Metamorphism
In geology, process of change in the structure, texture, or composition of rocks
caused by agents of heat, deforming pressure, shearing stress, hot, chemically active
fluids, or a combination of these, acting while the rock being changed remains essentially
in the solid state. Theoretically, rocks are formed when their constituents are in
equilibrium with ambient physical conditions. If the conditions are changed by crustal
movements or by igneous activity, metamorphism occurs to reestablish equilibrium and
changes the physical character of the rock mass.
Characteristics of Metamorphism
Metals in the Periodic Table
In general, a metamorphic rock is coarser and has a higher density and lower
porosity than the rock from which it was formed. Under low-grade metamorphic
conditions, the original rocks may only compact, as in the formation of slate from shale.
High-grade metamorphism changes the rock so completely that the source rock often
cannot be readily identified.
Foliation
Alteration of rock texture by metamorphism commonly results in a rearrangement
of mineral particles into a parallel alignment, called foliation, as a result of directed
stress. Foliation, called banding or layering, is probably the single most characteristic
property of metamorphic rocks. For example, slate is a metamorphic rock in which there
has been little recrystallization of fine-grained sedimentary shale, but mineral
realignment gives the rock a tendency to break along smooth planes termed slaty
cleavage. Further higher-grade metamorphic conditions lead to a foliation called
schistosity, resulting in schists, formed when tabular minerals, such as hornblende,
graphite, mica, or talc are aligned and tightly packed in a parallel fashion. High grade
metamorphism can segregate minerals, thereby forming bands. This foliation is called
gneissic layering and forms gneiss from such rock as granite. Foliation does not always
occur during metamorphism.
Changes in Chemical Constituents
Chemical changes occurring during metamorphism also can rearrange the
chemical constituents into assemblages stable in their new environment, thus often
forming new minerals of essentially the same chemical composition as those occurring in
the rock prior to metamorphism.
For example, hornblende can be changed into garnet or pyroxene. The mineral
composition of rocks may also be altered by the addition of new elements or by the
removal of elements formerly present through the action of circulating liquids or gases or
by recrystallization under pressure.
Types of Metamorphism
Local Metamorphism
Contact metamorphism occurs when local rocks are metamorphosed by the heat
from an igneous intrusion, such as limestone turning to marble along the contact zone.
Some of the changes that occur in the older rock are due simply to the heat radiated from
the igneous mass and to the pressures it creates. More extensive alterations are produced
by the fluids and gases given off by the igneous mass; metamorphism of this type rarely
causes foliation. Rocks around hot springs, or mineral-rich water, both of which are
common along active plate boundary ridges (see plate tectonics), are often changed by
hydrothermal metamorphism, (or metasomatism). This may, for example, transform
granite into china clay. “Black smokers”, which occur along mid-ocean ridges, are the
exit vents for extensive hydrothermal systems that alter basalts and can deposit mounds
of metalliferous sediments on the seafloor. Metamorphic rocks that develop by shearing
and crushing of the rock at low temperature are called cataclastic and are usually
associated with the mechanical forces, especially pressure, involved in faulting.
Regional Metamorphism
Metamorphism on a grander scale, called regional metamorphism, accompanies
mountain-building activity. These metamorphic rocks pervade regions that have been
subjected to intense pressures and temperatures during the development of mountain
chains along boundaries between crustal plates. Large scale, intense regional
metamorphism is particularly great in the "roots" of these mountains, which were at
considerable depths when the pressures forming the mountains were active. These kinds
of metamorphic rocks are most commonly exposed in old mountain chains, like the Blue
Ridge Mts., that have substantially eroded away over time, leaving only disturbed
structure and regional metamorphic rocks.