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CHAPTER 2: PLATE TECTONICS

2

Plate Tectonics

Shutterstock 14617147 Credit: Phil Emmerson

Introduction 2.1 Historical Development of the Theory 2.1.1 Conceptual Models of Earth 2.1.2 Continental Drift 2.2 Modern Developments 2.2.1 Marine Geology and Geophysics 2.2.2 Magnetism and Paleomagnetism 2.2.3 Seafloor Spreading 2.3 Plate Tectonics Theory 2.3.1 Tectonic Plates 2.3.2 Plate Boundaries 2.3.3 Plate Movement 2.3.4 The Supercontinent Cycle

KEY TERMS

CHAPTER 2: PLATE TECTONICS

CHAPTER 2: PLATE TECTONICS

PLATE TECTONIC THEORY

The theory that Earth’s lithosphere is broken into large, moving sections, or plates. CATASTROPHISM

The belief that the earth’s features were formed by sudden, violent, often worldwide events using processes that may have been different from those operating today. UNIFORMITARIANISM

The concept stating that the physical laws and processes in operation today have been there since the start and created incremental changes over long periods.

INTRODUCTION Among the attributes that characterize human beings, curiosity is certainly among the most important. It is curiosity that has propelled a great deal of the scientific investigation that has revealed to us how the world—and universe it inhabits—works. Part of that investigation has been devoted to explaining the many and varied landforms that make up the landscapes of our planet. How did the mountains rise? How did the oceans form? What are volcanoes and why do they erupt? Why does the ground sometimes shake so ferociously that buildings fall down? Many of these questions were answered with the development of plate tectonic theory, the theory that Earth’s lithosphere is broken into large, moving sections, or plates. This chapter discusses what evidence caused human minds to turn in the direction of such a revolutionary idea, and how scientists came to the conclusion that it accurately describes the way the world works.

2.1

HISTORICAL DEVELOPMENT OF THE THEORY

While plate tectonics theory is the currently accepted model for how Earth’s landforms were originally formed, it certainly wasn’t the first. Early humans developed a variety of myths that explained how the world formed and achieved its current landscape; these often involved animals, spirits and gods. The more scientifically minded looked for evidence that could help them to form ideas that could then be tested. Some of these models held the attention of scientists for quite some time.

2.1.1

Conceptual Models of Earth

Catastrophism versus Uniformitarianism One of the earliest scientific theories to explain Earth’s surface features was called catastrophism. This explanation for Earth’s features held that they achieved their current forms by sudden, violent events based on processes that don’t commonly occur today. For instance, the catastrophe might be an earthquake that caused a mountain range to suddenly rise or a single flood that carved river valleys or canyons. The main problem with catastrophism is that none of the scenarios were compatible with what was being learned about the laws of physics. Scientists were faced with a choice—either accept that physical laws once operated differently than they do today, or develop another model. In the late 1700s, geologist James Hutton introduced the concept of uniformitarianism, which stated that the physical laws and processes in operation today have been there since the start and have operated in the same way since the universe came into being. These processes cause small incremental changes that form Earth’s features over long periods of time. Hutton was not the first to introduce this concept; the Persian physician and philosopher, Ibn Sina (known in the West as Avicenna) had first suggested this idea in 1027. Even so, Hutton’s work brought the idea to the attention of western scientists. Two fellow geologists, John Playfair and Charles Lyell, further refined the model, and the idea that “the past is the key to the present” became widely accepted by the geologic community. Contracting Earth versus Expanding Earth Uniformitarianism limited the possible explanations for Earth’s landscapes to known natural laws and processes. In the late 1700s and early 1800s, the early geologists began to investigate

CHAPTER 2: PLATE TECTONICS

the earth by applying the scientific process of collecting data, then proposing and testing explanations based on that data. One of their first conclusions was that the earth was originally molten; as it cooled, rocks solidified into the continents we know today, locked into position for all time. This model dominated much of geological thinking for more than a century. Other explanations, however, continued to emerge. Around the early 1900s, the contracting earth model was proposed. This model was based on the idea that Earth does not have a constant size but is actually shrinking due to the cooling of its interior. The shrinking has caused Earth’s crust to form hills, valleys, and even mountain ranges in the same way that the skin of a dried up fruit withers and cracks. Once scientists were aware that the interior of the earth is not cooling down but is actually quite hot, a different idea was proposed—the expanding earth model. Because substances expand when heated, this model suggested that Earth’s interior heat was causing it to expand, breaking up the crust in the same way the skin or a lemon might tear apart if the fruit inside expanded. According to this model, such expansion has separated the continents, created oceans, and formed major land features such as mountain ranges. Even as geologists pondered whether the evidence supported an Earth that changed sizes, another idea was under formulation: continental drift.

2.1.2

Continental Drift

Ever since cartographers produced maps with a sufficient degree of accuracy to portray the shapes of the continents realistically, individuals have noticed the matching coastlines on opposite sides of the Atlantic Ocean. As early as 1620, the English philosopher, statesman, and scientist Francis Bacon commented on the similarity of the Atlantic coastlines of South America and Africa. The similarity had caused him and others to wonder whether the two continents had once been joined like two pieces of a giant jigsaw puzzle or if the apparent match was just a coincidence. Most scientists at the time were convinced that it was indeed just a coincidence, but one prolific geologist, Edward Suess, made a different suggestion. In the early 1800s, Suess noted not only the fit between the continents but also the discovery of identical Glossopteris fern fossils in South America, Africa, and Australia. He wondered how the same plants had ended up on different continents. The only explanation that made sense to him was that all the southern continents, including Antarctica, had once been joined into one larger continent that he called Gondwana or Gondwanaland. Suess’s ideas were not at all taken seriously and were discarded. In the early 1900s, however, several geologists took a second look, the most noteworthy of whom was German climatologist Alfred Wegener. Wegener went further than Suess, both in his hypothesis and in the evidence he gathered to develop it. Wegener noted other identical sets of fossils that appeared on different continents separated by oceans, and he wondered by what means these animals might have traveled overseas (see Figure 2.1 on page 70). He observed that the striations, long scratches in rock left behind by glaciers, appeared in areas that are now tropical. He also noted that the Appalachian Mountains in North America seemed to end abruptly in Newfoundland, while another range began at the coastline of Wales, and yet another extended up through Norway. Not only that, but the ranges were composed of rock so similar that the only possible explanation was that all of these ranges had been formed at the same time (see Figure 2.2).

CONTRACTING EARTH MODEL

The model proposing that Earth is cooling and shrinking, which causes the crust to crack and wrinkle, forming land features. EXPANDING EARTH MODEL

The model proposing that Earth is heating and expanding, causing the crust to crack and burst, creating continents. SUESS, EDWARD

The first geologist to envision some of the continents as having once been joined together. GONDWANA, GONDWANALAND

A supercontinent that included present-day Africa, South America, Australia, Antarctica, and India. WEGENER, ALFRED

The meteorologist-geologist who proposed that all the continents had once been joined together and have since separated and moved apart.

