The most obvious effect of plate tectonics is with the seafloor, where the crust is particularly thin and primarily basaltic in composition. The seafloor has mountain ranges known as midoceanic rises which are crossed by deep chasms known as midoceanic rifts. Scientists measured the residual magnetism of the seafloor and discovered sections which had recorded the periodic reversals of the Earth's magnetic field. These sections run parallel to the midoceanic rifts which indicates that new crust is being created along the rifts while the seafloor spreads outward. The crustal plates of the Atlantic ocean are spreading outward 2 - 4 cm per year.
If the crust is expanding in some places, it stands to reason that it destroying it somewhere else. There are trenches along the coasts of some continents where the seafloor is sliding downward where it eventually melts. The Andes Mountains of South America are the direct result of the Pacific seafloor sliding under the continent.
Sometimes the seafloor does not slide under the continent. The Atlantic seafloor is actually locked to North and South America and it is pushing the continents westward. Only 200 million years ago, North and South America were joined with Europe and Africa. About 250 million years ago, all the continents were joined together in one supercontinent called Pangea.
By plotting earthquakes and volcanoes on a world map, we can see the outline of the plate boundaries. The Pacific plate is surrounded by a ring of volcanic and earthquake zones and has been dubbed the "ring of fire."
Because the Earth's crust is active, it is constantly changing and the oldest crust has long since been wiped away. There are a few areas in Canada, Australia and Africa which have been relatively undisturbed for a long time. Here we have found the oldest samples of crust so far, they are about 3.9 billion years old.
Copyright © 1997 Kathy A. Miles and Charles F. Peters II