<P> The crust of the Earth is of two distinctive types: oceanic and continental . The oceanic crust is 5 km (3 mi) to 10 km (6 mi) thick and is composed primarily of basalt, diabase, and gabbro . The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick and is mostly composed of slightly less dense rocks than those of the oceanic crust . Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust . </P> <P> Both the continental and oceanic crust "float" on the mantle . Because the continental crust is thicker, it extends both to greater elevations and greater depth than the oceanic crust . The slightly lower density of felsic continental rock compared to basaltic oceanic rock contributes to the higher relative elevation of the top of the continental crust . As the top of the continental crust reaches elevations higher than that of the oceanic, water runs off the continents and collects above the oceanic crust . Because of the change in velocity of seismic waves it is believed that beneath continents at a certain depth continental crust (sial) becomes close in its physical properties to oceanic crust (sima), and the transition zone is referred to as the Conrad discontinuity . </P> <P> The temperature of the crust increases with depth, reaching values typically in the range from about 200 ° C (392 ° F) to 400 ° C (752 ° F) at the boundary with the underlying mantle . The crust and underlying relatively rigid uppermost mantle make up the lithosphere . Because of convection in the underlying plastic (although non-molten) upper mantle and asthenosphere, the lithosphere is broken into tectonic plates that move . The temperature increases by as much as 30 ° C (54 ° F) for every kilometer locally in the upper part of the crust, but the geothermal gradient is smaller in deeper crust . </P> <P> Partly by analogy to what is known about the Moon, Earth is considered to have differentiated from an aggregate of planetesimals into its core, mantle and crust within about 100 million years of the formation of the planet, 4.6 billion years ago . During the Hadean, the primordial crust was very thin and was probably destroyed by much more vigorous plate tectonics, volcanic activity and significant asteroid impacts, which were much more common in the early stages of the solar system . </P>

What is the average temperature of the crust