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Tall mountains created by tectonic uplift. Tall coastal mountains such as these in Glacier Bay National Park in southeastern Alaska have been uplifted by plate tectonic processes, creating a large amount of relief. Some of the uplifted rocks here have come from distant areas and include parts of the sea floor. M02_TRUJ3545_12_SE_C02.indd 38 16/12/15 3:49 AM 2 Before you begin reading this chapter, use the glossary at the end of this book to discover the meanings of any of the words in the word cloud Plate Tectonics above you don’t already know. and the Ocean Floor ach year at various locations around the globe, several thousand earthquakes Eand dozens of volcanic eruptions occur, both of which indicate how remarkably ESSENTIAL LEARNING CONCEPTS dynamic our planet is. These events have occurred throughout history, constantly At the end of this chapter, you should be able to: changing the surface of our planet, yet only a little over 50 years ago, most scientists believed the continents were stationary over geologic time. Since that time, a bold 2.1 Evaluate the evidence that supports new theory has been advanced that helps explain surface features and phenomena continental drift. on Earth, including: 2.2 Summarize the evidence that supports plate tectonics. • The worldwide locations of volcanoes, faults, earthquakes, and mountain building • Why mountains on Earth haven’t been eroded away 2.3 Discuss the origin and characteristics of features that occur at plate boundaries. • The origin of most landforms and ocean floor features 2.4 Show how plate tectonics can be used as a • How the continents and ocean floor formed and why they are different working model. • The continuing development of Earth’s surface 2.5 Describe how Earth has changed in the past • The distribution of past and present life on Earth and predict how it will look in the future. This revolutionary new theory is called plate tectonics (plate = plates of the lithosphere; tekton = to build), or “the new global geology.” According to the theory of plate tectonics, the outermost portion of Earth is composed of a patchwork of thin, rigid plates1 that move horizontally with respect to one another, like icebergs “It is just as if we were to refit the torn floating on water. As a result, the continents are mobile and move about on Earth’s pieces of a newspaper by matching their surface, controlled by forces deep within Earth. edges and then check whether the lines of The interaction of these plates as they move builds features of Earth’s crust print run smoothly across. If they do, there (such as mountain belts, volcanoes, and ocean basins). For example, the tallest is nothing left but to conclude that the mountain range on Earth is the Himalaya Mountains that extend through India, pieces were in fact joined in this way.” Nepal, and Bhutan. This mountain range contains rocks that were deposited millions of years ago in a shallow sea, providing testimony of the power and persistence —Alfred Wegener, The Origins of of plate tectonic activity. Continents and Oceans (1915) Plate tectonics is extensively supported by data from a variety of sciences, including geological, chemical, physical, and biological sources. Yet it wasn’t accepted by many scientists when it was first introduced. In fact, it is a classic example of the process of the scientific method: how a seemingly implausible hypothesis, when faced with a preponderance of evidence to support it, developed into a theory that now forms the basis of our understanding of fundamental Earth processes. 1These thin, rigid plates are pieces of the lithosphere that comprise Earth’s outermost layer and contain oceanic and/or continental crust, as described in Chapter 1. 39 M02_TRUJ3545_12_SE_C02.indd 39 16/12/15 3:49 AM 40 CHAPR TE 2 Plate Tectonics and the Ocean Floor 2. 1 What Evidence Supports Continental Drift? Alfred Wegener (Figure 2.1), a German meteorologist and geophysicist, was the first to advance the idea of mobile continents in 1912. He envisioned that the continents were slowly drifting across the globe and called his idea continental drift. Let’s examine the evidence that Wegener compiled that led him to formulate the idea of drifting continents. Fit of the Continents The idea that continents—particularly South America and Africa—fit together like pieces of a jigsaw puzzle originated with the development of reasonably accurate world maps. As far back as 1620, Sir Francis Bacon wrote about how the continents appeared to fit together. However, little significance was given to this idea until 1912, when Wegener used the shapes of matching shorelines on different continents as a supporting piece of evidence for continental