sustain permanent magnetization.2 Earth’s Magnetic Field Periodically Reverses. Before the discovery of plate tectonics, geologists working on land had already discovered that some volcanic rocks were magnetized in a direction opposite to the present-day field, suggesting that the polarity of Earth’s magnetic field was reversed at the time the volcanoes erupted and the rocks cooled (Figure 2.12b). The rocks were examined for whether their magnetic field was normal, as it is today, or reversed relative to that of today, for certain time intervals of the Earth’s history. A chronology for the last few million years was constructed on the basis of the dating of the “reversed” rocks. You can verify the current magnetic field of the Earth by using a compass; at this point in Earth’s history the needle points to the north magnetic pole. During a period of reversed polarity, the needle would point south! The cause of magnetic reversals is not well known, but it is related to changes in the convective movement of the liquid material in the outer core and processes occurring in the inner core. Reversals in Earth’s magnetic field are random, occurring on average every few hundred thousand years. The change in polarity of Earth’s magnetic field takes a few thousand years to occur, which in geologic terms is a very short time. What Produces Magnetic Stripes? To further explore the Earth’s magnetic field, geologists towed magnetometers, instruments that measure magnetic properties of rocks, from ships and completed magnetic surveys. The paleomagnetic record of the ocean floor is easy to read because of the fortuitous occurrence of the volcanic rock basalt (see Chapter 3) that is produced at spreading centers and forms the floors of the ocean basins of Earth. The rock is fine-grained and contains sufficient iron-bearing minerals to produce a good magnetic record. The marine geologists’ discoveries were not expected. The rocks on the floor of the ocean were found to have irregularities in the magnetic field. These irregular magnetic patterns were called anomalies or perturbations of Earth’s magnetic field caused by local fields of magnetized rocks on the seafloor. The anomalies can be represented as stripes on maps. When mapped, the stripes form quasi-linear patterns parallel to oceanic ridges. The marine geologists found that their sequences of stripe width patterns matched the sequences established by land geologists for polarity reversals in land volcanic rocks. Magnetic survey data for an area southwest of Iceland are shown on Figure 2.13. The black stripes represent normally magnetized rocks and the intervening white stripes represent reversed magnetized rocks.10 Notice that the stripes are not evenly spaced but have patterns that are symmetrical on opposite sides of the Mid-Atlantic Ridge (Figure 2.13). Why Is the Seafloor No Older than 200 Million Years? The discovery of patterns of magnetic stripes at various locations in ocean basins allowed geologists to infer PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 53 A Detailed Look at Seafloor Spreading 60° W 30° W 10° W 0° GREENLAND Arctic Circle ICELAND 60° N Mid ocean ridg e 60° N EUROPE 40° W 20° W Figure 2.13 Magnetic anomalies on the seafloor Map showing a magnetic survey southwest of Iceland along the Mid-Atlantic Ridge. Positive magnetic anomalies are black (normal) and negative magnetic anomalies are white (reversed). Note that the pattern is symmetrical on the two sides of the mid-oceanic ridge. (From Heirtzler, J. R., Le Pichon, X., and Baron, J. G. 1966. Magnetic anomalies over the Reykjanes Ridge. Deep-Sea Research 13:427–43) 10° W Rid g ax e is 50° W 53 numerical dates for the volcanic rocks. Merging the magnetic anomalies with the numerical ages of the rocks produced the record of seafloor spreading. The spreading of the ocean floor, beginning at a mid-oceanic ridge, could explain the magnetic stripe patterns.11 Figure 2.14 is an idealized diagram showing how seafloor spreading may produce the patterns of magnetic anomalies (stripes). The pattern shown is for the past several million years, which includes several periods of normal and reversed magnetization of the volcanic rocks. Black stripes represent normally magnetized rocks, and brown stripes are rocks with a reversed magnetic signature. Notice that the most recent magnetic reversal occurred approximately 0.7 million years ago. The basic idea illustrated by Figure 2.14 is that rising magma at the oceanic ridge is extruded, or pushed out onto the surface, through volcanic activity, and the cooling rocks become normally magnetized. When the field is reversed, the cooling rocks preserve a reverse magnetic signature, and a brown stripe (Figure 2.14) is preserved. Notice that the patterns of magnetic anomalies in rocks on both sides of the ridge are mirror images of one another. The only way such a pattern might result is through the process of