At the beginning of the Earth formation, it was a hot body having uniform composition which gradually cooled down and formed different layers, then how the remarkable variation in compositional between oceanic and continental crust is there? It should have been uniform composition throughout the earth crust.
I thinking this is belong to Isostasy idea, some time oceanic crust changing to Continental and some time continental to oceanic. Also as we know in the earth two type of tectonic activities 1- Epirogeny 2- Orogeny.
Oceanic crust represents the outermost layer of Earth’s lithosphere that is found under the oceans and formed at spreading centers on oceanic ridges, which occur at divergent plate boundaries. It is about 6 km (4 miles) thick. It is composed of several layers, not including the overlying sediment. The topmost layer, about 500 meters (1,650 feet) thick, includes lavas made of basalt (that is, rock material consisting largely of plagioclase [feldspar] and pyroxene). Oceanic crust differs from continental crust in several ways: it is thinner, denser, younger, and of different chemical composition. Like continental crust, however, oceanic crust is destroyed in subduction zones
While the origins of the most ancient continental crust - cratons, forming the deep-rooted flat-surfaced cores of continents - are highly debated and certainly originated in a tectonic and geothermal regime very different from today, continental crust has been growing in area over time (say 2Ga) mainly by accretion of volcanic island arcs to continental margins (e.g. Western USA, Avalonian half of Britain). Thus the origins of continental crust are, ultimately, the leakage of partial melt products above subduction zones, forming steadily thicker piles of steadily more evolved and less dense magmas and deeper and deeper roots. This crust is then slid around by tectonics, is interminably reworked by the rock cycle and acquires biogenic deposits (limestones, etc), so that the surface of the continental crust as we mostly experience it is familiarly sedimentary. The middle and lower levels of the continental crust, however, are crystalline and maybe of an andesitic composition (though we so rarely see it that it's hard to tell except through seismics).