Metamorphic rocks result when an existing rock is altered by some
combination of changes in pressure or temperature. That change occurs
while the rock in in a solid state--it doesn't actually melt, though
the molecules are re-organized.
The changes in pressure or temperature can come from a number of
possible circumstances. As the slow deposition of overlying sediment
buries a rock, those sediments may over time put enough pressure
on the rock to cause metamorphism. The stresses arising from the
interaction of plates, such as mountain-building plate collisions
or subduction, can put stress on a rock. Heat emanating from a nearby
magma body can alter rock, as does the intense heat and pressure
of a meteor impact.
Metamorphism will change the rock's mineralogic or textural features.
The shift in pressure or temperature may knock the rock's mineral
composition out of chemical equilibrium. As conditions change, the
molecules rearrange themselves, some forming different minerals,
until the rock reaches a new balance. The textural changes, meanwhile,
may include changes in grain size or the alignment of minerals as
a result of directional forces that can be manifested as a sort
of layering within the rock called foliation.
In some cases, significant amounts of hot water can infiltrate
the rock through cracks. This water, and the dissolved ions it contains,
can change the surrounding rock's mineralogy. In addition, the dissolved
material may precipitate from the water and remain as mineral deposits
in the cracks; sometimes, these veins are filled with deposits of
valuable ores of copper, silver, or gold.