Page 59 - Petrophysics

REVIEW OF COMPOSITION OF THE GLOBE 33

floor, it accumulates a layer of sediments that were eroded from the
continents. The sedimentary layer that accumulates on the ocean floor is
thin in comparison to the sedimentary layers on the continents because
the ocean floor is very young. Driven by convective, rotational, and
gravitational forces, the plates move around until they are eventually
drawn into the mantle at subduction zones before sedimentation has
time to form thick layers [ 1, 2, 61.
If two ocean plates of equal density collide, they will slowly deform at
the edges and become a range of mountains. If the colliding forces remain
active long enough, the range of mountains will rise above sea level.
The Alp Mountains in Switzerland constitute an example of this process
due to a collision between Eurasia and Africa that began about 80 million
years ago when the region was covered by a sea. Marine sediments can
be found high in the Alpine regions.
India was once a separate continent riding on a plate moving in a
northerly direction. The plate carrying the Indian continent was diving
under the Asian continental plate. Eventually, India collided with Asia
and pushed up the massive Himalayan Mountains range [3, 41.
Island arcs, such as those that have developed in the Pacific Ocean east
of Asia, also occur as a result of plate collisions. The Asian plate is more or
less stationary with respect to the Pacific Ocean plate, which is slipping
under the large land mass and forming a range of offshore islands. As the
more dense ocean plate returns to the high-temperature mantle, selective
melting of some of its material takes place, and the lighter materials are
squeezed upward as rising columns called diapirs. Diapirs are pushed
through the overriding plate and form chains of offshore volcanoes that
eventually rise above sea level to form islands. Lavas from the island arc
volcanoes are generally intermediate in composition between granitic
continental rocks and basaltic rocks. Deep-focused earthquakes occur
along the arcs, indicating deep fracture zones between the continent
and ocean plates.
The plates also may slip laterally with respect to each other, forming
transform faults. These faults may be very long (hundreds of miles) such
as the San Andreas Fault of California, where the Pacific plate abuts
the North American continental plate. The Pacific plate is moving in
a northwest direction with respect to the American plate, which is
moving west. The difference in the relative motions of the plates pro-
duces a shear-type phenomena at the junction and results in a transform
fault, many thrust faults parallel to the Earth’s surface, and devastating
earthquakes.
The ancient supercontinent known as Pangaea was formed by the
union of a number of other continents. North America apparently
moved east about 500 million years ago to collide into Pangaea, and the