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FIGURE 2.1 Various kinds of data exist to support the idea that the southern continents were once joined into a continent called Gondwana, some 200 million years ago. Illustration by John J. Renton.

FIGURE 2.2 Similarities in the rocks of North America’s Appalachian Mountains and the mountains on the northwestern edge of Europe, plus the way the continents seemed to fit together, caused Alfred Wegener to propose that the northern continents were once joined together into a larger continent called Laurasia. © 2014 by miha de. Used under license of Shutterstock, Inc.

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FIGURE 2.3 Alfred Wegener proposed that all of the present continents were once joined into a supercontinent he called Pangaea. Illustration by John J. Renton.

Wegener not only proposed that South America and Africa had once been joined but also that the northern continents had also been joined together to form a larger continent, Laurasia. His evidence seemed to indicate that all of the continents had once been connected together into one supercontinent he called Pangaea (Figure 2.3). As seen in Figure 2.4 on page 72, Pangaea provides a satisfactory explanation for much of Wegener’s evidence. The glacial striations on the southern continents, which point in contradictory directions today, all extended in the same direction when continents were oriented as they must have been to fit together to make Pangaea. The mountain ranges connected, and there were even matching flood basalts—large lava fields from volcanic eruptions—where the continents met. Not only did Wegener propose that this supercontinent existed, but he also contended that Pangaea had split up and, over millions of years, the continents had moved to their current positions in a process he called continental drift. North and South America moved to the west, Europe and Asia moved to the north, Australia moved to the east, and India moved to the northeast until it met the Asian continent, which had twisted into a new position (see Figure 2.5 on page 73). Wegener’s proposal was a bold one with one major problem: Wegener could not identify a scientifically defensible source of energy and a mechanism to move the continents. Consequently, Wegener’s proposal, like those of Suess before him, met with considerable doubt and dismissal, even though individual scientists continued to be intrigued with his ideas and the evidence he accumulated that Pangaea had once existed.

LAURASIA

A supercontinent that included present-day North America and Eurasia. PANGAEA

A supercontinent composed of all of the present-day continents joined together in a single land mass. CONTINENTAL DRIFT

The model stating that continents move around the surface of the earth rather than staying in one fixed position.

CHAPTER 2: PLATE TECTONICS

FIGURE 2.4 The striations, or grooves, found in southern continents indicate glacial movement from a central point as part of Gondwana. © 2014 by miha de. Used under license of Shutterstock, Inc.

CHAPTER 2: PLATE TECTONICS

FIGURE 2.5 In the millions of year since Pangaea broke up, North and South America have moved west, Europe and Asia have moved north, Australia has moved east, and India has moved northeast to collide with Asia. Illustration © Kendall Hunt Publishing Company.

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2.2

MODERN DEVELOPMENTS

Wegener, of course, was right in his assertions, but it would take some time for evidence to surface that would provide evidence of a possible mechanism for continental drift. Most of that evidence was found not on the continents but on floor of the ocean.

FIGURE 2.6 The modern exploration of the ocean began in 1872, when the HMS Challenger set sail loaded with instrumentation. The voyage lasted four years and obtained hundreds of depth soundings and samples. Image: NOAA.

2.2.1

SONAR

A technology that creates images by emitting sound waves and interpreting the returning echoes. BATHYMETRY

The measurement of the depth of water in lakes, rivers, and oceans. OCEANIC RIDGE

Long, undersea, linear volcanic mountain chain containing a rift where ocean floor is made, and the crust is pulling apart. RIFT

A location where Earth’s crust and lithosphere are being pulled apart. MAGMA

Molten rock from within the earth’s interior.

Marine Geology and Geophysics

Modern exploration of the oceans began in 1872, when two British naturalists loaded a small British Navy warship, HMS Challenger, with laboratories, microscopes, sounding and sampling devices, and other scientific equipment and set sail. During the next four years, they traveled 68,890 miles, took samples at 362 stations, and measured the depth at 492 different sites (Figure 2.6). Among the discoveries were over a thousand new species of marine animals, a mountain range, and the deepest spot in the ocean, now called the Challenger Deep in a trench near the Pacific island of Guam. While this was an impressive accomplishment for the time, given the extent of the ocean it was barely a beginning. Until the 1940s, little was really known about the ocean bottom, aside from the depths of various locations. Water depth measurements were taken with sounding lines, which were ropes or chains with a weight attached. Once the rope lost its tension, geologists knew that they had reached the bottom and a measurement of the depth could be read from the line. This technique did not work well for the deep ocean basins because it took so long to collect a single depth measurement. Because of the lack of data about the ocean floor, it was considered to be almost entirely flat as a billiard table from continent-to-continent, with the exception of the features previously discovered by Challenger. Our ignorance about the ocean was to change, however, with the invention of sonar by the U.S. Navy following World War I (Figure 2.7). Sonar uses sound waves in water to measure depth in the same way an echo returns to a person on land. It is possible to determine the distance to an object by counting the amount of time it takes for a sound to echo back to its source. Sonar made possible bathymetry, the science of measuring the depth of water in

CHAPTER 2: PLATE TECTONICS

FIGURE 2.7 The invention of sonar allowed scientists, for the first time, to “see” and map the topography of the ocean bottom. Illustration by John J. Renton.

oceans, lakes, and rivers. Not only could bathymetry provide a continuous depth profile of the ocean bottom to be made, but it also had the capability of generating a visual cross section of the bottom sediments and the underlying crustal rock surface (Figure 2.8). For the first time, scientists were able to “see” to the bottom of the ocean to discern the contours of oceanic crust. Oceanic Ridges The first major feature to be discovered on the ocean floor was the mountain range previously noted by Challenger, a range far more extensive than had previously been thought possible. In fact, bathymetry showed that every ocean has such a mountain range, which scientists named the oceanic ridge. This is the longest mountain range on Earth, with a length of 65,000 kilometers (40,000 miles), and it extends beneath every ocean (see Figure 2.9 on page 76), encircling the planet much like the seams of a baseball. Not only are the oceanic ridges the most dominant feature on the ocean bottom, they are the most dominant feature on Earth. It is said that if one were approaching Earth from space (and the ocean basins were empty), the oceanic ridges would be the first surface feature to be identified, even before major mountain ranges such as the Rockies, the Alps, and the Himalayan range. This extensive feature turned out to be more than a mountain range, however. Further investigation—including samples and visual inspections through the use of submersibles— revealed that at the center of the oceanic ridge lies a rift, a break in the ocean floor, through which magma, or molten rock, is continually rising and solidifying into basalt, the rock that composes oceanic crust. The importance of this discovery will be described later in this chapter.

