<P> Because the surface of continental crust mainly lies above sea level, its existence allowed land life to evolve from marine life . Its existence also provides broad expanses of shallow water known as epeiric seas and continental shelves where complex metazoan life could become established during early Paleozoic time, in what is now called the Cambrian explosion . </P> <P> There is little evidence of continental crust prior to 3.5 Ga, and there was relatively rapid development on shield areas consisting of continental crust between 3.0 and 2.5 Ga . All continental crust ultimately derives from the fractional differentiation of oceanic crust over many eons . This process has been and continues today primarily as a result of the volcanism associated with subduction . </P> <P> In contrast to the persistence of continental crust, the size, shape, and number of continents are constantly changing through geologic time . Different tracts rift apart, collide and recoalesce as part of a grand supercontinent cycle . There are currently about 7 billion cubic kilometers of continental crust, but this quantity varies because of the nature of the forces involved . The relative permanence of continental crust contrasts with the short life of oceanic crust . Because continental crust is less dense than oceanic crust, when active margins of the two meet in subduction zones, the oceanic crust is typically subducted back into the mantle . Continental crust is rarely subducted (this may occur where continental crustal blocks collide and overthicken, causing deep melting under mountain belts such as the Himalayas or the Alps). For this reason the oldest rocks on Earth are within the cratons or cores of the continents, rather than in repeatedly recycled oceanic crust; the oldest intact crustal fragment is the Acasta Gneiss at 4.01 Ga, whereas the oldest oceanic crust (located on the Pacific Plate offshore of Kamchatka) is from the Jurassic (~ 180 Ma). Continental crust and the rock layers that lie on and within it are thus the best archive of Earth's history . </P> <P> The height of mountain ranges is usually related to the thickness of crust . This results from the isostasy associated with orogeny (mountain formation). The crust is thickened by the compressive forces related to subduction or continental collision . The buoyancy of the crust forces it upwards, the forces of the collisional stress balanced by gravity and erosion . This forms a keel or mountain root beneath the mountain range, which is where the thickest crust is found . The thinnest continental crust is found in rift zones, where the crust is thinned by detachment faulting and eventually severed, replaced by oceanic crust . The edges of continental fragments formed this way (both sides of the Atlantic Ocean, for example) are termed passive margins . </P>

Why is the oceanic crust subducted under continental crust