Metamorphic Petrology Revision Content

Metamorphic Processes

Metamorphism refers to the transformation of existing rocks into new types under conditions of high pressure, high temperature, and chemically active fluids.
There are two main types: contact metamorphism (in which the metamorphism occurs due to the heat from nearby molten rock) and regional metamorphism (in which the metamorphism occurs over a large area usually due to tectonic forces).
During the metamorphic process, minerals can remain stable or they can react to form new minerals. This forms the basis for mineral stability diagrams.
New textures called metamorphic textures can form. These can be foliated (layered or banded appearance) or non-foliated (massive and homogeneous appearance).

Foliated Metamorphic Rocks such as slate, schist, and gneiss form when pressure squeezes the flat or elongate minerals within the rock so they become aligned, giving the rock a banded appearance.
Non-foliated Metamorphic Rocks such as marble and quartzite do not exhibit a banded or layered appearance.

Metamorphic grade refers to a scale of metamorphic intensity which goes from low grade (low temperature and pressure) to high grade (high temperature and pressure).
Certain minerals known as index minerals can be used to estimate the metamorphic conditions a rock has been subjected to.
Examples include chlorite (low-grade metamorphism), garnet (intermediate-grade metamorphism), and sillimanite (high-grade metamorphism).

Metamorphic zones are regions where certain index minerals are present.
Metamorphic facies are sets of metamorphic mineral assemblages indicative of particular combinations of pressure and temperature.

Certain metamorphic rocks, especially those with high mineral wealth like garnet and mica schists, are of economic value for extraction of non-metal mineral resources.
Others, such as marble and slate, have been widely used in the construction industry.
Metamorphic rocks also host important ore deposits, such as lead, zinc, and copper.
Many gemstones, such as garnets and rubies, are found in metamorphic rocks.