Metamorphosis require heat, pressure and time and is an extension of lithification, but it can occur on any rocks type (including metamorphic rocks). There is a continuum of type from heat dominated (marble) to pressure dominated (blueschist). Metamorphism is ranked in terms of a grade. A high grade metamorphic rock is a gneiss, which has undergone intense heat and pressure. It is important to note metamorphism occurs in the solid state - there is no melting. If the rock starts to melt it is called a migmatite (Mixed IGneous and metamorphic rock).
GeologyRocks