<P> The top of the mantle is defined by a sudden increase in seismic velocity, which was first noted by Andrija Mohorovičić in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho". The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, an irregular layer with a maximum thickness of perhaps 200 km (120 mi). Below the lithosphere the upper mantle becomes notably more plastic . In some regions below the lithosphere, the seismic shear velocity is reduced; this so - called low - velocity zone (LVZ) extends down to a depth of several hundred km . Inge Lehmann discovered a seismic discontinuity at about 220 km (140 mi) depth; although this discontinuity has been found in other studies, it is not known whether the discontinuity is ubiquitous . The transition zone is an area of great complexity; it physically separates the upper and lower mantle . Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous . The D" layer at the core--mantle boundary separates the mantle from the core . In 2015, research using gravitational data from GRACE satellites and the long wavelength nonhydrostatic geoid indicated viscosity increases by a factor of ten to 150 about 1,000 kilometres (620 mi) below earth's surface; separate research also indicates sinking tectonic plates stall at this depth, leading Robert van der Hilst to speculate "In term's of structure and dynamics, 1,000 kilometers could be more important" (than the currently accepted 660 km depth upper--lower division). The lower mantle also contains some discontinueous zones, called "thermochemical piles" which have been interpreted as either thermally differentiated, upwellings bringing warmer material towards the surface, or as chemically differentiated material . A principal source of the heat that drives plate tectonics is the radioactive decay of uranium, thorium, and potassium in Earth's crust and mantle . </P> <P> The mantle differs substantially from the crust in its mechanical properties as the direct consequence of the difference in composition (expressed as different mineralogy). The distinction between crust and mantle is based on chemistry, rock types, rheology and seismic characteristics . The crust is a solidification product of mantle derived melts, expressed as various degrees of partial melting products during geologic time . Partial melting of mantle material is believed to cause incompatible elements to separate from the mantle, with less dense material floating upward through pore spaces, cracks, or fissures, that would subsequently cool and solidify at the surface . Typical mantle rocks have a higher magnesium to iron ratio and a smaller proportion of silicon and aluminium than the crust . This behavior is also predicted by experiments that partly melt rocks thought to be representative of Earth's mantle . </P> <P> Mantle rocks shallower than about 410 km (250 mi) depth consist mostly of olivine, pyroxenes, spinel - structure minerals, and garnet; typical rock types are thought to be peridotite, dunite (olivine - rich peridotite), and eclogite . Between about 400 km (250 mi) and 650 km (400 mi) depth, olivine is not stable and is replaced by high pressure polymorphs with approximately the same composition: one polymorph is wadsleyite (also called beta - spinel type), and the other is ringwoodite (a mineral with the gamma - spinel structure). Below about 650 km (400 mi), all of the minerals of the upper mantle begin to become unstable . The most abundant minerals present, the silicate perovskites, have structures (but not compositions) like that of the mineral perovskite followed by the magnesium / iron oxide ferropericlase . The changes in mineralogy at about 400 and 650 km (250 and 400 mi) yield distinctive signatures in seismic records of the Earth's interior, and like the moho, are readily detected using seismic waves . These changes in mineralogy may influence mantle convection, as they result in density changes and they may absorb or release latent heat as well as depress or elevate the depth of the polymorphic phase transitions for regions of different temperatures . The changes in mineralogy with depth have been investigated by laboratory experiments that duplicate high mantle pressures, such as those using the diamond anvil . </P> <Table> Composition of Earth's mantle in weight percent <Tr> <Th> Element </Th> <Th> Amount </Th> <Th> </Th> <Th> Compound </Th> <Th> Amount </Th> </Tr> <Tr> <Td> O </Td> <Td> 44.8 </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> Mg </Td> <Td> 22.8 </Td> <Td> SiO </Td> <Td> 46 </Td> </Tr> <Tr> <Td> Si </Td> <Td> 21.5 </Td> <Td> MgO </Td> <Td> 37.8 </Td> </Tr> <Tr> <Td> Fe </Td> <Td> 5.8 </Td> <Td> FeO </Td> <Td> 7.5 </Td> </Tr> <Tr> <Td> Ca </Td> <Td> 2.3 </Td> <Td> Al O </Td> <Td> 4.2 </Td> </Tr> <Tr> <Td> Al </Td> <Td> </Td> <Td> CaO </Td> <Td> 3.2 </Td> </Tr> <Tr> <Td> Na </Td> <Td> 0.3 </Td> <Td> Na O </Td> <Td> 0.4 </Td> </Tr> <Tr> <Td> </Td> <Td> 0.03 </Td> <Td> K O </Td> <Td> 0.04 </Td> </Tr> <Tr> <Td> Sum </Td> <Td> 99.7 </Td> <Td> Sum </Td> <Td> 99.1 </Td> </Tr> </Table>

What is earth's upper mantle made of
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