Geography Reference
In-Depth Information
One of the most perplexing theories about our so-called solid earth is the notion that all of the land on
earth isn't sitting still but sloshing around like toy boats in a wash tub on which small boys are banging
hammers!
This notion is technically known as
global plate tectonics
, an almost universally accepted theory. Al-
though the idea that pieces of the earth are in constant motion and that the continents had once been at-
tached to each other goes back hundreds of years, it was formally put forward first by Alfred Wegener in
his 1915 book,
The Origin of the Continents and Oceans.
A German meteorologist and naturalist, Wegener
was interested in the seeming alignment of the continents. Like a jigsaw puzzle waiting to be put togeth-
er, the Atlantic coastlines of South America and Africa looked like they could be snapped together into
a fairly neat arrangement. When Wegener learned of the discovery of fossils in Brazil that were similar
to those found in Africa, it added fuel to his notion that the two places were once connected. Based on
these and other bits of physical evidence, Wegener proposed the idea that the continents had once been a
single mass, which later split. He even christened his theoretical giant landmass Pangaea (“all land”) and
surrounded it by an all-encompassing sea, Panthalassa (“all seas”).
Like so many other new ideas in science, Wegener's theory of what he called “continental drift” was
largely dismissed in his day, principally because he was unable to explain the forces that could propel such
enormous landmasses. Wegener thought that tidal pull might have something to do with the process. A
visionary and a hero of science, Wegener died in 1930 in Greenland as he attempted to establish a mid-ice
observatory. For decades to come, his ideas were simply dismissed as the left-field notions of a crackpot.
But somewhere in science heaven, Wegener is having the last laugh. A large body of evidence collected
since the 1960s has shown that Wegener was indeed on the right path. British geologist S. Keith Runcorn
was one of the first champions of the revised notion of continents being connected. Wegener's theory of
continental drift has evolved into what is now called plate tectonics theory. Continental drift is out because
it is now known that more than just the continents are on the move. The earth's crust is divided into mobile
sections called plates. Some of these plates contain continents, or large parts of them; others carry the sea
floor. The plates—Canadian geophysicist J. T. Wilson first used the term in 1965—move over the earth's
superheated, molten core, pushed and pulled by convection currents in the molten material generated by
the heat of the core. The study of the movements of these large plates is called
tectonics
(from the Greek
word
tekton,
“to build”). Think of cooking tapioca pudding. You get it to boiling and you can see bubbles
move up to the top. If you put something that could float on top of the pudding into the top, it would start
bucking and jostling around the cooking pot. The inside of the earth is like a big vat of bubbling tapioca,
except that the pudding is magma, the liquefied matter within the earth's core that gets blasted out in vol-
canoes. Floating on top of the magma are the plates—pieces of the earth's crust.
The plates average from thirty to fifty miles in thickness and move at rates as great as a few inches
per year. They may be as large as a few thousand miles across—the North American plate stretches for
six thousand miles from the Pacific coast to the mid-Atlantic—or as little as a few hundred miles across.
People tend to associate rock with the idea of solidity. It is hard to imagine such massive pieces of rock
cruising around the planet's surface. The key to their movement is the flexibility of the upper part of the
earth's mantle—the layer beneath the crust—which is partly molten. As the plates “float” on this elastic,
moving mantle, they play a planetary game of hockey—jostling each other for position, rubbing each other
the wrong way, pushing one another down, or occasionally banging straight into each other with dramatic
and sometimes catastrophic effects. And they are not finished yet. The world we recognize today will be
quite different in, say, fifty million years. (Again, this is a blink of the eye in the geological time frame.)
For instance, a good-sized portion of East Africa will probably break off. You can see where it will happen
if you stand in the Great Rift Valley, which extends from Syria to Mozambique. And Baja California will
have detached itself from the Mexican mainland.