CHAPTER 2: PLATE TECTONICS

FIGURE 2.9 The oceanic ridge is the longest mountain range on Earth, extending 65,000 kilometers (40,000 miles) through all of the world’s ocean basins. Map: © 2014 by VanHart. Used under license of Shutterstock, Inc.

Deep-Sea Trenches The second major discovery made through bathymetry also reflected Challenger’s earlier voyage: a series long, narrow, very deep troughs that parallel the coasts of some continents. These are appropriately named deep-sea trenches. The deepest trench is the Mariana Trench east of the Philippines, China, and Japan and near Guam Island; the Mariana Trench extends for 2,550 km (1,560 miles) and is seven miles deep (Figure 2.10). At one end is the Challenger Deep, the deepest spot in the ocean and named for the Challenger on whose voyage it was first discovered.

DEEP-SEA TRENCH

A long, narrow, deep depression in the ocean floor where subduction takes place. MAGNETIC ALIGNMENT

The orientation of crystals within magnetic rocks with regard to Earth’s magnetic north and south poles.

As of 2010, 22 trenches have been discovered. Of that number, 18 are in the Pacific Ocean, including the Middle America Trench sitting just off the coast of Central America, and the Aleutian trench that borders Alaska. The Atlantic Ocean has three trenches, the deepest of which is the Puerto Rico Trench skirting the Caribbean Sea. The Indian Ocean has two trenches that run together to form the second longest in the word, Java and Sundra trenches. The importance of the oceanic ridges and deep-sea trenches was not immediately understood. At the time they were discovered, these were simply interesting ocean features. A third major discovery, one based on the magnetism in rocks, revealed their importance and tied the two of them together.

MAGNETOMETER

An instrument capable of detecting the magnetic alignment of rock crystals. PALEOMAGNETISM

The study of the changes that have occurred in Earth’s magnetic field.

2.2.2

Magnetism and Paleomagnetism

The information stored by magnetism in rocks was revealed by another new technology developed during World War II, a technology that allowed scientists to detect the magnetic alignments of the crystals that form some rocks.

CHAPTER 2: PLATE TECTONICS

FIGURE 2.10 The Mariana Trench in the South Pacific is the site of the deepest spot on Earth, the Challenger Deep, which lies 10,900 meters (35,760 feet) below sea level. Image: NOAA.

FIGURE 2.11 Earth’s magnetic field is similar to having a slightly tilted bar magnet in its interior. A “wobble” in the tilt causes the locations of north and south magnetic poles to vary slightly from year to year. © 2014 by Milagli. Used under license of Shutterstock, Inc.

Earth has a magnetic field that is the result of rotation and convection within Earth’s liquid outer core. It is as if there were a bar magnet centered at Earth’s core, one that leans slightly compared to the planet’s rotational axis (Figure 2.11). At one end of the “magnet” is the north magnetic pole and at the other is the south pole. This imaginary bar magnet wobbles a bit, so the north and south magnetic poles are not stationary but vary in location by about 15 kilometers (9 miles) per year. For example, in 1838, the north magnetic pole was located on the west coast of the Boothia Peninsula in the Arctic (95° West longitude and 70° North latitude); since then, it has moved at least 1,100 km (690 miles) into the Arctic Ocean. This means that, from the point of view of a compass needle, the direction of magnetic north has changed; it points in a slightly different direction now than it did in 1838, because the location of magnetic north has changed (Figure 2.12). The orientation of Earth’s magnetic field, called its magnetic alignment, is frozen in rocks that contain magnetic minerals, such as those containing iron in their crystal structures. Basalt, which composes the oceanic lithosphere, has large amounts of iron, but there are also some rocks

FIGURE 2.12 The location of magnetic north (indicated by the red dots) has moved about 690 miles since 1831. Image: NOAA.

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POLAR WANDERING

The ongoing changes in the location of the magnetic north and south poles. MAGNETIC REVERSAL

The reversing of the earth’s north and south magnetic poles. MAGNETIC POLARITY

The state of being a north or south pole of a magnet or of the earth. MAGNETIC ZONATION

A striped pattern of alternating positive and negative magnetic polarity parallel to and on both sides of a midoceanic ridge. SEAFLOOR SPREADING

The widening of the seafloor as a result of crust added by the outpouring of magma at a rift zone.

in the continental lithosphere that contain magnetic minerals. When the iron crystals in these rocks solidify from the magma, they capture alignment of magnetic north and south. A magnetometer is an instrument that can detect the direction of that magnetic alignment. This new technology allowed geologists to determine the orientation of Earth’s magnetic field at the time the rock formed, a capability that has launched the field of paleomagnetism, the study of the changes that have occurred in Earth’s magnetic field. The rocks have stored the story of Earth’s magnetic history, a story that provided evidence regarding the break up of Pangaea and supporting continental drift. Polar Wandering Paleomagnetic scientists spent years taking samples of rocks from various areas of each continent to measure their magnetic alignment. What they found out was that, at any given time, all the rocks of a certain age on the same continent would agree on where magnetic north was when they solidified. However, the location frozen in the rocks was not where magnetic north is now. Furthermore, the captured location of magnetic north changed with the age of the rock. One set of rocks showed magnetic north to have once been located where the Hawaiian Islands are today. There were only two possible conclusions that could be made from this data. The first was that the location of magnetic north varied far more widely over millions of years than the limited movement scientists had already observed, a phenomenon they called polar wandering. The second possible conclusion was that the magnetic poles had only limited movement, in which case the continents on which the rocks were found must have done the moving. To find out which was the case, geologists plotted maps of the magnetic alignments found in rocks of a similar age on each continent. They then compared the maps. If the continents were fixed while the poles moved, the plotted pathways of magnetic north over time would be nearly identical for all of the continents. If the continents had moved, however, the map of each continent would have its own unique plot.

FIGURE 2.13 Paleomagnetic studies indicated that the location of magnetic poles changed dramatically over time. There were only two possible explanations: either the poles did move that extensively or the poles stayed relatively fixed and the continents moved. Studies on separate continents yielded the conclusion that it was the continents, not the poles, that moved.

The maps turned out to be substantially different (Figure 2.13), which supported Wegener’s concept of continental drift. Further plotting of the data allowed geologists to determine the positions of each continent at various times. The results showed that the continents had moved along the very paths Wegener had calculated them to have taken after the breakup of Pangaea. This, in turn, confirmed the supercontinent’s existence some 200 million years ago. However, it still did not address the nagging problem of how its subcontinents had since managed to move. The answer to that question was found on the seafloor.

