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Imagine a very simple situation - a tectonically stable continental craton bordered by ocean basins all around. The continent is eroded down nearly to sea level everywhere (a peneplain); it is dead flat from edge to edge and corner to corner and there is no tectonic activity anywhere. On the surface is a blanket of mature quartz sandstone (QFL, yellow field), the result of millions of years of weathering and erosion and sorting. Limestones are probably also well developed, if the climate is warm, but 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 not rise or sink. Nothing exciting is happening; no earthquakes or volcanic activity - unrelenting boredom, perhaps for tens or hundreds of millions of years.
Continents are composed of relatively light weight felsic igneous rock (granites, granodiorites, etc.). Light enough that when eroded to a peneplain, and "floating" in isostatic equilibrium, it's surface is a few hundred feet above sea level. Thus, granite gives us the dry land we live on.
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 are thus natural divisions on the earth, not only because they are composed of very different rocks, but also because one lies naturally above sea level, and the other naturally far below sea level.
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