Plate Tectonics

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Plate tectonics

Anyone who looks carefully at a map of the Atlantic Ocean is likely to be struck by an interesting point. The eastern coastline of South America bears a striking similarity to the western coastline of Africa. Indeed, it looks almost as if the two continents could somehow fit together--provided, of course, one could find a way to slide the land masses across the ocean floor.

The match of coastlines was noticed by scholars almost as soon as good maps were first available in the late fifteenth century. In 1620, for example, the English philosopher Sir Francis Bacon commented on this fact and argued that the match was "no mere accidental occurrence."

One obvious explanation for the South America-Africa fit was that the two continents had once been connected and had somehow become separated in the past. A few early scientists tried to use the Biblical story of The Flood to show how that might have happened. But as Biblical explanations for natural phenomena began to lose credibility, this approach was discarded.

That made the concept of moving continents even more difficult to believe. The earth's crust was generally thought to be solid and immoveable. How could one or two whole continents somehow slide through such a material?

Yet, over time, more and more evidence began to accumulate, supporting the notion that South America and Africa might once have been joined to each other. Much of that early evidence came from the research of the German geographer, Alexander von Humboldt. Von Humboldt spent a number of years traveling through South America, Africa, and other parts of the world. In his journeys, he collected plant and animal specimens and studied geological and geographic patterns. He was struck by the many similarities he observed between South America and Africa, similarities that went far beyond an obvious geographic fit of continental coastlines.

For example, he observed that mountain ranges in Brazil that end at the sea appear to match other mountain ranges in Africa that began at the coastline. He noted similar patterns among mountain ranges in Europe and North America.

During the nineteenth century, similarities among fauna and flora on either side of the Atlantic were observed. Although species in eastern South America do differ to some extent from those in western Africa, their similarities are often striking. Before long, similarities across other oceanic gaps began to be noted. Plant and animal fossils found in India, for example, were often remarkably similar to those found in Australia .

Attempts to explain the many similarities in various continental properties were consistently stymied by the beliefs that the earth's crust was solid and immoveable. One way around this problem was the suggestion that mammoth land bridges existed between continents. These large bridges would have allowed the movement of plants and animals from one continent to another. But no evidence for such bridges could be found, and this idea eventually fell into disrepute.

By the mid-1850s, an important breakthrough in geological thought began to occur. A few geologists started to accept the hypothesis that the earth's crust is not as solid and immovable as it appears. In fact, they said, it may be that the earth's outer layer is actually floating (and, thus, is moveable) on the layer below it, the mantle.

Still, it was not until the early years of the twentieth century that a new theory of "floating continents" was seriously proposed. Then, in a period of less than four years, two distinct theories of this kind were suggested. The first was offered in a December 29, 1908, paper by the American geologist Frank B. Taylor. Taylor outlined a theory that described how the continents had slowly shifted over time with a "mighty creeping movement."

Taylor's paper met largely with indifference. Such was not the fate, however, of the ideas of a German astronomer and meteorologist, Alfred Wegener. While browsing through the University of Marburg library in the fall of 1911, Wegener was introduced to the problem of continental similarities. Almost immediately, he decided to devote his attention to this question and began a study that was to dominate the rest of his professional life and to revolutionize the field of geology.

By January 1912, Wegener had developed a theory to explain continental similarities. Such similarities cannot be explained by sunken land bridges, he said, but are the result of continents having moved slowly across the face of the planet. Three more years of research were needed before Wegener's theory was completed. In 1915, he published The Origins of Continents and Oceans, summarizing his ideas about continental similarities.

According to Wegener's theory, the continents were once part of one large land mass, which he called Pangaea. Eventually this land mass broke into two parts, two super-continents, which he called Gondwanaland and Laurasia. Over millions of years, Gondwanaland broke apart into South America, Africa, India, Australia, and Antarctica , he suggested, while Laurasia separated into North America and Eurasia.

