A tectonically stable continental Craton is typically described as one that is broad and flat and bordered by ocean basins on all sides The continent is usually eroded down to slightly above sea level (a peneplain) with no indication of ongoing tectonic activity. On the surface of the stable Craton is a blanket of mature quartz sandstone (QFL, yellow field) which has been produced as a result of millions of years of weathering, erosion and sorting. Limestones are sometimes also well developed if the climate is warm. Most shales (clays) have been wind blown or washed off the continent into the surrounding ocean basins. The continent is in perfect isostatic equilibrium; by itself it will neither rise nor sink.
Continents are composed of relatively light weight felsic igneous rock (granites and granodiorites), which are light enough that when eroded to a peneplain and "floating" in isostatic equilibrium, the surface is a few hundred feet above sea level. Thus, granite gives us the dry land we live on.
In contrast, ocean basins are composed of mafic igneous rocks (basalt and gabbro) and because these are relatively heavy rocks, they isostatically "float" on the underlying mantle a little over 5 miles below sea level. Continents and oceans form natural divisions on the earth, not only because they are composed of different rock types, but also because one lies above sea level, and the other below.
Contributed by Lynn Fichter
Thursday, August 21, 2014