Recall that any rock can be changed to a metamorphic rock.  Whatever the beginning rock is, it is called the PROTOLITH of the metamorphic rock it is transformed into.  Three things contribute in varying proportions to the transformation from a protolith to a metamorphic rock: heat, pressure, and fluids (mostly water with dissolved ions).

1. HEAT contributes to the process in two ways.  First, atoms may combine differently at different temperatures.  This means that a mineral stable at one temperature might become unstable at a higher (or lower) temperature and be converted to a different mineral with a more stable atomic structure.  This may or may not involve changing the exact elemental composition.  Second, heat makes practically all chemical reactions go faster, meaning that mineral transformations are much easier at higher temperature.

2. PRESSURE also has two effects.  As with heat, it can control which minerals or forms of minerals are stable.  Some minerals may be converted to minerals with similar composition but different atomic packing simply because pressure is increased.  The exact nature of the pressure is not important in this case, only the amount.  Thus the CONFINING (or LITHOSTATIC) PRESSURE created by deep burial of rocks under sediment may have this effect as well as the DIRECTED (or DIFFERENTIAL) PRESSURE produced by converging plates.  The second effect of pressure is to reorient minerals with linear or platy structure or to create a preferred orientation of them as they form.  Thus elongate minerals such as amphiboles, or platy minerals such as clays or micas tend to align themselves parallel to each other when under pressure.  This only happens when there is directed pressure; confining pressure does not accomplish it.  The diagram illustrates the effect.  A texture of this sort in a metamorphic rock is called FOLIATION and the rocks are said to be FOLIATED.

3. FLUIDS serve only to speed up other metamorphic processes, or perhaps even allow them to happen at all.  Chemical reactions require water, and most proceed much faster as the amount of water goes up.  Dissolved ions in the fluid also make those mineral transformations that require chemical changes in the minerals to occur, whether by supplying needed ions or flushing away excess ones.