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Magnetic Reversals In 1920, a Japanese scientist, Motonori Matuyama was studying the magnetism of lava flows in Japan and discovered that Earth’s magnetic field appeared to have undergone reversals— that is, the north and south magnetic poles had reversed locations a number of times. Since Matuyama’s original discovery, we have learned that magnetic reversals have occurred over the past 10 million years at an average rate of 4 to 5 reversals per million years with the last reversal occurring about 780,000 years ago. With all that we know about Earth’s magnetic field and magnetic reversals, there is still no agreement as to what causes reversals to take place. Which pole is north and which pole is south is called magnetic polarity. Today, the north pole is in the Arctic region while the south pole is in the Antarctic region. This is called normal polarity. Reverse polarity occurs when poles switch places so that what a compass points to as the north pole is in the Antarctic and the south pole is in the Arctic. FIGURE 2.14 Oceanic crust contains striping called magnetic zonation that is invisible to the eye but not to the magnetometer. Each stripe is composed of rocks with the same magnetic polarity, captured at the time they were formed before polarity reversed to form a new stripe; the pattern of stripes on one side of a rift is the mirror image of pattern on the other side. Magnetic zonation not only led to the concept of seafloor spreading but also provided proof of this concept. Illustration by John J. Renton.

2.2.3

Seafloor Spreading

While Matuyama’s data came from lava flows in Japan, even more evidence for magnetic reversals was found on the seafloor. There the magnetic reversals appear as parallel bands of rock on the seafloor with magnetic polarities that alternate—first there is a band of rocks with normal polarity, then a band of rocks with reverse polarity, then one that has normal polarity and so on. The alternating bands are called magnetic zonation, or magnetic striping (Figure 2.14). Collection of the magnetic zonation data from ocean surveys increased after World War II, and it greatly puzzled geologists. A Princeton University geologist named Harry Hess undertook the task of explaining how such striping could occur. According to Hess, the only possible explanation for the banding patterns was that the rocks of the ocean floor were constantly spreading away from the oceanic ridge. Hess asserted that outflow of magma at the rift zones is continually forming new oceanic crust, and that its addition to existing crust actually widens the seafloor, a concept called seafloor spreading (see Figure 2.15 on page 80). As newly erupted magma solidified into new crust, it recorded Earth’s magnetic field and then moved away from the rift as more rock formed at the rift. When a magnetic reversal occurred, the basalts that formed after the reversal then recorded the altered magnetic field, with the iron

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FIGURE 2.15 Seafloor spreading creates mirror images of magnetic striping on each side of the rift. The width of each pair of stripes depends on the length of time from one magnetic reversal to the next and on the rate of formation of new oceanic lithosphere. Illustration by John J. Renton.

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crystals in that rock pointing to the new north pole. The longer the time between magnetic reversals, the wider the magnetic stripe that was created. Hess conjectured that if seafloor spreading were a fact, there should be two such patterns of magnetic striping, one on each side of the rift, that are mirror images of each other. The patterns were there. By 1963, evidence had been collected that conclusively supported Hess’s hypothesis. This was the beginning of the end of arguments against continental drift. What followed was the formulation of plate tectonics theory.

2.3

PLATE TECTONICS THEORY

2.3.1

Tectonic Plates

Plate tectonics theory developed courtesy of yet another new technology, tomography. FIGURE 2.16 The lithosphere is broken into about a dozen major plates and a number of minor plates. Image: USGS.

Tomography analyzes the speed of waves to create an image of what lies beneath the surface of Earth. What tomographic studies revealed was that Earth’s lithosphere is not one solid unit, but is broken up into pieces, called tectonic plates. The continents are not separate units moving through ocean waters, as Wegener had thought; the continents are each part of a larger plate, the part that is exposed above sea level. The plate of which the continent is a part includes both oceanic and continental crust resting on lithosphere. As the lithosphere composing the plate moves, the continental crust riding on the lithosphere moves as well. There are twelve major plates plus a number of minor plates, each of which is composed of lithosphere that is moving over the asthenosphere and following its own path. Figure 2.16 illustrates the plates and the direction of movement. The forces involved where plates meet have created much of Earth’s landscape.

2.3.2

Plate Boundaries

The dividing line between two plates is called a plate boundary. There are three types of plate boundaries: (1) divergent boundaries, where two plates are moving apart from each other;

TECTONIC PLATE

A rigid, moving piece of the earth’s lithosphere. DIVERGENT BOUNDARY

Boundary where tectonic plates are moving away from each other.

CHAPTER 2: PLATE TECTONICS

FIGURE 2.17 The continuous creation of crust in a rift zone increases the area of the ocean floor, creating an opening ocean. The Atlantic, Indian, and Arctic oceans are opening oceans. Illustration © Kendall Hunt Publishing Company.

(2) convergent boundaries, where the plates are moving toward each other; and (3) transform boundaries, where the plates are sliding past each other. Each type of boundary has distinctive features associated with it. Divergent Plate Boundaries Divergent boundaries have a significant feature in common: a rift zone where magma erupts from the underlying mantle, forming new crust that moves away from the rift on each side. At this point, there is only crust; the mantle portion of the lithosphere is still molten. As the new crust cools and moves away from the rift zone, it becomes more dense. As discussed in Chapter 1, denser materials have less buoyancy, so the cooler the oceanic crust becomes, the more deeply it sinks into the asthenosphere. At a certain distance, the portions of the mantle that will become part of the oceanic lithosphere also cool enough to solidify, adding to the solid mass and causing it to further sink into the asthenosphere. Consequently, the older the lithosphere, the cooler and denser it is and the more deeply it sinks into the asthenosphere. From our point of view, this appears as increasing ocean depths: the ocean is shallowest at the oceanic ridges and deepest at the deep-sea trenches, which are the points farthest away from the rift zone. The importance of this is explained later in this chapter. FIGURE 2.18 The oceanic rift is not a continuous break but occurs in segments. Fractures in the rift zone called transform faults allow segments of crust to slide past each other as the seafloor spreads. Illustration by John J. Renton.

Because the movement of oceanic lithosphere is away from the rift zone, seafloor spreading produces an ocean area that is getting wider. Such an ocean is referred to an opening ocean (Figure 2.17). The Atlantic, Indian, and Arctic oceans are all opening oceans.

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FIGURE 2.19 Oceanic crust sinks back into the mantle at a subduction zone, forming a deepsea trench where the converging plates meet. Illustration © Kendall Hunt Publishing Company.

