Because the slope of topography/bathymetry is so flat, relatively small changes in sea level (order of 10s-100s of meters) can dramatically change the connectiv

Because the slope of topography/bathymetry is so flat, relatively small changes in sea level (order of 10s-100s of meters) can dramatically change the connectivity of land (and oceans). This allows for both the migration and isolation of species on earth, notably the migration of humans to the Americas[0]. The major differences in elevation/bathymetry are dominantly due to the density change that accompanies the change from continental crust to oceanic crust. Continents are basically rafts of granite that float on more dense mantle rocks; oceanic crust is pretty close chemically and physically to the mantle, and is a lot thinner and more dense, and basically floats lower. This equilibrium is called isostasy[1], which is more or less Archimedes' principle[2] applied to the earth. It's basically happenstance that the volume of water on earth is enough to just barely overtop the ocean basin. Very fortuitous happenstance, though (life might have evolved pretty differently if the ocean/land transition was several km high). [0]: http://ows.edb.utexas.edu/site/hight-kreitman/land-bridge-theory http://ows.edb.utexas.edu/site/hight-kreitman/land-bridge-th... [1]: http://en.wikipedia.org/wiki/Isostasy http://en.wikipedia.org/wiki/Isostasy [2]: http://en.wikipedia.org/wiki/Archimedes'_principle http://en.wikipedia.org/wiki/Archimedes'_principle