<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>
why is the oceanic crust subducted under continental crust
Because continental crust is less dense than oceanic crust