The rift zone itself is not a continuous break in Earth’s crust. Rather, it is a series of breaks that are slightly offset from each other. As Figure 2.18 illustrates, the moving crust causes fractures along lines roughly perpendicular to rift segments. The fractures, called transform faults, allow the jagged edges of the plates to slide past each other as the seafloor spreads. One way to picture this is to bring your hands together, alternating the fingers of each hand. As you pull your hands apart, notice that the fingers are sliding along each other; the sliding is similar to the motion at transform faults as oceanic lithosphere pulls apart at the rift zones. Convergent Plate Boundaries Convergent boundaries occur where two plates meet that are moving toward each other. What happens then depends on whether the crust on the leading edge of the plate is oceanic crust or continental crust. The meeting of two plates with continental crust on their leading edges results in a collision that has formed the world’s highest mountains; an event that will be discussed in Chapter 3.

OPENING OCEAN

An ocean that is becoming wider due to the production of new lithosphere at a rift zone. TRANSFORM FAULT

A fracture in the lithosphere where plates slide horizontally past each other. CONVERGENT BOUNDARY

Boundary where tectonic plates are moving toward each other. SUBDUCTION

A process by which one tectonic plate slides beneath another plate and sinks back into the mantle at a convergent boundary.

When at least one of the two plates has oceanic crust at its leading edge, the coming together of the two plates means that one plate will slip beneath the other plate in a process called subduction. As the subducting plate sinks, it forms a steep depression in the ocean floor—a deep-sea trench (Figure 2.19). The realization that the deep-sea trenches were subduction zones solved two puzzles. First, it explained the presence of such deep troughs. Second, it explained where the oceanic crust continuously being formed at the rift zone ultimately went—it is being consumed at a subduction zone. Because total subduction equals the total amount of new crust being formed at rift zones, the surface area of Earth stays constant. Subduction also explained

FIGURE 2.20 Tectonic plates are typically bounded by a rift zone, which forms lithosphere, and a subduction zone, which consumes lithosphere. The lithosphere slowly moves from the rift zone to the subduction zone. Continents between the rift and subduction zones ride on top of moving lithosphere as if on a conveyor belt. Illustration © Kendall Hunt Publishing Company.

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FIGURE 2.21 California’s San Andreas Fault marks the transform boundary where the Pacific and North American plates slide past each other. Photo: USGS; Map: Shutterstock 15180868, credit Map Resources.

continental drift, although the movement of continents is no longer called that. In general, a plate is bounded on one side by an oceanic ridge and on the other by a zone of subduction. As new oceanic lithospheric rocks are added to the plate at the oceanic ridge and old oceanic lithospheric rocks are being consumed at the zone of subduction, the plate physically moves from the divergent plate margin to the convergent plate margin. Should a continent be located between the two plate margins, it will be carried along as a passive passenger much like airline passengers are carried by moving sidewalks down terminal corridors (Figure 2.20). TRANSFORM BOUNDARY

Boundary where tectonic plates slide past each other. CONVECTION

The movement within a fluid in which warm particles rise, lose heat, and sink again. CONVECTION CELL

A circular current caused by convection. MANTLE DRAG

A proposed plate tectonic mechanism in which friction from a mantle convection cell drags a plate in the direction of the cell’s current. RIDGE PUSH

A proposed plate tectonic mechanism in which cooled crust at oceanic ridges slides from the higher elevation to a lower elevation, pushing the plate ahead of it.

Subduction involves tremendous forces. Friction that occurs as one plate slides beneath the other builds up stresses that, when released, generate the largest earthquakes on Earth. The magnitude 9.3 Sumatra earthquake in 2004, the magnitude 8.8 Chilean quake in 2010, and the magnitude 9.2 earthquake in 1964 in Alaska all occurred in subduction zones. The forces also create magma, which in turn, erupts to form volcanoes. It is no accident that a large proportion of Earth’s volcanoes are near subduction zones. Transform Boundaries The third type of plate boundary is the transform boundary, where two tectonic plates slide past each other. The San Andreas Fault that extends from Mexico to northern California is a transform boundary at the meeting of the Pacific and North American tectonic plates (Figure 2.21). Here, the Pacific plate is moving northwest relative to the North American plate at a rate of about 34 millimeters (1.33 inches) per year; the stresses that are involved account for the large number of earthquakes that occur in California. Other transform boundaries include the Alpine Fault in New Zealand, which marks the meeting of the Pacific and Australian plates, and the North Anatolian Fault in Turkey. Both areas experience large numbers of earthquakes. The magnitude 7.0 earthquake that devastated Haiti in 2010 occurred along a transform boundary. The Caribbean plate is a smaller plate with two

CHAPTER 2: PLATE TECTONICS

FIGURE 2.22 Convection warms

this living area as air currents carry heat from the fireplace to other parts of the room. Photo: Shutterstock 70250692, credit photobank.ch.

large plates moving around it—the North American and South American plates. The resulting stress and strain were enough to cause the earthquake in Haiti.

2.3.3

Plate Movement

The identification of tectonic plates and the discovery of a mechanism for their movement explained how continents once joined together could now be separated by thousands of miles of ocean. However, what has yet not been determined with certainty is the source of the energy driving the movement. One likely explanation is the presence of convection cells in the mantle. Mantle Convection Cells Convection is a circular movement that occurs when a substance is heated and subsequently cooled. Warmer substances have less density and so they are more buoyant; consequently, they tend to rise. Cooler substances are denser and less buoyant; they sink. If a fluid substance, like air or water—or, in this case, magma—is heated, it rises away from the source of the heat, where it then cools, becomes denser, and sinks. As it moves, other fluid moves in to take its place and undergoes the same heating and cooling. This process of rising and sinking causes a circular current, called a convection cell. You may have noticed convection cells in boiling water or sauces, or even have used a convection oven. Convection also carries heat through homes and offices during cold weather

FIGURE 2.23 The energy needed to move tectonic plates may come from convection cells within the asthenosphere. Three possible mechanisms for transferring the energy are ridge push, mantle drag, and slab pull. Illustration by John J. Renton.

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S ee I t S idebar JOSHUA TREE NATIONAL PARK

Inselbergs at Hidden Valley Campground display the rectangular joint system that is prevalent in the White Tank monzogranite. Photo: Shutterstock 35119738, credit Andrew F. Kazmierski; Map: Shutterstock 15180868, credit Map Resources.

Joshua Tree National Park lies at the eastern end of a series of East-West trending mountains in California called the Transverse Ranges. The earliest rocks in the park are around 1.7 billion years old. These rocks are metamorphic rocks that were caught up in the Mesozoic tectonic arc (about 65 million years ago) along the Pacific margin of California. Igneous granite plutons rose up through these older rocks created by the subduction of oceanic crust under the North American continent. That continental collision has, of course, ceased, but the rocks that were created then still exist and are widely scattered across California as the plate tectonic environment has changed.

that few other national parks can demonstrate. They are plutonic (intrusive) rocks formed by the Mesozoic subduction off California and are the deep basement rocks that intruded under the Andesitic volcanoes of that time. The ending of subduction and the onset of a plate boundary based transformation and extensional motion has thinned the crust and brought these rocks to the surface. The entire Joshua Tree National Park has had these plutonic bodies unroofed by erosion that continues to cause crustal stretching and clockwise block rotation because of the park’s close proximity to the plate boundary.

