Encyclopedia of Evolution.pdf - Online Reading Center

plate tectonics
The Earth’s surface consists of eight major and several smaller crustal plates. Arrows indicate direction of plate movement. Most volcanoes (dots) occur
near plate interfaces. Most earthquakes also occur in the same locations as the volcanoes.
from Cambridge University presented this evidence in 1963,
their paper was denied publication at first, but when the evidence
was finally accepted, it became perhaps the single most
important factor in convincing geologists that the theory of
plate tectonics was correct.
The Eurasian Plate and the North American Plate have
been separating for more than 120 million years, at about the
same rate that fingernails grow. Eurasia and North America,
parts of which remained connected until about 50 million
years ago, are now separated by the North Atlantic Ocean.
During this same time, South America and Africa have separated.
The Mid-Atlantic Ridge runs along the ocean floor
like a gigantic backbone. In Iceland, the Mid-Atlantic Ridge
emerges.
When crustal plates collide, two things happen. First,
one of the plates is thrust underneath the other plate, a process
known as subduction. The subducted materials melt
and recirculate in the magma. This happens primarily with
the heavier materials that make up the ocean floors. Because
ocean floor crust is recirculated by subduction, no portion of
the ocean floor is much older than 200 million years. Second,
the continents, composed of lighter material, usually move up
rather than down, forming mountain ranges. Because continental
rock is less frequently subducted than ocean floor
material, many continental rocks are billions of years old.
One of the subduction zones occurs where the Pacific Plate is
thrust beneath the Indian-Australian Plate, forming the Marianas
Trench, which at over seven miles (11 km) is the deepest
part of the ocean. The plate that carries the subcontinent
of India has collided with the Eurasian Plate, resulting in the
relatively rapid and dramatic rise of the Himalayas.
Two plates can rub against one another. For example, the
Pacific Plate is moving northwest and rubbing against the North
American Plate, along the famous San Andreas fault line.
Geological events accompany tectonic movements, such as:
• All three kinds of plate movements can produce volcanic
eruptions. The locations of volcanoes correspond closely to
the fault lines where plates meet. Most famous is the ring
of fire around the Pacific Ocean, with volcanoes in South
America, Mexico, the western United States, Alaska, Japan,
and southeast Asia. Volcanic gas consists largely of carbon
dioxide and water vapor, from the ocean water and limestone
that are subducted along with ocean floor rock.
• Earthquakes occur as plates move. When two plates rub
against one another, as in the San Andreas fault, the movement
is uneven; they can stick, then slip suddenly, resulting
in major earthquakes. For this reason, frequent small