Metamorphic Rocks

Metamorphic rocks are one of the three types of rocks found on earth. The other two are sedimentary and igneous rocks. Metamorphic rocks account for 12% of the earth’s land surface and comprise a large part of the earth’s crust.

How are Metamorphic Rocks formed?

Metamorphic rocks are created from existing rocks. These rocks are not deposited like igneous rocks. This conversion process is known as metamorphism. Some metamorphic rocks are also created as a result of metasomatism.

The transformation or conversion happens when an existing rock undergoes extreme temperature and pressure. This rock is known as original rock, protolith, or parent rock. The temperature and pressure are usually above 150 to 200°C (300 to 400°F) and 100 megapascals (1,000 bar). The exposure to higher temperature and pressure causes big changes in the rock, and it recrystallizes to a new texture and mineral composition. The rock remains solid during the transitional change.

Types of Metamorphic Rocks

Metamorphic rocks are usually of two types.

Foliated Metamorphic Rocks: Foliated metamorphic rocks have a layered or banded appearance. This appearance results due to exposure to heat and directed pressure. Common foliated rocks are slate, phyllite, schist, and gneiss.

Non-Foliated Metamorphic Rocks: These rocks don’t have aligned mineral crystals and form as a result of low pressure and temperature. These rocks usually form when parent rock consists of blocky minerals like quartz and calcite. Common non-foliated rocks are marble, quartzite, and hornfels.

Metamorphic Rocks Classification

Metamorphic rocks are usually classified based on the following properties.

• Texture
• Protolith
• Chemical composition

Metamorphic Rocks Uses

Metamorphic rocks have different uses. Some rocks are best for the construction industry, home décor, roofing, dimension stone, sculpturing, and as a host for precious gemstones and marbles.

The exact use of the rock depends on hardness, quarrying ease, and how easily it can be processed.

Levels of Metamorphism

The level of metamorphism a rock receives dominates the transformation process and recrystallization. There are two levels of metamorphism that create metamorphic rocks.

Low-grade Metamorphism: The temperatures are between 200 to 350°C with relatively low pressure. Low-grade metamorphic rocks are characterized by hydrous minerals.

High-Grade Metamorphism: The temperature is above 350°C with higher pressures. These rocks have fewer hydrous minerals because the resultant rock loses H20, and non-hydrous minerals become more common.

Types of Metamorphism

Contact Metamorphism: It occurs adjacent to igneous intrusions with high temperatures due to superheated magma. The metamorphism process is limited to a small area because only a small area is affected by magma. Contact metamorphism has low pressure because the temperature contrast between the surrounding rock and magma is high at shallow levels in the crust.

Regional Metamorphism: It occurs over large areas and is generally not linked with igneous bodies. Regional metamorphism is accompanied by deformation due to non-hydrostatic or differential stress conditions. Regional metamorphism creates strongly foliated rocks like slate, schist, and gneiss. Regional metamorphic rocks are usually found in eroded mountain ranges or the cores of fold/thrust mountain belts.

Cataclastic Metamorphism: It happens when two rock bodies slide one another along a fault line. The rocks are created as a result of mechanical deformation. When rocks slide past one another, heat is generated, mechanically deforming the rock. Cataclastic metamorphism is uncommon and happens in narrow zones where shearing has occurred.

Hydrothermal Metamorphism: This type of metamorphism happens when hydrothermal fluids with high temperatures and pressure pass through already deposited rocks. It usually happens in basaltic rocks that don’t have hydrous minerals. Hydrothermal metamorphism creates ore deposits.

Burial Metamorphism: Burial metamorphism creates metamorphic rocks from sedimentary rocks buried several kilometers below the earth’s surface. The temperature may go up to 300C. Burial metamorphism is often considered regional metamorphism when temperatures increase.

Metasomatism: When fluids alter a preexisting igneous, sedimentary, or metamorphic rock, it is known as metasomatism. The new rock has different compositional and mineralogy.