acasta gneiss is formed on top of continental crust.
acasta gneiss is formed on top of continental crust.
what is formed on top of continental crust?
acasta gneiss
as explained above, tectonic plates may include continental crust or oceanic crust, and most plates contain both. for example, the african plate includes the continent and parts of the floor of the atlantic and indian oceans. the distinction between oceanic crust and continental crust is based on their modes of formation. oceanic crust is formed at sea-floor spreading centers, and continental crust is formed through arc volcanism and accretion of terranes through tectonic processes, though some of these terranes may contain ophiolite sequences, which are pieces of oceanic crust considered to be part of the continent when they exit the standard cycle of formation and spreading centers and subduction beneath continents. oceanic crust is also denser than continental crust owing to their different compositions. oceanic crust is denser because it has less silicon and more heavier elements ("mafic") than continental crust ("felsic"). as a result of this density stratification, oceanic crust generally lies below sea level (for example most of the pacific plate), while continental crust buoyantly projects above sea level (see the page isostasy for explanation of this principle).
plate tectonics
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.
continental crust