interior. This idea explained why oceanic crust consisted mainly of volcanic
basalts, and why it was so much younger than most continental crust.
These discoveries laid the foundations for the modern theory of plate
tectonics. (The term “plate tectonics” is used to distinguish this theory
from Wegener’s theory of “continental drift.”) According to plate tectonics,
the Earth’s crust consists of a dozen or so plates, like sections of a
cracked eggshell.
Convection currents in the semimolten regions beneath them move the
plates around. Most geologically interesting events occur at the plates’
edges. At “divergent margins,” such as suboceanic ridges, magma from
the interior rises and forces adjacent plates apart. This is why the Atlantic
Ocean is widening by about 2 cm each year, about the speed at which your
¿ ngernails grow.
Elsewhere, at “convergent margins,” plates are forced together. There are
two main types of convergent margins. If both plates are continental, they
may buckle up at “collision margins” to form mountain chains such as the
Himalayas, which were formed when the Asian and India plates collided.
If an oceanic plate, consisting of heavy basaltic materials, meets a lighter
continental plate, the oceanic plate will dive under the continental plate
at “subduction margins.” As it does so, the plates will grind together and
Harry Hess, a geology professor at Princeton and World War II naval
commander, used sonar to detect submarines —and volcanoes on the seaÀ oor.
Corel Stock Photo Library.