They are the rocks
that have formed by changes from other rocks such as sedimentaries,
igneous rocks, or other metamorphics.
As opposed to
igneous processes, metamorphism
is a solid-state
process, which means
the rocks are exposed to mechanical stress, or rising pressures and
temperatures, but not so high that the rock would melt and turn into
Neither do they form from eroded or organic material like
sedimentaries - they
from other rocks.
metamorphic processes the
mineral crystal size increases, porocity decreases, and new minerals
and old ones disappear.
rocks that experience different kinds of mechanical stress, can be
compressed, elongated and/or twisted, which creates the impressive
they are famous for.
Others are "schisty"
clearly visible layers or plates which have a perfect cleavage. This is
not to be confused with the layers of sedimentary rock, which have no
cleavage or the wavy foliation of metamorphics.
While you can
be pretty sure a foliated or "schisty" rock is a metamorphic, not all
metamorphics have foliation or shistiness.