<P> 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 . </P> <P> 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 discontinuous zones, called "thermochemical piles" which have been interpreted as either thermally differentiated, upwellings bringing warmer material towards the surface, or as chemically differentiated material . </P> <P> 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>

Which of the following statements describe(s) the earth's mantle (select all that apply.)