Answer to Question 1
The decompression of rising rock lowers its melting point while only lowering its temperature a small amount. The rock is now above its melting point and it begins to melt. Upward movement to cause decompression results from convection.
Answer to Question 2
Viscosity, gravity, and temperature are important controls on convection. Gravity causes the cooler, denser material to sink, whereas viscosity inhibits motion of the mass. Convection is favored by those conditions which overcome viscosity, including large mass, long time, and higher temperatures.
Scaling the Earth down to the size of a basketball, the Moon is roughly the size of a tennis ball. T
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