drift. Wegener suggested that during the geologic past, the continents collided to form a large landmass, which he named Pangaea (pan = all, gaea = Earth) (Figure 2.2). Further, a huge ocean, called Panthalassa (pan = all, thalassa = sea), surrounded Pangaea. Panthalassa included several smaller seas, including the Tethys Sea (Tethys = a Greek sea goddess). Wegener’s evidence indicated that as Pangaea began to split apart, the various continental masses started to drift toward their present geographic positions. Wegener’s attempt at matching shorelines revealed considerable areas of crustal overlap and large gaps. Some of the differences could be explained by material deposited by rivers or eroded from coastlines. What Wegener didn’t know at the time was that the shallow parts of the ocean floor close to shore are underlain by materials similar to those beneath continents. In the early 1960s, Sir Edward Bullard and two associates used a computer program to fit the continents together (Figure 2.3). Instead of using the shorelines of the continents as Wegener had done, Bullard achieved the best fit (for example, with minimal overlaps or gaps) by u sing a depth of 2000 meters (6560 feet) below sea level. This depth corresponds to halfway between the shoreline and the deep-ocean basins; as such, it represents the true edge of the continents. By using this depth, the continents fit together remarkably well. Matching Sequences of Rocks and Mountain Chains If the continents were once together, as Wegener had hypothesized, then evidence should appear in rock sequences that were originally continuous but are now separated by large distances. To test the idea of drifting continents, geologists began comparing the rocks along the edges of continents with rocks found in adjacent positions on matching continents. They wanted to see if the rocks had similar types, ages, and structural styles (the type and degree of deformation). In some areas, younger rocks had been deposited during the millions of years since the continents separated, covering the rocks that held the key to the past history of the continents. In other areas, the rocks had been eroded away. Neverthe- Figure 2.1 Alfred Wegener, circa 1912–1913. Alfred Wegener (1880–1930), shown here in his research station in Greenland, less, in many other areas, the key rocks were present. developed the idea of continental drift. He was one of the first Moreover, these studies showed that many rock sequences from one continent scientists to use multiple lines of evidence to suggest that were identical to rock sequences on an adjacent continent—although the two were continents are mobile. separated by an ocean. In addition, mountain ranges that terminated abruptly at M02_TRUJ3545_12_SE_C02.indd 40 16/12/15 3:49 AM 2.1 What Evidence70°N Supports Continental70°N Drift? 41 60°N 60°N 50°N 50°N 40°N 50˚ 70°N 70°N 40°N 60°N 60°N 50˚ 40˚ the edge of a continent continued on another continent across an ocean basin, with 30°N 50°N NORTH EUROPE ASIA 50°N 40˚ 30°N 30˚ 50˚ identical rock sequences, ages, and structural styles. Figure 2.4 shows, for example, 40°N AMERICA 50˚ 40°N30˚ 20°N 40˚ 20°N 30°N NORTH EUROPE ASIA 40˚ 30°N20˚ 30˚ AFRICA how similar rocks from the Appalachian Mountains in North America match up with 10°N 10˚ AMERICA 30˚ 10˚ 20°N 20°N 140˚ 120˚ 100˚ 40˚ 20˚ 0˚ 140˚ 160˚ 180˚ 60˚ 80˚ 20˚ identical rocks from the British Isles and the Caledonian Mountains in Europe. AFRICA 10°N 10˚ SOUTH 10˚ 10°S 10˚ AMERICA 10˚ 140˚ 120˚ 100˚ Wegener noted the similarities in rock sequences on both sides of the Atlantic 40˚ 20˚ 0˚ 140˚ 160˚ 180˚ 60˚ 80˚ 20˚ 20°S 20˚ SOUTH AUSTRALIA 20°S and used the information as a supporting piece of evidence for continental drift. 10°S 10˚ 10˚ 30˚ AMERICA 30˚ 30°S 30°S20˚ He suggested that mountains such as those seen on opposite sides of the Atlantic 20°S 20˚ 40˚ AUSTRALIA 20°S 40°S 50˚ 50˚ 40°S30˚ 30˚ 60˚ formed during the collision when Pangaea was formed. Later, when the continents 50°S 60˚ 50°S 30°S 40˚ 70˚ ANTARCTICA 30°S 60°S 60°S 40°S 50˚ 50˚ 40°S split apart, once-continuous mountain ranges were separated. Confirmation of 70°S 70°S 60˚ 50°S 60˚ 50°S 70˚ ANTARCTICA this idea exists in a similar match with mountains extending from South America (a) The positions60°S of the continents today. 60°S 70°S 70°S through Antarctica and across Australia. (a) The positions of the continents today. 60˚ 60˚ 50˚ 50˚ Glacial Ages and Other Climate Evidence 40˚ 60˚ 40˚ 60˚ 30˚ 50˚ EURASIA Wegener also noticed the occurrence of past glacial activity in areas that are now tropi- 50˚ 30˚ 40˚ NORTH AMERICA 40˚ 20˚ cal and suggested that it, too, provided supporting evidence for drifting continents.

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