Why does partly molten material... ?

Question

rise toward earth's surface?

Answer 1

The mass of the solid rock above it exerts a downward pressure on a cavity of molten rock.  The molten rock then flows towards a path of least resistance ( a few dozen miles to the surface in low density rock is less resistive thousand miles down into high density rock)

Denisty: the mass of a substance per unit volume

http://www.merriam-webster.com/cgi-bin/d...

This means you must take into account both "how heavy it is" and "how much space it occupies".

Answer 2

The first answer was close.  He used the word density but did not quite use it correctly.  Density means how heavy it is. Molten material often has lighter volatiles that make it molten and thus it rises.  A heavy molten material will not rise.  It is lighter so gravity essentially puts more force on the surrounding rocks and it moves upwards as a consequence.