The most famous rocks of the Joshua Tree National Park are the granite hills and plutonic boulders of the Jumbo Rocks area. The rocks are tied to plate tectonics in ways

Joshua Tree National Park is named after the magnificent strands of Joshua trees that are unique to the Mojave Desert. It is this harsh desert climate that has polished

CHAPTER 2: PLATE TECTONICS

N ational P arks

Skull Rock at Jumbo Rocks Campground illustrates cavernous weathering and undercutting by subsoil notching.

Photo: Shutterstock 34733728,

credit Hanka B.

nearly all the rock surfaces with some degree of desert varnish, a thin layer of insoluble clay, plus iron and manganese oxides that creates a metallic-looking rind on most of the exposed rocks.

The northern end of the Salton Trough as seen from the Keys View in Joshua Tree National Park. The high peak in the distance is Mount San Jacinto, which rises 10,786 feet in elevation. The communities of Palm Springs and Cathedral City are located at the base of the mountains. The Indio Hills, running from left to right in the middle distance, mark an uplifted block wedged between two branches of the San Andreas Fault. Photo: Shutterstock 33024874, credit Caitlin Mirra.

There is so much going on geologically on the surface of this harsh habitat that it is easy to sometimes forget that this desert owes its existence to ocean plate boundary tectonics and volcanic activity. But anytime one forgets the region’s briny past or superheated volcanic depths hiding beneath the surface of the Mojave Desert, remember the Salton Trough. The Salton Trough is a linear rift valley within the continental crust, and the normal faults are nearly parallel to each other on opposite sides of the Salton Sea. Because of the crustal thinning and overlying sedimentation, magma has moved near the surface and is the source of the heat that feeds numerous hot springs in the area. The hot springs are located in a line on the eastern side of the Salton Sea extending from Desert Hot Springs (just outside the boundaries of Joshua Tree National Park) south into Northern Mexico. Joshua Tree National Park is a favorite of extreme See It geology fans.

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SLAB PULL

A proposed plate tectonic mechanism in which subducting crust pulls the plate along behind it. MANTLE PLUME

A column of abnormally hot rock rising from the mantle. TRIPLE JUNCTION

Three fractures in the lithosphere that occur as rifting begins. RIFT VALLEY

A narrow, linear depression caused by rifting in continental crust. LINEAR OCEAN

A long, narrow arm of the ocean formed when the ocean breaches a widening rift valley. FAILED ARM

A fracture at a triple junction that fails to become a fullfledged rift zone.

with warm air near a fireplace or vent rising, then losing energy near the ceiling, and then dropping back down to be heated again (Figure 2.22). Geologists suspect that there are many convection cells inside the earth that form when rock heated deep within Earth rises at the rift zones, loses energy as it moves along under the lithosphere, then drops back into the earth’s interior at subduction zones. The moving rock of the convection cell has been compared to a conveyor belt within the asthenosphere upon which move the cooler, more brittle tectonic plates. However, there is not yet a scientific consensus about the exact mechanisms involved and the extent to which the convection cell moves the plates or vice versa. Three possible mechanisms have received the most attention in recent years: mantle drag, ridge push, and slab pull (Figure 2.23). Mantle drag is described as a frictional force exerted on a tectonic plate by the moving magma in the convection cell below the plate. As the magma moves, it would literally drag the plate above it along with it. Ridge push is a two-fold force that occurs when new oceanic crust is formed at the rift zone. First, the emergence of magma that then solidifies would push the older crust out of the way to make room for the newly formed crust. Second, the new oceanic crust rides high on the asthenosphere, which forms the oceanic ridge. Here, the force of Earth’s gravity would cause the crust to, in effect, slide downhill, pushing the older oceanic crust ahead in the same way that pushing on the end car of a train would push all the others ahead of it. The third possible mechanism, slab pull, also is the result of gravity. In this case, the cooler, denser, heavier old crust at the subduction zones sinks into the asthenosphere, pulling the younger crust behind it. Whether tectonic plates move as the result of mantle drag, ridge push, or slab pull is still under discussion. These forces may work separately, or together, or perhaps there is some new mechanism at work that is yet to be discovered. Either way, mantle convection cells offer an explanation of how the supercontinent of Pangaea not only came together but also broke apart hundreds of millions of years later. Rift Valleys Mantle convection cells under oceanic lithosphere help to explain the conveyor belt of oceanic crust, which forms at the rift zones and disappears in the subduction zones. But what happens if convection cells appear in the midst of an existing plate? Models indicate that the forces involved with such convection cells would stretch continental crust, eventually breaking it to form a new rift zone. In fact, this is actually occurring several places on Earth, although there is not universal agreement as to the mechanism that initiates

FIGURE 2.24 The formation of three fracture zones, called a triple junction, signals the initial splitting of the lithosphere above a mantle plume. With time, some of the fractures will widen to form a divergent plate boundary that will eventually open to form an ocean basin. The third fracture zone, called a failed arm, may not progress beyond a rift valley. Illustration by John J. Renton.

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FIGURE 2.25 Satellite (left) and map (right) views of the triple junction in east Africa. The Red Sea, the Gulf of Aden, and the East African Rift Valley are the three fracture zones of this triple junction. Of the three, the East African Rift Valley appears to be the failed arm, while the Gulf of Aden and the Red Sea are linear oceans formed by diverging plate margins. These bodies of water will eventually become an ocean basin when Africa becomes completely detached from the northern continental mass. Photo: Courtesy of Image Science Analysis Laboratory, NASA-Johnson Space Center, “The Gateway to Astronaut Photography of Earth”; Map: © Kendall Hunt Publishing Company.

the rifting. Some geologists have suggested that when continents remain stationary for long periods, heat accumulates beneath the continental lithosphere and initiates the development of mantle plumes, upwellings of magma from the mantle. As the lithosphere begins to lift as the result of the heat and pressure of either a convection current or mantle plume, a three-pronged fracture called a triple junction develops in the continental crust. This triple junction is not unlike the fractures that develop in the surface of a muffin or cupcake during the baking process. According to the theory, two of the fractures propagate laterally and join similar fractures generated at adjacent hot spots to create a rift zone that opens and causes the continent to break apart (Figure 2.24). As rifting continues, the linear zone develops into a rift valley which floods when one end of the valley depression reaches the edge of the continent. Once seawater begins to fill the valley, it forms a linear ocean. When the landlocked end of this linear ocean is finally breached, the linear ocean becomes an opening ocean. The third fracture, called a failed arm, usually does not progress beyond the development of a rift zone or rift valley. FIGURE 2.26 The Rio Grande River runs through the Rio Grande Rift Zone (map, right), which extends from Leadville, Colorado, to south of El Paso, Texas. The Rio Grande Gorge (photo, left) lies in a segment of the rift zone near Taos, New Mexico. Photo: Shutterstock 56626609, credit ejwhite; Map: © Kendall Hunt Publishing Company.

