METAMORPHIC ROCKS
DEFINITION: Metamorphic rocks are formed through the transformation
of preexisting rock into texturally or
mineralogically distinct new rock as a result of high temperature, high pressure or both, but without
the rock melting in the process. The depth and temperature of the metamorphism define the grade of
metamorphism.
Metamorphic rocks have been chemically altered by heat, pressure and deformation,
while buried deep in the earth's crust. These rocks show changes in mineral
composition or texture or both. This area of rock classification is highly
specialised and complex.
Slates are foliated rocks representing low-grade metamorphic alteration of shales (laminated clay).
Argillite is a mudstone, much hardened by pressure.
Schists are foliated medium-grade metamorphic rock with parallel layers, vertical to the direction
of compaction.
Gneiss are banded rocks consisting of alternating layers of quartz and feldspar, of high metamorphic
grade.
Quartzites represent metamorphosed sandstone.
Greywacke is a severely hardened sandstone with mica and feldspar, sometimes containing fossils.
Chert is a siliceous rock deposited chemically, often common among greywacke.
Marble is metamorphosed limestone, just recrystallised.
Metamorphic rock may be of sedimentary origin or stem from igneous rocks.
Rocks formed under high temperatures (basalt, gabbro) are less sensitive
to metamorphosis than those solidified at low temperatures (quartz &
felspar minerals).
The following are causes of metamorphism:
Pressure from sinking deeper while overlaid by other sediments.
Pressure from continental collision and consequent folding and overthrusting
of the crust (dynamometamorphism).
Temperature from sinking deeper, into warmer layers of the crust (metamorphism).
Temperature from igneous hot lava running nearby, either overhead or from intrusions (contact or
thermal metamorphism).
Temperature from steam escaping from hot vents.
Repetitive metamorphism (polymetamorphism).