Source: A Science Odyssey Web site
The theory of plate tectonics has come a long way since Alfred Wegener first proposed his theory of continental drift. Geologists now have strong evidence to show not only that tectonic plates have moved and are continuing to move, but also to describe what happens when they meet. This interactive activity adapted from A Science Odyssey Web site illustrates what happens at the three types of boundaries where plates meet.
Alfred Wegener based his theory of continental drift on a varied collection of evidence that showed striking similarities between continents separated by vast distances. Wegener's theory began with his and others' observations that the coastlines of continents on opposite sides of the Atlantic appear as if they would fit neatly against each other like the pieces of a puzzle. Wegener bolstered his theory between 1912 and his death in 1930 with fossil evidence that showed that the same species of plants and animals had existed in both South America and Africa. He also noted the occurrence of fossils and rock types in North America that matched those in Europe, and in Madagascar that matched those in India. In addition, coal deposits from tropical plants on Arctic islands supported the idea that northern continents had once occupied tropical latitudes.
As interesting as Wegener's evidence was, fossil and geological records said nothing about the mechanism that might have moved continents across Earth's surface. In 1928, two years before Wegener's death, Arthur Holmes, one of the few proponents of the theory of continental drift, proposed such a mechanism. Holmes proposed that circular convection currents of molten material deep within Earth might provide ample force to divide and move continents. This idea received little attention when Holmes proposed it, but 30 years later scientists found evidence that suggested his theory was correct all along.
By the late 1960s, scientists had developed the theory of plate tectonics based on a range of new evidence. Technological advances had helped reveal that the ocean floor was not essentially flat, as once assumed, but instead was marked by 50,000-kilometer-long (31,000-mile), 3,000-meter-high (9,800-ft) ridges and 11-kilometer-deep (7-mile) trenches. Scientists found striking patterns related to these features. They found that the youngest oceanic crust is located nearest the mid-ocean ridge and the oldest crust is nearest the trenches. They also detected a pattern of alternating magnetic polarity along the ocean floor, which emanated from the ridge tops. These two pieces of evidence, coupled with the fact that volcanic activity and island-building occurred most commonly at ocean trenches, suggested that new crust was created at mid-ocean ridges and destroyed at ocean trenches. Scientists Harry Hess and Robert Dietz used this evidence to revive and expand Holmes' convection theory into the theory they called "seafloor spreading." Finally, Wegener's notion of continental drift was coupled with a mechanism that could explain the movement of tectonic plates.
Today, geologists identify three types of boundaries that occur between tectonic plates. The first, called a divergent boundary, occurs most often at mid-oceanic ridges. Here, two plates move away from each other, creating a break, or rift, in Earth's crust. Magma from the mantle rises to fill the gap, thus creating new crust. The second type, called a convergent boundary, occurs when two plates moving toward each other meet. If at least one of the plates is made up of relatively dense oceanic crust, the denser of the two plates subducts, or slides beneath the other. If both plates are made up of continental crust, very little subduction occurs. Instead, the plates buckle and fold to create valleys, ridges, and high mountain ranges such as the Himalayas. The last type is called a transform boundary. At these locations, plates slide roughly alongside each other in opposite directions. The plates often get stuck as they move past each other, leading to a build-up of tension. This tension is ultimately released in the form of an earthquake.
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