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Figure 2.25 shows an example of a triple junction where two fractures opened to form the Gulf of Aden and the Red Sea, resulting in the separation of Africa and the Arabian Peninsula, while what may be a failed arm became the East African Rift Valley. However, there are two pieces of evidence indicating that the East African Rift is not failed, but instead is still active. First, the valley is the site of active volcanism with one of Earth’s best-known volcanoes, Mount Kilimanjaro, being located within the rift. Second, as seawater has begun to make its way into the northern end of the valley through fractures that connect the valley and the Red Sea, inland saltwater lakes grow larger daily. North America has its own rift valleys. About 30 million years ago, a rift zone formed in North America along the easternmost portion of the Basin and Range Province. Called the Rio Grande Rift Zone after the Rio Grande River that follows a portion of the zone, this rift extends more than 1,000 km (620 miles) from Chihuahua, Mexico, to Leadville, Colorado (Figure 2.26). While the most recent volcanic eruption along the zone occurred about 5 million years ago, rifting persists today but at a slower rate. FIGURE 2.27 Although geologists do not universally agree on the mechanism that initiates rifting, they do agree that the divergence of plates is the result of stretching forces within the lithosphere. Illustration by John J. Renton.

Another North America rift zone lies in the Gulf of California, which is a linear ocean. This rift runs well into Southern California. An ancient, failed rift extends from northeastern Kansas through Iowa, Minnesota, and Upper Michigan, and then drops down to central Michigan. Lake Superior lies within what was once a rift valley. Had the rift successfully split the continent, North America might have had a very different history. HOT SPOT

A location in the lithosphere where a mantle plume is near the surface.

Another theory to explain the breakup of continents proposes that the continents rift along zones of weakness in the continental lithosphere caused by past tectonic activity or as the plate moves over a mantle plume. The movement over the hot spot not only results in the chains of volcanoes that are characteristic of rifting, but also weakens the plate. The weakened lithosphere makes the plate conducive to rifting by subsequent mantle convection. At the

CHAPTER 2: PLATE TECTONICS

FIGURE 2.28 The movement of the Pacific plate over stationary hot spots produces chains of volcanoes, such as the Hawaiian Islands. Illustration © Kendall Hunt Publishing Company.

present time, it is not possible to determine which, if either, of these theories is correct. While we lack evidence as to which explanation is responsible for the initial rifting, the breakup of the continents is the result of stretching forces that develop within the continental lithosphere. Figure 2.27 illustrates how the process of rifting proceeds. Hot Spots While many of Earth’s earthquakes and volcanoes have been explained by plate tectonics, there are other features and processes for which plate tectonics has not provided a complete explanation. Active volcanic activity far from the plate edges, such as that in the Hawaiian Islands, is one such mystery. The Hawaiian Islands are older in the northwest and younger in the southwest; in fact, all of Hawaii’s active volcanoes are on or near the southernmost island. The source of the magma that formed them seems to be from a mantle plume that has nothing to do with the processes responsible for plate tectonics. The plume, called a hot spot, terminates at the top of the asthenosphere, and does not move with a lithospheric plate, but is FIGURE 2.29 Volcanic chains in the Pacific Ocean show similar orientations indicating the direction the Pacific plate is moving. Illustration © Kendall Hunt Publishing Company.

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S ee I t S idebar

WRANGELL-ST. ELIAS NATIONAL PARK AND PRESERVE

With a height of 18,008 feet, Mount St. Elias towers over the landscape as seen from Icy Bay. Photo: Shutterstock 13960162, credit Mariusz S. Jurgielewicz; Map: Shutterstock 15180868, credit Map Resources.

Alaska is often described as having one of the most rugged terrains on Earth. From a geologic perspective it is the terranes that make Wrangell-St. Elias National Park and Preserve one of the most significant national parks for the study of plate tectonics. Geologists use the word terrain for its common meaning of general landscape characteristics or “lay of the land.” Pronounced the same, but spelled differently, the term terrane refers to a block of crustal rock (having surface dimensions of tens to a few thousand kilometers) with a unique sequence of rock units that have resulted from its particular geologic history.

One terrane might have massive granites at its base overlain by a thick sequence of carbonate sedimentary rocks from the Paleozoic Era (about 250 million years ago) with warm-climate fossils and an uppermost zone of basalt flows interbedded with terrestrial sediments. Another terrane might consist of oceanic crust. Finding such dissimilar rock sequences on adjacent ridges inside Wrangell-St. Elias National Park and Preserve leaves field geologists with a significant problem except when one accounts for the differences in the blocks by knowing that the terranes originated in different parts of the globe and had completely different geologic histories until they

N ational P arks

“drifted” via the mechanism of plate tectonics to collide with, and accrete to, North America. Such terranes apparently existed either as microplates or as islands of continental materials within the upper part of an oceanic plate, moving toward and subducting beneath, the North American continental plate. When they collided with North America, the terranes were “scraped off” and attached to the main continental mass. It now appears that a wide strip of western North America, including Wrangell-St. Elias National Park and Preserve, is made up of accreted terranes. Wrangell-St. Elias National Park and Preserve, which is located in south central and southeastern Alaska, is by far the largest unit in the National Park System; it is larger than several of the smaller states, and, in conjunction with the adjacent Kluane National Park of Canada, is larger in size than all of the national parks in the other 49 states and American Samoa. Wrangell-St. Elias National Park and Preserve is located in one of the world’s most active plate tectonic zones, where the Pacific plate is being subducted beneath North America and the Yakutat terrane and is accreting to North America. The associated volcanic belts, active faults, frequent earthquakes, and high mountains are expected results of the high heat flows and stresses in crustal rocks of this plate tectonic settling. The major zone of volcanoes is in the Wrangell Mountains, with Mount Wrangell (14,163 feet), one of the world’s largest steep volcanic cones made of the igneous rock, andesite, being the park’s only active cone. Its last reported eruption was in 1930. Signs of more recent activity include an increase in the heat flow near the summit of Mountt Wrangell. Presently, there are a

