The temperature of Earth generally increases with depth. The increase in degrees per kilometer is called the geothermal gradient, and geothermal gradients vary from place to place. A crudely average value for the continents is 20°C/km. (a) What is the average temperature at the base of a normal continental crust of 20 km thickness?

(b) At the same geothermal gradient, what would be the temperature of the basal crust thickened to 50 km in a mountain belt?

(c) If the geothermal gradient at a mid-ocean ridge flank were 75°/km, what would be the temperature at a depth of 10 kilometers, the base of the normal oceanic crust?

(d) If melting occurs at temperatures between 600°C and 1300°C, depending on rock composition, pressure, and abundance of fluids, would any of the sites in (a), (b), or (c) be a possible site of melting?