The basic problem Wegener faced was to explain how huge land masses like continents can flow. His answer was that the materials of which the earth's crust is made are of two very different types. One, then call "sial," is relatively light, but strong. The other, then called "sima," can be compared to very thick tar. Continents are made of sial, he said, and sea floors of sima. The differences in these materials allows continents to "ride" very slowly across sea floors.

Wegener's theory was met with both rejection and hostility. Fellow scientists not only disagreed with his ideas, but also attacked him personally even for suggesting the ideas. The theory of continental drift did not totally disappear as a result of these attitudes, but it fell into disfavor for more than three decades.

Research dating to the mid-1930s revealed new features of the sea floor which made Wegener's theory more plausible. Scientists found sections of the ocean bottoms through which flows of hot lava were escaping from the mantle, somewhat like underwater volcanoes. These discoveries provided a crucial clue in the development of plate tectonics, the modern theory of continental drift.

According to the theory of plate tectonics, the upper layer of the earth is made of a number of plates, large sections of crust, and the upper mantle. About ten major plates have been identified. The largest plate is the Pacific Plate, underlying the Pacific Ocean. The North and South American, Eurasian, African, Indian, Australian, Nazca, Arabian, Caribbean, and Antarctica are the other major plates.

Scientists believe that plates rest on an especially plastic portion of the mantle known as the "asthenosphere." Hot magma from the asthenosphere seeps upward and escapes through the ocean floor by way of openings known as rifts. As the magma flows out of the rift, it pushes apart the plates adjoining the rift. The edge of the plate opposite the rift is ultimately forced downward, back into the asthenosphere. The region in which plate material moves down into the mantle is a trench.

Plates move at different speeds in different directions at different times. On an average, they travel 0.4–2 in (1-5 cm) per year. To the extent that this theory is correct, a map of the earth's surface ten million years from now will look quite different from the way it does now.

The theory of plate tectonics explains a number of natural phenomena that had puzzled scientists for centuries. Earthquakes, for example, can often be explained as the sudden, rather than gradual, movement of two adjacent plates. One of the world's most famous earthquake zones, the San Andreas Fault, lies at the boundary of the Pacific and the North American plates. Volcanoes often accompany the movement of plates and earthquakes. The boundaries of the Pacific Plate, for example, define a region where volcanoes are very common, a region sometimes called the Ring of Fire.

Plate tectonics is now accepted as one of the fundamental theories of geology. Its success depends not only on the discovery of an adequate explanation for continental movement (sea-floor spreading, rifts, and trenches), but also on the discovery of more and more similarities between continents.

For example, modern geologists have re-examined Humboldt's ideas about the correlation of mountain ranges in South America and Africa. They have found that rock strata, or layers, in Brazil lie almost exactly where they should be expected if strata in Ghana are projected to the west. If the Atlantic Ocean could somehow be removed, the two strata could be made to coincide almost perfectly.

Some interesting data have come from studies of paleomagnetism also. Paleomagnetism refers to the orientation of iron crystals in very old rock. Since the earth's magnetic poles have shifted over time, so has the orientation of iron crystals in rock. Scientists have found that the orientation of iron crystals in one continent correspond very closely to those found in rocks of another continent thousands of miles away.

Fossils continue to be a crucial way of confirming continental drift. For example, scientists have learned that the fossil population of is much less like the fossil population of Africa (300 miles [483 km] to the west) than it is like the fossil population of India (3,000 miles/4,830 km to the northeast). This finding would make almost no sense at all unless one recognized that the theory of continental drift has Madagascar breaking off from India millions of years ago, not off Africa.

The flow of magma out of the asthenosphere often results in the formation of ores, regions that are rich in some mineral. For example, the high temperatures characteristic of a rift or a trench may be sufficient to cause the release of metals from their compounds. The flow of magma across rock may also result in a phenomenon known as contact metamorphism, a mechanism that also results in the formation of metals such as lead and silver.

[David E. Newton ]