<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> <P> The inner core is solid, the outer core is liquid, and the mantle solid / plastic . This is because of the relative melting points of the different layers (nickel--iron core, silicate crust and mantle) and the increase in temperature and pressure as depth increases . At the surface both nickel--iron alloys and silicates are sufficiently cool to be solid . In the upper mantle, the silicates are generally solid (localised regions with small amounts of melt exist); however, as the upper mantle is both hot and under relatively little pressure, the rock in the upper mantle has a relatively low viscosity . In contrast, the lower mantle is under tremendous pressure and therefore has a higher viscosity than the upper mantle . The metallic nickel--iron outer core is liquid because of the high temperature, despite the high pressure . As the pressure increases, the nickel--iron inner core becomes solid because the melting point of iron increases dramatically at these high pressures . </P> <P> In the mantle, temperatures range between 500 to 900 ° C (932 to 1,652 ° F) at the upper boundary with the crust to over 4,000 ° C (7,230 ° F) at the boundary with the core . The geothermal gradient of the mantle increases rapidly in the thermal boundary layers at the top and bottom of the mantle, and increases gradually through the interior of the mantle . Although the higher temperatures far exceed the melting points of the mantle rocks at the surface (about 1200 ° C for representative peridotite), the mantle is almost exclusively solid . The enormous lithostatic pressure exerted on the mantle prevents melting, because the temperature at which melting begins (the solidus) increases with pressure . </P>

What does the mantle do for the earth