Travel Reference
In-Depth Information
way as well, and unless these great Ice Age elephants of North America were
swimmers far from shore (doubtful), or had their teeth somehow transported
to locales many miles off the shoreline (equally doubtful), we can conclude
that during the Ice Ages, the level of the sea was far lower from what it is now.
The sea has indeed been quite unstable in its position telative to the
land's surface. Sea level routinely changes, although the rate of change is so
slow that no change would be observable in a human lifetime, or even many
lifetimes. How does this happen? How did science make this discovery?
That the sea does change in level over time has been known for nearly
two centuries. When nineteenth-century geologists discovered the reality of
the Ice Ages—when it became clear that the enormous quantity of ice cov-
ering so much of continental land surfaces could have come only from the
sea—they realized that the consequences of such an event would be a lower-
ing of sea level. So large are the volumes of water required that fresh water
on land could not have produced the necessary volumes of ice. Large por-
tions of water were extracted from the sea to be turned into ice. Normal run-
off back into the sea did not take place. More and more water became conti-
nental ice, and the level of the ocean began to fall.
The most recent Ice Age was not a single event, but the alternation of
more than 20 individual glacial advances and retreats covering the last 2
million years of earth history. Each glacial advance and retreat caused a cor-
responding retreat and advance of sea level. What is so extraordinary is the
rapidity and magnitude of these changes: drops and rises of as much as 150
meters.
We have arrived at two causes of change in sea level—tides, which
cause changes of up to 15 meters within a single day, and continental glacia-
tion (and growth of icecaps), which causes changes over thousands of years.
But there is a third type of sea level change that is far slower. It is currently
the subject of intense research by a phalanx of earth scientists, and the im-
petus for this sustained research comes from the oil companies. According to
many earth scientists, this third type of sea level change is brought about by
long-term changes in the volume of the ocean basins. These changes, called