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number of inactive volcanoes made up of Quaternary lavas, such as Mount Bona, Mount Blackburn, and Mount Sanford, all of which are more than 16,000 feet in height. If volcanoes and active plates aren’t enough to create geologic interest, Wrangell-St. Elias National Park and Preserve also boasts a past history of earthquakes and a present full of glaciers and ice. Some of the world’s strongest earthquakes have occurred in this area. In September of 1899, two earthquakes of Richter magnitudes 8.3 and 8.6 were recorded with epicenters near the city of Yakutat. The several earthquakes with magnitudes in the 7 and 8 range that have occurred in the area during the past century give ample evidence that the Yakutat microplate is actively colliding with, and accreting to, the North American continent. Relatively warm waters of the Japanese current wash the shores of southern Alaska. The air masses moving over these warm currents take up water vapor until they become completely saturated. As these air masses move inland, the high mountains force them to rise. While the air is rising, it cools and releases precipitation, mostly in the form of snow. Much of the snow is converted to ice, with the result that the Wrangell, St. Elias, and Chugach mountain ranges have the largest concentration of glaciers in North America. A particularly interesting aspect of the glaciers in this area is their high level of activity. Many of them sustain high rates of flow. Additionally, many of the glaciers in this area tend to undergo periodic surges. These surges are times of rapid advance that last from a few weeks to a year or more and are usually followed by rapid meltback.

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fixed, and more or less continuously provides magma that erupts to produce volcanoes. The Hawaiian Islands and a string of underwater volcanoes called the Emperor Seamounts were formed as the Pacific plate passed over such a stationary hot spot (Figure 2.28). The fact that other chains of islands in the Pacific Ocean have similar orientations strongly suggests the hot spots that formed the chains have not moved relative to each other (Figure 2.29), but have remained in the same location as the Pacific plate has passed over them. Meanwhile, the volcanic chains themselves provide evidence that the plates do move. Not all hot spots are under ocean basins. Beginning at the southwestern end of the Snake River Valley in southern Idaho and extending northeastward, there are a series of overlapping, basalt-filled areas that formed following enormous volcanic eruptions. This indicates that North America has moved over a hot spot that is now located under Yellowstone Park). The geology of Yellowstone Park is the result of three massive volcanic eruptions triggered by the underlying hot spot. With all we know about hot spots, however, there is still much that is unknown. For example, once a hot spot forms, it appears to remain stationary for its entire life, even though the mantle plume that provides the heat to create the hot spot magma is passing through the mobile asthenosphere. Why is the plume not affected by convection within the asthenosphere? Also, for some unknown reason, most of the active hot spots are located under ocean basins. Perhaps someday, we will have answers for all these yet unexplained hot spot characteristics.

WILSON CYCLE

A plate tectonics cycle in which moving continents join to become supercontinents, separate to move apart, then once again move together to form a new supercontinent.

2.3.4

The Supercontinent Cycle

Plate tectonics and its mechanisms supported Wegener’s idea that all of the current continents were originally combined into a supercontinent called Pangaea, surrounded by a single superocean. Was Pangaea the only supercontinent to ever form during Earth’s 4.5-billion-year history? Such a possibility would essentially make its formation a unique event, an occurrence that would go against the concept of uniformitarianism. The Wilson Cycle J. Tuzo Wilson (1908–1993), a Canadian geologist who played a major role in defining plate tectonics, proposed the idea that supercontinents are part of a natural cycle that has been going on since the onset of modern plate movements. In his honor, the cycle is called the Wilson cycle. According to Wilson, supercontinents form and, after existing for a period of time, break up to form a number of continents separated by newly formed oceans that widen at the expense of the once single superocean. Following the breakup, the cycle enters an opening phase during which the continents move apart and the ocean basins widen. After a period of about 250 million years, the opening phase of the cycle comes to an end, and the cycle enters a closing phase as zones of subduction consume oceanic lithosphere faster than it is created, closing the oceans and causing the continents to move closer together. After another period of about 250 million years, all of what were newly formed oceans close again and the continents collide to form another supercontinent, completing the cycle. The cycle then repeats. In fact, there is evidence of another supercontinent that existed prior to Pangaea. Rodinia is a suggested supercontinent that began to form 1.2 billion years ago, and broke up some 750 million years ago. Today, although technically we are still in the opening phase of the present Wilson cycle, some plate collisions are already occurring. The convergence of India and Asia has already resulted in a plate collision that has created the Himalayan range of mountains (which will be further

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discussed in Chapter 3). Similarly, the northward movement of the Arabian plate against the Iranian plate has resulted in the formation of the Zagros Mountains; and the Alps continue to rise as the African plate approaches from the south to converge with the Eurasian plate. While the Atlantic Ocean continues to open at the rate at which human fingernails grow, the Arctic Ocean appears to have reached its maximum area, as has the Indian Ocean. In fact, the presence of subduction zone between Australia and Asia indicates that the easternmost portion of the Indian Ocean has already begun to close (see Figure 2.15 on page 80). The Pacific Ocean is a closing ocean, and in fact, one tectonic plate, the Farallon plate, has already almost completely subducted under the North American continent. All that remains of this major plate are two remnants, the Cocos plate just west of Mexico and the Juan de Fuca plate off the coast of Washington, Oregon, and British Columbia. As to what lies ahead, according to Wilson, in another 50 million years, the Atlantic, Arctic, and Indian oceans will all be closing with the creation of a new supercontinent predicted to occur about 300 million years from now. This cycle will continue as long as Earth retains enough internal energy for divergent plate boundaries to break apart and plate subduction reclaims old oceanic crust.

KEY TERMS bathymetry (p. 74)

hot spot (p. 90)

opening ocean (p. 83)

Suess, Edward (p. 69)

catastrophism (p. 68)

Laurasia (p. 71)

paleomagnetism (p. 76)

tectonic plate (p. 81)

continental drift (p. 71)

linear ocean (p. 88)

Pangaea (p. 71)

transform boundary (p. 84)

contracting earth model (p. 69)

magma (p. 74)

transform fault (p. 83)

convection (p. 84)

magnetic alignment (p. 76)

plate tectonic theory (p. 68)

convection cell (p. 84)

magnetic polarity (p. 78)

convergent boundary (p. 83)

magnetic reversal (p. 78)

deep-sea trench (p. 76)

magnetic zonation (p. 78)

divergent boundary (p. 81)

magnetometer (p. 76)

expanding earth model (p. 69)

mantle drag (p. 84)

failed arm (p. 88)

mantle plume (p. 88)

Gondwana, Gondwanaland (p. 69)

oceanic ridge (p. 74)

polar wandering (p. 78) ridge push (p. 84) rift (p. 74) rift valley (p. 88) seafloor spreading (p. 78) slab pull (p. 88) sonar (p. 74) subduction (p. 83)

triple junction (p. 88) uniformitarianism (p. 68) Wegener, Alfred (p. 69) Wilson cycle (p. 94)