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showed that in many places on earth, oceanic crust dips downward into the
earth itself along long linear arcs of the earth's crust called subduction zones.
Subduction invariably leads to mountain chains and active volcanic moun-
tains inland of these features. A scientific revolution had occurred within a
few short years. We now know that continents indeed have drifted and that
they drift still. They do so because they float.
All continents are masses of relatively low-density rock embedded in a
ground of more dense material; continents essentially float on a thin (com-
pared with the diameter of the earth) bed of basalt. Earth scientists like to
use the analogy of an onion. The ocean crust can be likened to the thin, dry,
and brittle onion skin sitting atop a concentric globe of higher-density, wet-
ter material that itself is moving relative to the much thicker interior of our
global "onion." Continents are like thin smudges of slightly different mate-
rial embedded in the onion skin. Unlike an onion, however, the earth has a
radioactive core and constantly generates great quantities of heat as the ra-
dioactive minerals, entombed deep in the eatth, break down into theit vari-
ous isotopic by-products, liberating heat in the process. As this heat tises to-
ward the surface, it creates gigantic convection cells of hot, liquid rock in
the mantle—a molten layer of material directly beneath the outermost re-
gion of the earth, the crust. Like boiling water, the viscous upper mantle
rises, moves parallel to the surface of the earth for great distances at rates of
several inches each year (all the while losing heat) and then, much cooled,
settles back down into the depths of the earth. These gigantic convection
cells carry the thin, brittle outer layer of the earth—known as plates—along
with them. Sometimes this outermost layer of crust is composed only of
ocean bed. Sometimes, however, one or more continents or smaller land
masses are trapped in the mewing outer skin. This process, which is termed
continental drift or plate tectonics, is one of the great unifying theories of all
science.
The spreading centers produce new crust as giant plates drift apart. Yet
these plates are not always diverging. They also converge, sometimes causing
the collision of great continents. The collision of two continents is a slow,
majestic process each land mass moves at only a few centimeters pet year; so