seafloor spreading. Thus, the pattern of magnetic reversals found on rocks of the ocean floor is strong evidence that the process of spreading is happening. Mapping of magnetic anomalies, when combined with age-dating of the magnetic reversals in land rocks creates a database that suggests exciting inferences; Figure 2.15 shows the age of the ocean floor as determined from this database. The pattern, showing that the youngest volcanic rocks are found along active midoceanic ridges, is consistent with the theory of seafloor spreading. As distance from these ridges increases, the age of the ocean floor also increases, to a maximum of about 200 million years, during the early Jurassic period (see Table 1.1). Thus, it appears that the present ocean floors of the world are no older than PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 54 2/2/07 Chapter 2 7:31 PM Page 54 Internal Structure of Earth and Plate Tectonics Magnetic field anomaly observed at sea surface Age (millions of years) Seafloor 0 Magnetized rocks 2 4 6 8 Black = normal polarity Brown = reversed polarity 200 0 200 Distance from spreading center (kilometers) (b) Seafloor spreading (a) Polarity reversal time scale Figure 2.14 Magnetic reversals and seafloor spreading Idealized diagram showing an oceanic ridge and the rising of magma, in response to seafloor spreading. As the volcanic rocks cool, they become magnetized. The black stripes represent normal magnetization; the brown stripes are reversed magnetization. The record shown here was formed over a period of several million years. Magnetic anomalies (stripes) are a mirror image of each other on opposite sides of the mid-oceanic ridge. Thus, the symmetrical bands of the normally and reversely magnetized rocks are produced by the combined effects of the reversals and seafloor spreading. (Courtesy of Tanya Atwater) 200 million years. In contrast, rocks on continents are often much older than Jurassic, going back about 4 billion years, almost 20 times older than the ocean floors! We conclude that the thick continental crust, by virtue of its buoyancy, is more stable at Earth’s surface than are rocks of the crust of the ocean basins. Continents form by the processes of accretion of sediments, addition of volcanic materials, and collisions of tectonic plates carrying continental landmasses. We will continue this discussion when we consider the movement of continents during the past 200 million years. However, it is important to recognize that it is the pattern of magnetic stripes that allows us to reconstruct how the plates and the continents embedded in them have moved throughout history. Hot Spots What Are Hot Spots? There are a number of places on Earth called hot spots, characterized by volcanic centers resulting from hot materials produced deep in the mantle, perhaps near the core-mantle boundary. The partly molten materials are hot and buoyant enough to move up through mantle and overlying moving tectonic plates.3,6 An example of a continental hot spot is the volcanic region of Yellowstone National Park. Hot spots are also found in both the Atlantic and Pacific Oceans. If the hot spot is anchored in the slow-moving deep mantle, then, as the plate moves over a hot spot, a chain of volcanoes is produced. Perhaps the best example of this type of hot spot is the line of volcanoes forming the Hawaiian-Emperor Chain in the Pacific Ocean (Figure 2.16a). Along this chain, volcanic eruptions range in age from present-day activity on the big island of Hawaii (in the southeast) to more than 78 million years ago near the northern end of the Emperor Chain. With the exception of the Hawaiian Islands and some coral atolls (ringlike coral islands such as Midway Island), the chain consists of submarine volcanoes known as seamounts. Seamounts are islands that were eroded by waves and submarine landslides and subsequently sank beneath the ocean surface. As seamounts move off the hot spot, the volcanic rocks PRELIMINARY P R O O F farther S Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:31 PM Page 55 Pangaea and Present Continents 55 Age of Ocean Floor (millions of years) 0–2 58–66 2–5 66–84 5–24 84–117 24–37 117–144 37–58 144–208 Figure 2.15 Age of the ocean floor Age of the seafloor is determined from magnetic anomalies and other methods. The youngest ocean floor (red) is located along oceanic ridge systems, and older rocks are generally farther away from the ridges. The oldest ocean floor rocks are approximately 180 million years old. (From Scotese, C. R., Gahagan, L. M., and Larson, R. L. 1988. Plate tectonic reconstruction of the Cretaceous and Cenozoic ocean basins. Tectonophysics 155:27–48) the islands are composed of cool and the oceanic crust they are on becomes denser, and sinks. Seamounts constitute impressive submarine volcanic mountains. In the Hawaiian Chain the youngest volcano is Mount Loihi, which is still a submarine volcano, presumably