Metamorphic Rock Characteristics

Metamorphic rocks are the types of rocks that change considerably from their original igneous, sedimentary or metamorphic structure and composition. These metamorphic rocks develop under heat and pressure near a pre-existing rock or collision, etc.

What are the Characteristics of Metamorphic Rocks?

Metamorphic rocks undergo metamorphism that involves change in existing rocks structure or composition by heat, pressure or even chemical action. This change causes chemical or structural modification of the minerals in the rock. The continental crust of the earth is made up of metamorphic and igneous rocks. The base layer of the core is made of metamorphic and igneous rocks, that is covered by a thick layer of sedimentary rocks. In some regions, the base layer is exposed and is known as shields. The metamorphic rocks are also found in the core of the mountain ranges around the world.

Fact: The Acasta gneiss is a metamorphic rock on the Canadian Shield that is one of the oldest rock on the planet. It is about 3.96 billion years old.

Characteristics Due to Heat
The important factor in metamorphic rock modification of a rock is due to heat. The rocks begin to change their chemical structure when the temperatures reach above 200° C . The minerals in the rock break down and get transferred into different elements and compounds that gives rise to a new rock all together.

Fact: If the rocks are heated to a temperature that turns them into magma, they become igneous rocks after cooling down.

Heat applied on the rocks can be through two processes; the tectonic subduction and intrusion of magma. The rocks at the surface get transported into the crust and the upper mantle at the tectonic subduction zone. The temperature beneath earth's surface increases at the rate of 25° C per kilometer and can reach about 800 to 1200° C at the base crust. This heat causes the metamorphism of the rocks.

Sometimes, magma migrates through the crust and forms igneous intrusion that is seen along the western side of North America where this subduction occurs. The rocks surrounding the magma body metamorphose into a different rocks due to heat dissipation.

Characteristics due to Pressure
The rocks that are deep within the earth are subjected to a lot weight of the matter above it. The tectonic forces also cause a lot of pressure over the rocks. This pressure can cause mineral crystals in the rocks to reorient themselves. Thus, rearrangement of crystalline structure causes changes in the rock from its original form.

Characteristics due to Chemicals
The metamorphism of rocks due to chemical reaction also causes them to change from their original characteristics. There is often presence of water and carbon dioxide found in small amounts in between the mineral crystals or in the pore spaces of the rocks. When they are mixed they cause chemical reactions and dissolving ions. Thus, with substitution, removal and addition of chemical ions leads to formation of a new rock.

Types of Metamorphism

Metamorphism of the rocks can occur by three process. These three process that lead to metamorphic rocks are as follows.

Thermal Metamorphism
The thermal metamorphism involves heating, structural and chemical alteration of rocks due to a process associated with the plate tectonics. The thermal metamorphism involves two sub-categories, regional and contact metamorphism. In regional metamorphism, the large-scale heating and modification of rocks takes place through creations of plutons at the tectonic subduction zones. This metamorphism is seen in large areas and volumes of rock. The contact metamorphism is a small-scale heating and changes in the rock seen in case of igneous intrusion.

Dynamic Metamorphism
The dynamic metamorphism is a structural change in rock that occurs through excretion of pressure. The minerals do not change chemically and is seen in mountain building where extreme pressure is excreted.

Metasomatic Metamorphism
The chemical changes in the elements of the rock minerals when gases and liquids are absorbed into the bedrock lead to metasomatic metamorphism.

Types of Metamorphic Rocks

The types of metamorphic rocks are classified according to their texture and composition. They are foliated rocks and non-foliated rocks. The types of foliated rocks have a distinct planar structure that is because of the mineral grains. The foliated metamorphic rocks are:

• Slate
• Phyllite
• Schist
• Gneiss
• Amphibolite
• Migmatites

The non-foliated rocks are formed due to regional metamorphism of monomineralic rocks or even by contact metamorphism. The types of non-foliated rocks are:

• Marble
• Quartzite
• Greenstone
• Hornfels
• Anthracite

Thus, this was a brief answer related to the question what are some characteristics of metamorphic rocks. The metamorphic rock characteristics include classification by texture and composition. They rarely contain fossils and may react with acid. They have alternate bonds of light and dark minerals. They are either composed of only one mineral like marble. They have visible layers of crystal and made up of mineral crystals of different sizes. They rarely have pores and may have a bent or cured foliation.