Answer:

Definition of Metamorphism

The word "Metamorphism" comes from the Greek:  meta = after, morph = form, so metamorphism means the after 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.

The original rock that has undergone metamorphism is called the protolith.   Protolith can be any type of rock and sometimes the changes in texture and mineralogy are so dramatic that is difficult to distinguish what the protolith was.

Note that diagenesis and weathering are also a changes in form that occur in 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 3,000 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 they become buried deeper in the Earth.  Such burial usually takes place as a result of tectonic processes such as continental collisions or subduction.

The upper limit of metamorphism occurs at the pressure and temperature of wet partial melting of the rock in question.  Once melting begins, the process changes to an igneous process rather than a metamorphic process.

During metamorphism the protolith undergoes changes in texture of the rock and the mineral make up of the rock.   These changes take place mostly in the solid state and are caused by changes in physical or chemical conditions, which in turn can be caused by such things as burial, tectonic stress, heating by magma or interactions with fluids. 

Factors that Control Metamorphism

Metamorphism occurs because rocks undergo changes in temperature and pressure and may be subjected to differential stress and hydrothermal fluids.   Metamorphism occurs because some minerals are stable only under certain conditions of pressure and temperature.  When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions.  But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism.

Temperature

Temperature increases with depth in the Earth along the Geothermal Gradient.  Thus higher temperature can occur by burial of rock.

Temperature can also increase due to igneous intrusion.

Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth.  Pressure is defined as a force acting equally from all directions.  It is a type of stress, called hydrostatic stress, or uniform stress. 

If the stress is not equal from all directions, then the stress is called a differential stress.



There are two kinds of differential stress.  Normal stress causes objects to be compressed in the direction of maximum principal stress and extended in the direction of minimal stress.  If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock.  Shear stress causes objects to be smeared out in the direction of applied stress.

Differential stress if acting on a rocks can have a profound affect on the appearance or texture of the rock.

Rounded grains can become flattened in the direction of maximum stress.



Minerals that crystallize or grow in the differential stress field can have a preferred orientation.  This is especially true of the sheet silicate minerals (the micas: biotite and muscovite, chlorite, talc, and serpentine).