directly over a hot spot, as idealized on Figure 2.16b. The ages of the Hawaiian Islands increase to the northwest, with the oldest being Kauai, about 6 million years old. Notice in Figure 2.16a that the line of seamounts makes a sharp bend at the junction of the Hawaiian and Emperor Chains. The age of the volcanic rocks at the bend is about 43 million years, and the bend is interpreted to represent a time when plate motions changed.12 If we assume that the hot spots are fixed deep in the mantle, then the chains of volcanic islands and submarine volcanoes along the floor of the Pacific Ocean that get older farther away from the hot spot provide additional evidence to support the movement of the Pacific plate. In other words, the ages of the volcanic islands and submarine volcanoes could systematically change as they do only if the plate is moving over the hot spot. 2.5 Pangaea and Present Continents Plate Tectonics Shapes Continents and Dictates the Location of Mountain Ranges. Movement of the lithospheric plates is responsible for the present shapes and locations of the continents. There is good evidence that the most recent global episode of continental drift, driven by seafloor spreading, started about 180 million years ago, with the breakup of a supercontinent called Pangaea (this name, meaning “all lands,” was first proposed by Wegener). Pangaea (pronounced pan-jee-ah) was enormous, extending from pole to pole and over halfway around Earth near the equator (Figure 2.17). PRE L I M I N APangaea R Y P R O O F Shad two parts Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD Chapter 2 7:31 PM Page 56 Internal Structure of Earth and Plate Tectonics RUSSIA Alaska Aleutian Kuril Trench 50˚N 40˚N Islands Aleutian Trench 78 MY Emperor Seamounts 56 2/2/07 Volcanoes of Hawaiian– Emperor Chain Seamounts Subduction zone Hawaiian–Emperor Bend Present plate motion 43 MY 30˚N Haw PACIFIC OCEAN aiian Ridg e Hawaii 20˚N 6 MY 0 0 160˚E 325 325 Present volcanic activity 750 Miles 750 Kilometers 170˚E 180˚ 170˚W 160˚W (a) Kauai 3.8–5.6 MY Oahu 2.2–3.3 MY Hawaiian Islands Molokai 1.3–1.8 MY Maui All less than 1.0 MY Hawaii 0.8 to present MY Loihi (submarine) present Oceanic lithosphere Hot spot (deep in mantle) Dates in millions of years MY = Million years old (b) Figure 2.16 Hawaiian hot spot (a) Map showing the Hawaiian-Emperor Chain of volcanic islands and seamounts. Actually, the only islands are Midway Island and the Hawaiian Islands at the end of the chain, where present volcanic activity is occurring. (Modified after Claque, D. A., Dalrymple, G. B., and Moberly, R. 1975. Petrography and K-Ar ages of dredged volcanic rocks from the western Hawaiian Ridge and southern Emperor Seamount chain. Geological Society of America Bulletin 86:991–98) (b) Sketch map showing the Hawaiian Islands, which range in age from present volcanic activity to about 6 million years old on the island of RELIMINARY PROOFS Kauai. (From Thurman, Oceanography, 5thUnpublished ed. PColumbus, OH: plate 2) Work © 2008 by Merrill, Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:32 PM Page 57 Pangaea and Present Continents 57 70˚ A A U S I R A L P E NORTH AMERICA EU 160˚ RO A S I A 120˚ 80˚ SOUTH AMERICA 120˚ 160˚ 120˚ 160˚ TETHYS SEA AFRICA G ON DW A N INDIA A AUSTRALIA ANTARCTICA (a) 180 million years ago Direction of plate motion Subduction zone 70˚ U R A S I A P E L NORTH AMERICA A EU 160˚ RO A S I A 120˚ 80˚ AFRICA SOUTH AMERICA G O N A AN DW INDIA AUSTRALIA ANTARCTICA (b) 135 million years ago Figure 2.17 Two hundred million years of plate tectonics (a) The proposed positions of the continents at 180 million years ago; (b) 135 million years ago; (c) 65 million years ago; and (d) at present. Arrows show directions of plate motion. See text for further explanation of the closing of the Tethys Sea, the collision of India with China, and the formation of mountain ranges. (From Dietz, R. S., and Holden, J. C. 1970. Reconstruction of Pangaea: breakup and dispersion of continents, Permian to present. Journal of Geophysical Research 75(26):4939–56. Copyright by the American Geophysical Union. Modifications and block diagrams from Christopherson, R. W. 1994. Geosystems, 2nd ed. Englewood Cliffs, NJ: Macmillan) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 58 2/2/07 Chapter 2 7:32 PM Page 58 Internal Structure of Earth and Plate Tectonics 160˚ E NORTH AMERICA EU RO P A S I A 120˚ 80˚ 120˚ 160˚ 120˚ 160˚ AFRICA SOUTH AMERICA INDIA AUSTRALIA ANTARCTICA (c) 65 million years ago ce O ic an Convergent plate boundary— plates converge, producing a subduction zone, mountains, volcanoes, and earthquakes h nc tre Plate Plate E Asthenosphere NORTH AMERICA EU RO P A S I A AFRICA 160˚ 20˚ 60˚ SOUTH AMERICA Divergent plate boundary— plates diverge at mid-ocean ridges AUSTRALIA 40˚ Plate ge ANTARCTICA rid M id - oc ea n 60˚ (d) Present Plate Transform fault— plates move laterally past each other between