Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 33

FIGURE 2.10 Distribution of island arcs (solid lines) and deep-sea trenches (dashed lines). Both phe-
nomena occur mostly around the Pacific, a closing ocean. (Adapted from various sources.)
Plate tectonics provides a broad and unifying framework into which virtually all as-
pects of Earth science fit. Although details are still being added, especially in the more
complex and older continental areas, the theory explains the movement of continents,
as well as such formerly perplexing problems as the youthfulness of the bottoms of the
ocean basins. The origin of the oceans has long been a great enigma: They are thought
to have formed soon after initial accretion, when the crust had cooled enough to allow
liquid water. The Earth is over 4 billion years old, yet the sea-floor rocks are less than
180 million years old. This disparity can be explained easily in plate tectonic theory.
New sea-floor crust is continually produced at the mid-ocean ridges and destroyed at
the subduction zones; as a result, it can never be very old. The continents, on the oth-
er hand, can be broken apart and reassembled in various ways as continental plates
are rifted apart or sutured together in collisions, and because of their low density they
are not subducted easily; the basic amount of continental material stays the same or
increases through time. Erosion may wear down the land and transport the low-density
sediments to the sea, but much of this material is reconstituted by being carried down
subduction zones, melted, and then rising up as igneous intrusions and added to exist-
ing continental masses through mountain building. Continents thus display accretion of
younger material and active tectonism at their edges, whereas their interiors are com-
monly older and more stable.
Mountain Building and Plate Tectonics