LITHOSPHERE-4 ESO
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 50,000kilometer-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.
Discussion Questions
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How do scientists explain why, if new crust is constantly being
formed from volcanic materials, Earth's crust stays the same size?
Why are continental volcanoes associated with oceanic-continental
convergent boundaries?
Why are the Appalachian Mountains not as high as the Himalayan
Mountains even though they were formed in the same way?
Why do scientists predict that Los Angeles will be north of San
Francisco in 16 million years?