The word "Metamorphism" comes from the Greek: Meta =
change, Morph = form, so metamorphism means to change
form. In geology this refers to the changes in mineral
assemblage and texture that result from subjecting a rock to
pressures and temperatures different from those under which
the rock originally formed.
Note that Diagenesis is also a change in form that occurs in
sedimentary rocks. In geology, however, we restrict diagenetic
processes to those which occur at temperatures below 200oC
and pressures below about 300 MPa (MPa stands for Mega
Pascals), this is equivalent to about 3000 atmospheres of
pressure.
Metamorphism, therefore occurs at temperatures and
pressures higher than 200oC and 300 MPa. Rocks can be
subjected to these higher temperatures and pressures as