seafloor spreading centers Fracture zone Transform fault Asthenosphere Figure 2.17 Two hundred million years of plate tectonics (Continued) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:32 PM Page 59 Pangaea and Present Continents 59 (Laurasia to the North and Gondwana to the South) and was constructed during earlier continental collisions. Figure 2.17a shows Pangaea as it was nearly 200 million years ago. Seafloor spreading over the past 200 million years separated Eurasia and North America from the southern land mass; Eurasia from North America; and the southern continents (South America, Africa, India, Antarctica, and Australia) from one another (Figure 2.17b–d). The Tethys Sea, between Africa and Europe-Asia (Figure 2.17a–c), closed, as part of the activity that produced the Alps in Europe. A small part of this once much larger sea remains today as the Mediterranean Sea (Figure 2.17d). About 50 million years ago India crashed into China. That collision, which has caused India to forcefully intrude into China a distance comparable from New York to Miami, is still happening today, producing the Himalayan Mountains (the highest mountains in the world) and the Tibetan Plateau. Understanding Plate Tectonics Solves Long-Standing Geologic Problems. Reconstruction of what the supercontinent Pangaea looked like before the most recent episode of continental drift has cleared up two interesting geologic problems: 쐍 Occurrence of the same fossil plants and animals on different continents that would be difficult to explain if they had not been joined in the past (see Figure 2.18). Fossil remains of Cynognathus, a Triassic land reptile approximately 3 m long, have been found in Argentina and southern Africa. Remains of the freshwater reptile Mesosaurus have been found in both Brazil and Africa. Fossils of the fern Glossoptens, found in all of the southern continents, are proof that they were once joined. Africa India South America Evidence of the Triassic land reptile Lystrosaurus have been found in Africa, Antarctica, and India. Australia Antarctica Figure 2.18 Paleontological evidence for plate tectonics This map shows some of the paleontological (fossil) evidence that supports continental drift. It is believed that these animals and plants could not have been found on all of these continents were they not once much closer together than they are today. Major ocean basins would have been physical barriers to their distribution. (From Hamblin, W. K. 1992. Earth’s dynamic systems, 6th ed. New York: Macmillan) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 60 Chapter 2 2/2/07 7:32 PM Page 60 Internal Structure of Earth and Plate Tectonics 쐍 Evidence of ancient glaciation on several continents, with inferred directions of ice flow, that makes sense only if the continents are placed back within Gondwanaland (southern Pangaea) as it was before splitting apart (see Figure 2.19). Late Paleozoic glacial boundary Late Paleozoic glacial deposits Direction of glacier motion (a) Late Paleozoic glacial boundary Late Paleozoic glacial deposits Direction of glacier motion GO ND N WA ALA ND (b) Figure 2.19 Glacial evidence for plate tectonics (a) Map showing the distribution of evidence for late Paleozoic glaciations. The arrows indicate the direction of ice movement. Notice that the arrows are all pointing away from ocean sources. Also these areas are close to the tropics today, where glaciation would have been very unlikely in the past. These Paleozoic glacial deposits were formed when Pangaea was a supercontinent, before fragmentation by continental drift. (b) The continents are restored (it is thought that continents drifted north away from the South Pole). Notice that the arrows now point outward as if moving away from a central area where glacial ice was accumulating. Thus, restoring the position of the continents produces a pattern of glacial deposits that makes much more sense. (Modified after Hamblin, W. K. 1992. Earth’s dynamic systems, 6th ed. New York: Macmillan) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:32 PM Page 61 How Plate Tectonics Works: Putting It Together 61 2.6 How Plate Tectonics Works: Putting It Together Driving Mechanisms That Move Plates. Now that we have presented the concept that new oceanic lithosphere is produced at mid-oceanic ridges because of seafloor spreading and that old, cooler plates sink into the mantle at subduction zones, let us evaluate the forces that cause the lithospheric plates to actually move and subduct. Figure 2.20 is an idealized diagram illustrating the two most likely driving forces, ridge push and slab pull. The mid-oceanic ridges or spreading centers stand at elevations of 1 to 3 km (3,000 to 9,000 ft) above the ocean floor as linear, gently arched uplifts (submarine mountain ranges; see Figure 2.21) with widths greater than the distance from Florida to Canada. The total length of mid-oceanic ridges on Earth