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eustatic sea level changes, take place over time scales measured in hundreds
of thousands of years. Eustatic changes appear to he caused by the same
processes that bring about continental drift.
Through plate tectonic processes (so goes the most popular theory),
the volume of the ocean basins increases or decreases, and these volumetric
changes are sufficient to cause the oceans to spill onto land surfaces and flood
the lowland areas of all continents. The mechanism that causes this volu-
metric change is the rise and fall of mid-ocean "spreading centers," such as
the mid-ocean ridge extending the length of the Atlantic. We know that sea
floor spreading is caused by convection of the earth's mantle region and that
the giant plates (which can include both oceans and continents on the same
plate) sit atop these gigantic convection cells. Creation of new oceanic crust
occurs at the spreading centers, and if the rate at which heat rises in these re-
gions increases, the spreading centers themselves increase in volume. When
heat flow diminishes, the spreading centers contract. They act like gigantic
mountain ranges that inflate or deflate in response to the heat within them.
The changing volume of rock within the spreading centers causes the
changes in sea level. When spreading centers receive more heat and increase
in volume, the net effect is to
decrease
the ocean basin volume and cause
water to flow out of the oceans onto land surface, much like a bathtub over-
flowing. Sea level rises. The opposite occurs when heat flow decreases, and
the spreading center subsides. Sea level drops.
The magnitude of these changes can be enormous. Near the end of the
Age of Dinosaurs, the sea may have been at its "highest" stand ever; half of
North America, as well as huge regions of all other continents, were covered
with shallow inland seas. North America was really two continents: one
east, one west, with a seaway extending from the Rocky Mountains to near
the Appalachians and from the Arctic Circle to the tropics. And because the
"seven seas" are really a single global ocean of interconnected water,
any
change in sea level is global.
The concept that long-term global changes occur in sea level has
launched a rising star among the geosciences; the field of sea level change. A
