Types of rocks

There are different types of rocks that form the Earth’s crust. On the basis of origin, these rocks can be classified as

  • Igneous rocks
  • Sedimentary rocks
  • Metamorphic rocks

Types of rocks

Igneous rocks

Cooling and solidification of magma (molten material inside the Earth) forms the igneous rocks. Igneous rocks have a crystalline structure and a non layered appearance. These rocks do not contain any fossils.

Igneous rocks are said to be acidic if they have a high silica (SiO2) content. Acidic rocks have a lighter appearance and a low density.

Basic igneous rocks have a low silica content. These rocks have a darker appearance as compared to acidic rocks.

Igneous rocks can be further classified as – Volcanic or plutonic rocks.

El Capitan Granite
El Capitan granite rock formation in Yosemite National Park

Volcanic rocks

The cooling of lava and tephra coming out of a volcano forms the volcanic igneous rocks. These rocks have smaller crystals. Some examples are Basalt, rhyolite, etc.

Volcanic basalt rock
Volcanic basalt rocks

Plutonic rocks

Plutonic rocks are formed when the magma solidifies inside the Earth’s crust. These rocks come to the surface after years of erosion or denudation.

The crystals of these rocks are larger as compared to volcanic rocks. Some examples are granite, diorite, gabbro, etc.

Sedimentary rocks

Sedimentary rocks are formed by the accumulation of sediments from different rocks or organic materials. These rocks have a non crystalline structure and they may contain fossils.

Over 70% of Earth’s surface rocks are sedimentary. However, they make less than 10% of the total Earth’s crust. Sedimentary rock are also known as stratified rocks because of their layer formation.

These rocks are further classified as – mechanically formed, organically formed and chemically formed sedimentary rocks.

Mechanically formed sedimentary rocks (clastic sedimentary rocks)

Clastic sedimentary rocks form when the fragments from other rocks accumulate and cement together. Some examples are sandstone, cobblestone, etc.

Sandstone
Sandstone – a mechanically formed sedimentary rock

Organically formed sedimentary rocks

These rocks are formed when decayed materials from dead plants and animals accumulate and cement together. Some examples are limestones, coal, etc.

Chemically formed sedimentary rocks

These rocks precipitate from solutions e.g. Rock salts.

Metamorphic rocks

These are igneous and sedimentary rocks which change their appearance and properties under extreme heat and pressure due to the crust’s movement. Some examples are marble, quartzite, gneiss, etc.

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