Geoscience Reference
In-Depth Information
magma breaks through the crust and bubbles to the surface, resulting in volcanic eruptions on land and under
the oceans.
Today a series of volcanic islands in the Mid-Atlantic stretches from Iceland in the north to Bovet Island
near the Antarctic ice pack. These islands mark the Mid-Atlantic Ridge and are the only emergent part of a
16,000-kilometer-long (1,000-mile) mountain chain. The Mid-Atlantic Ridge occurs at a plate boundary, sep-
arating the Eurasian Plate from the North American Plate, where the seafloor is spreading as the molten rock
wells up and spreads out and is then replaced by more molten rock. Compelling evidence for sea-floor spread-
ing surfaced in the 1950s when magnetic readings, obtained by the U.S. Coast Guard, revealed that the farther
from an oceanic ridge you go, on either side, the older the rocks.
The oceanic crust acts much like a conveyor belt. New crust is formed at oceanic ridges by upwelling of
magma but is destroyed when it reaches the edge of another plate. This latter area is a subduction zone, where
one plate slides beneath the other and is melted and incorporated again into the molten layer underneath. (The
addition and subtraction of crust is, in this way, kept in balance, and therefore the Earth itself does not expand
in size.) Currently, the spreading of the sea floor at the Mid-Atlantic Ridge is causing the Atlantic Ocean to
widen by roughly 2 centimeters (less than an inch) per year. The opposite process is happening in the Pacific,
where the ocean floor is being swallowed under the North American Plate.
The plates, which can consist of both oceanic and continental crust, are in constant motion relative to one
another in a kind of millennial bump and grind. The area where two plates meet is called a plate boundary, and
a number of things can happen there, depending upon the nature of the two plates. If a plate of oceanic crust
meets one of continental crust, the heavier oceanic plate is subducted. If two plates of entirely oceanic crust
meet head-on, a subduction occurs randomly, with either plate being subducted. This encounter is often ac-
companied by the creation of an arc of volcanic islands, such as the Aleutian Archipelago. But if the plates are
composed mostly of continental material, subduction may not occur at all when they collide. They may, in-
stead, crumple at their edges and set off a mountain-building episode, also known as orogeny. Today the Indo-
Australian Plate is being thrust under the Eurasian Plate, a cataclysmic process that began 50 million years ago
and is still pushing up the Himalayas. In the distant past, such a collision of continental plates was responsible
for creating the Appalachians, the great mountain chain that runs parallel to the eastern seaboard of North
America, then arcs across the North Atlantic to Scotland.
