<Tr> <Td> 2,890--5,150 </Td> <Td> 1,790--3,160 </Td> <Td> Outer core </Td> </Tr> <Tr> <Td> 5,150--6,360 </Td> <Td> 3,160--3,954 </Td> <Td> Inner core </Td> </Tr> <P> The layering of Earth has been inferred indirectly using the time of travel of refracted and reflected seismic waves created by earthquakes . The core does not allow shear waves to pass through it, while the speed of travel (seismic velocity) is different in other layers . The changes in seismic velocity between different layers causes refraction owing to Snell's law, like light bending as it passes through a prism . Likewise, reflections are caused by a large increase in seismic velocity and are similar to light reflecting from a mirror . </P> <P> The crust ranges from 5--70 kilometres (3.1--43.5 mi) in depth and is the outermost layer . The thin parts are the oceanic crust, which underlie the ocean basins (5--10 km) and are composed of dense (mafic) iron magnesium silicate igneous rocks, like basalt . The thicker crust is continental crust, which is less dense and composed of (felsic) sodium potassium aluminium silicate rocks, like granite . The rocks of the crust fall into two major categories--sial and sima (Suess, 1831--1914). It is estimated that sima starts about 11 km below the Conrad discontinuity (a second order discontinuity). The uppermost mantle together with the crust constitutes the lithosphere . The crust - mantle boundary occurs as two physically different events . First, there is a discontinuity in the seismic velocity, which is most commonly known as the Mohorovičić discontinuity or Moho . The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below). Second, in oceanic crust, there is a chemical discontinuity between ultramafic cumulates and tectonized harzburgites, which has been observed from deep parts of the oceanic crust that have been obducted onto the continental crust and preserved as ophiolite sequences . </P>

Describe the layers of the earth defined by composition