Types of Metamorphic Rocks: What are Metamorphic Rocks - Formation

Before learning about types of metamorphic rocks, let us take a look at what are metamorphic rocks.

Metamorphic Rocks: The term Metamorphic is derived from �metamorphose� which means a change in form.

Formation: Metamorphic rocks are formed through the process of metamorphism. As a result of changes in temperature, pressure, or the composition of pore fluids, these rocks have been crystallized in a solid-state to such an extent that the diagnostic features of the original rocks are modified. These original rocks can be igneous, sedimentary, or even metamorphic. These conditions are most often found either deep in Earth�s crust or at plate boundaries where tectonic plates collide.

 

For example, Sandstone changes into quartzite, limestone into marble, clay, and shale into slate, granite changes into gneiss, basalt into schist, and coal into graphite.

 

Types of Metamorphic Rocks

 

On the basis of the agency involved, these rocks are categorized as:

 

1. Thermal Metamorphism

 

When the changes take place due to high temperature, the new rock formed is due to Thermal Metamorphism. The heat may be from hot magma or from the friction of moving rock layers. For example- slate is formed into clay and graphite is formed from coal.

 

2. Dynamic Metamorphism:

 

When the changes are caused due to high pressure, it is called Dynamic Metamorphism. Chemically active hot gases while passing through the rocks change their chemical composition and the minerals get arranged in a series of bands known as foliation. Rocks used in the Taj Mahal at Agra are marbles that have been dynamically metamorphized from the Dolomites.

 

On the basis of the extent of the area, these rocks are categorized as

 

� Contact Metamorphism: This type of Metamorphism is related to volcanic activity. When extremely molten magma or lava rises, it passes through a volcanic pipe or enters the weak points and cavities between different layers. The rocks which come in contact with this lava are baked through the operation of heat. Marble is nothing but baked limestone.

 

� Regional Metamorphism: Due to the mountain building process, igneous and sedimentary rocks are buried deep inside the crust. The pressure of overlying rocks and intense heat changes such rocks. This process is known as regional metamorphism. Many metallic minerals such as gold and silver are found in metamorphic rocks.

Read More: Types of Sedimentary Rocks: What are Sedimentary Rocks? - Class 9