Poorly understood The earth is still a hot planet Lots of hot rocks below the surface But it needs more than just heat for the hot rocks to melt Must also have a reduction in pressure so the rock can expand enough to become a liquid 
(the rock; can expand; enough) (the rock; to become; a liquid) (it; needs; more than just heat for the hot rocks; to melt)
the rock the rock it
can expand to become needs
enough
a liquid
more than just heat for the hot rocks to melt