Travel Reference
In-Depth Information
thousands of lifetimes must pass before any change in position would be ap-
parent. But as millennia pass, the continents do move relative to one an-
other, and sometimes they collide. The first contact of the opposing conti-
nental shelves does little. But year after year, as the two giant continental
blocks of earth's crust coalesce, enormous forces of compression act on the
continental edges until the outermost regions buckle. Mountains begin to
form along the two edges as the collision progresses, often creating high,
spewing volcanoes amid the contorted mass of sediment and rock that was
once a tranquil coastline of wide sandy beaches. Finally, the two continents
are incapable of further compression, yet they are still driving against each
other. Slowly, one of the continents slides over the other, often doubling the
thickness of their crustal edges in the process; then they lock together, no
longer able to give any more ground. A relatively recent and dramatic ex-
ample is the collision of India and Asia. Forty million years ago, India was a
small fugitive of the ancient Gondwana supercontinent, fleeing northward
from its origins in the Southern hemisphere until it collided with mainland
Asia. In the process, the edge of the Indian continent rode up onto the
Asian mainland, resulting in the formation of the world's highest mountains,
the Himalayas, which are composed of the thickest continental crust known
on earth.
Geologists know now that plates, those enormous lithic fragments that
do the drifting of continental drift, can interact with other plates in only
three ways: They can pull apart, smash together, or run side by side. The first
two phenomena—the divergence of plates at the mid-ocean ridge spreading
centers, and their convergence at subduction zones—were the subject of the
first pioneering wave of plate tectonics research, which demonstrated the re-
ality of continental drift. Yet the third, the side-by-side motion of plate mar-
gins, is equally important. The San Andreas fault in California is perhaps the
best example of side-by-side motion. The San Andreas has been operating
now for millions of years, and as the thin slice of California on the outboard
side scrapes northward, earthquake by earrhquake, it separates rock units
that once were geographically together.