is about twice the circumference of Earth. Ridge push is a gravitational push, like a gigantic landslide, away from the ridge crest toward the subduction zone (the lithosphere slides on the asthenosphere). Slab pull results because as the lithospheric plate moves farther from the ridge, it cools, gradually becoming denser than the asthenosphere beneath it. At a subduction zone, the plate sinks through lighter, hotter mantle below the lithosphere, and the weight of this descending plate pulls on the entire plate, resulting in slab pull. Which of the two processes, ridge push or slab pull, is the more influential of the driving forces? Calculations of the expected gravitational effects suggest that ridge push is of relatively low importance compared with slab pull. In addition, it is observed that plates with large subducting slabs attached and pulling on them tend to move much more rapidly than those driven primarily by ridge push alone (for example, the subduction zones surrounding the Pacific Basin). Thus, slab pull may be more influential in moving plates than ridge push. Volcanoes Spreading center (mid-oceanic ridge) offset by transform fault Transform fault Lit ho sp he re As th en os ph er e Trench ll ab Sl pu n tio c du z e on Ridge push Lithosphere b Su Figure 2.20 Push and pull in moving plates Idealized diagram showing concepts of ridge push and slab pull that facilitate the movement of lithospheric plates from spreading ridges to subduction zones. Both are gravity driven. The heavy lithosphere falls down the mid-oceanic ridge slope and subducts down through the lighter, hotter mantle. (Modified after Cox, A., and Hart, R. B. 1986. Plate tectonics. Boston: Blackwell Scientific Publications) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 62 2/2/07 Chapter 2 7:32 PM Page 62 Internal Structure of Earth and Plate Tectonics Transform fault England Canada U.S.A. Mid-Atlantic Ridge Subduction zone Pacific Ocean Subduction zone and trench Atlantic Ocean 0 1,500 3,000 km Figure 2.21 Mid Atlantic Ridge Image of the Atlantic Ocean basin showing details of the seafloor. Notice that the width of the Mid-Atlantic Ridge is about one-half the width of the ocean basin. (Heinrich C. Berann/NGS Image Collection) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:32 PM Page 63 Plate Tectonics and Environmental Geology 63 2.7 Plate Tectonics and Environmental Geology Hudso n River Plate Tectonics Affects Us All. The importance of the tectonic cycle to environmental geology cannot be overstated. Everything living on Earth is affected by plate tectonics. As the plates slowly move a few centimeters each year, so do the continents and ocean basins, producing zones of resources (oil, gas, and minerals), as well as earthquakes and volcanoes (Figure 2.4b). The tectonic processes occurring at plate boundaries largely determine the types and properties of the rocks upon which we depend for our land, our mineral and rock resources, and the soils on which our food is grown. For example, large urban areas, including New York and Los Angeles, are developed on very different landscapes, but both have favorable conditions for urban development. New York (Figure 2.22a) is sited on the “trailing edge” of the North American plate, and the properties of the coastline are directly related to the lack of collisions between plates in the area. The divergent plate boundary at the Mid-Atlantic Ridge between North America and Africa is several thousand kilometers (over 1,500 miles) to the east. The collision boundaries between the North American and Caribbean plates and between the North American and Pacific plates are several thousands of kilometers (over 1,500 miles) to the south and west, respectively (see Figure 2.4a). The passive processes of sedimentation from rivers, glaciers, and coastal processes, depositing sediments on rifted and thinned continental crust, instead of the more active crustal deformation that produces mountains, have shaped the coastline of the eastern United States north of Florida. The breakup of Pangaea about 200 million years ago (Figure 2.17) produced the Atlantic Ocean, which, with a variety of geologic processes, including erosion, deposition, and glaciation over millions San Gabriel Mts. Santa Monica Mts. Long Island Jersey City Los Angeles Long Beach Coney Island N 0 10 km 0 10 km N (b) (a) Figure 2.22 Los Angeles and New York Satellite images of (a) New York City and (b) the city of Los Angeles. Both are coastal cities; however, Los Angeles is surrounded by mountains, whereas New York is sited in a relatively low-relief area characteristic of much of the Atlantic coastal environment. For these images, healthy vegetation is red, urban development is blue, beaches are off-white, and water is black. (Science Source/Photo Researchers, Inc.) PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 64 Chapter 2 2/2/07 7:32 PM Page 64 Internal Structure of