The planet we call earth is made of three types of rocks. The rocks are classified according to the way they form. The three types are: sedimentary, igneous and metamorphic rocks.
Sedimentary and Igneous Rocks
Sedimentary rocks are formed through the gradual deposition and accumulation of various materials, which could include organic and inorganic matter over a substantial period of time. They are so called because of the sediments that are deposited which eventually develop into a rock. Sedimentary rocks are of three types: clastic, chemical and organic. Igneous rocks are made of magma, either on the surface of the earth or inside the crust. Igneous rocks are of two types: intrusive and extrusive. Both sedimentary and igneous rocks are the primary types of rock formation. Metamorphic rock is not a primary rock.
What is Metamorphic Rock?
As the name implies, metamorphic rock is formed after a process of metamorphosis or metamorphism, in other words transformation. The metamorphosis or transformation in existing rocks can be brought about by the changes in atmospheric conditions or the immediate environment that includes heat and pressure.
It must be noted that metamorphic rocks are formed from existing rocks. They are not the first stage of any rock. Sedimentary and igneous rocks can become metamorphic rocks subjected to heat and pressure over time. When any rock is subjected to more heat and pressure than it can bear, there will be changes in its form or structure, chemical composition, texture and various properties. The rock may break down, it may become a completely different rock due to the changes of its components and the physical or chemical characteristics will be transformed. While sedimentary and igneous rocks are generally the earlier phase of metamorphic rock, there are many metamorphic rocks that have formed from an older or existing metamorphic rock.
Types of Metamorphic Rock
There are two types of metamorphic rock: foliate and non-foliate. Foliate metamorphic rocks will have a banded or layered appearance. Examples are slate, gneiss, schist and phyllite. Non-foliate metamorphic rocks do not have any layered appearance. Examples are marble, hornfels, novaculite and quartzite.
Metamorphic rocks are usually formed as a result of two types of metamorphism: contact metamorphism and regional metamorphism. Contact metamorphism is a process in which the exact cause for metamorphism is in direct contact with the existing rock. Regional metamorphism doesn’t require direct contact. The causal factor or factors will have an indirect impact and cause the metamorphism.