<Tr> <Td> U </Td> <Td> 5.69 × 10 </Td> <Td> 7.04 × 10 </Td> <Td> 0.22 × 10 </Td> <Td> 1.25 × 10 </Td> </Tr> <Tr> <Td> Th </Td> <Td> 2.64 × 10 </Td> <Td> 1.40 × 10 </Td> <Td> 124 × 10 </Td> <Td> 3.27 × 10 </Td> </Tr> <Tr> <Td> </Td> <Td> 2.92 × 10 </Td> <Td> 1.25 × 10 </Td> <Td> 36.9 × 10 </Td> <Td> 1.08 × 10 </Td> </Tr> <P> The geothermal gradient is steeper in the lithosphere than in the mantle because (a) crustal minerals have lower thermal conductivity than mantle minerals, (b) the crust produces more radiogenic heat than the mantle and (c) the crust contains numerous fractures . </P>

What is the geothermal gradient and how does it relate to the melting curve of peridotite