<P> Partial melting occurs when only a portion of a solid is melted . For mixed substances, such as a rock containing several different minerals or a mineral that displays solid solution, this melt can be different from the bulk composition of the solid . </P> <P> Partial melting occurs where the solidus and liquidus temperatures are different . For single minerals this can happen when they exhibit solid solution, for example in olivines between iron and magnesium . In rocks made up of several different minerals, some will melt at lower temperatures than others . </P> <P> Partial melting is an important consideration in geology with respect to the chemical differentiation of crustal rocks . Virtually all rocks on Earth derive from material from the interior of the earth that has been partially melted . The main places where partial melting occurs are subduction zones, mid-ocean ridges and hotspots . In all these places partial melting is often associated with volcanism, although some melts do not make it to the surface . Partial melts are thought to play an important role in enriching old parts of the continental lithosphere in incompatible elements . Partial melts produced at depth move upwards due to the compaction of the surrounding matrix . </P>

Where do volatiles cause partial melting of the mantle