Earth and Plate Tectonics of years, eventually led to the development of the beautiful but subdued topography of the coastal New York area. In contrast, the Los Angeles metropolitan area is near the “leading edge” of the boundary between the North American and Pacific plates (Figure 2.10), characterized by active, vigorous crustal deformation (uplift; subsidence, or sinking of the ground’s surface; and faulting) associated with the San Andreas fault, a transform boundary. The deformation has produced the Los Angeles Basin, rimmed by rugged mountains and uplifted coastline (Figure 2.22b). Plate motion over millions of years can change or modify flow patterns in the oceans and the atmosphere, influencing or changing global climate as well as regional variation in precipitation. These changes affect the productivity of the land and its desirability as a place to live. Plate tectonics also determines, in part, what types of minerals and rocks are found in a particular region. We will explore how rocks and minerals are influenced by plate tectonics in Chapter 3. SUMMARY Our knowledge concerning the structure of Earth’s interior is based on the study of seismology. Thus we are able to define the major layers of Earth, including the inner core, outer core, mantle, and crust. The uppermost layer of Earth is known as the lithosphere, which is relatively strong and rigid compared with the soft asthenosphere found below it. The lithosphere is broken into large pieces called plates that move relative to one another. As these plates move, they carry along the continents embedded within them. This process of plate tectonics produces large landforms, including continents, ocean basins, mountain ranges, and large plateaus. Oceanic basins are formed by the process of seafloor spreading and are destroyed by the process of subduction, both of which result from convection within the mantle. The three types of plate boundaries are divergent (midoceanic ridges, spreading centers), convergent (subduction zones and continental collisions), and transform faults. At some locations, three plates meet in areas known as triple junctions. Rates of plate movement are generally a few centimeters per year. Evidence supporting seafloor spreading includes paleomagnetic data, the configurations of hot spots and chains of volcanoes, and reconstructions of past continental positions. The driving forces in plate tectonics are ridge push and slab pull. At present we believe the process of slab pull is more significant than ridge push for moving tectonic plates from spreading centers to subduction zones. Plate tectonics is very important in environmental geology because everything living on Earth is affected by it. PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458. KELLMC02_0132251507.QXD 2/2/07 7:32 PM Page 65 Critical Thinking Question 65 Key Terms asthenosphere (p. 41) isostasy (p. 48) seafloor spreading (p. 42) continental drift (p. 42) lithosphere (p. 41) seismology (p. 41) convection (p. 41) magnetic reversal (p. 52) spreading center (p. 42) convergent boundary (p. 46) mantle (p. 39) subduction zone (p. 44) core (p. 39) mid-oceanic ridge (p. 42) submarine trench (p. 46) crust (p. 40) Moho (p. 40) transform boundary (p. 48) divergent boundary (p. 46) paleomagnetism (p. 52) triple junction (p. 48) hot spot (p. 54) plate tectonics (p. 42) Wadati-Benioff zone (p. 44) Review Questions 1. What are the major differences between the inner and outer cores of Earth? 4. What is the major process that is thought to produce Earth’s magnetic field? 2. How are the major properties of the lithosphere different from those of the asthenosphere? 5. Why has the study of paleomagnetism and magnetic reversals been important in understanding plate tectonics? 3. What are the three major types of plate boundaries? 6. What are hot spots? 7. What is the difference between ridge push and slab pull in the explanation of plate motion? Critical Thinking Question 1. Assume that the supercontinent Pangaea (Figure 2.17) never broke up. Now deduce how Earth processes, landforms, and environments might be different than they are today with the continents spread all over the globe. Hint: Think about what the breakup of the continents did in terms of building mountain ranges and producing ocean basins that affect climate and so forth. PRELIMINARY PROOFS Unpublished Work © 2008 by Pearson Education, Inc. From the forthcoming book Introduction to Environmental Geology, Fourth Edition, by Edward A. Keller, ISBN 9780132251501. To be published by Pearson Prentice Hall, Pearson Education, Inc., Upper Saddle River, New Jersey. All rights reserved. This publication is protected by Copyright and written permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form or by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permission(s), write to: Rights and Permissions Department, Pearson Education, Inc., Upper Saddle River, NJ 07458.