Metamorphic rocks form when pre-existing, solid rock is heated and/or subjected to high pressures, causing physical and/or chemical changes.
(pre-existing, solid rock; is; heated and/or subjected to high pressures) (pre-existing, solid rock; subjected; to high pressures)
pre-existing, solid rock pre-existing, solid rock
is subjected
heated and/or subjected to high pressures
to high pressures