Igneous Rock

Due to various reasons for example tectonic movements of plates, crustal loading, density differences, etc. the magma is forced upwards on the surface as lava through the crust along lines of weakness.

Thus after cooling of lava two types of igneous rocks formed, the first type of rock spill out on the surface and consolidate to form extrusive igneous rocks, and another one does not find an outlet, sometimes it solidifies in cavities and fissures within the crust to form intrusive igneous rocks.

Types of Igneous Rock

There are two types of igneous rocks:

1. Intrusive igneous rock- Intrusive indigenous rocks are those which are crystallized below the surface of the earth which results in the formation of large crystals as the process of cooling takes place very slowly some examples include diorite, granite, etc.
2. Extrusive igneous rock- Extrusive igneous rocks refer to those rocks which erupt onto the surface which results in small crystals as cooling takes place at a faster rate and for some rocks, it is so quick, that it leads to the formation of amorphous glass. A few examples include basalt, tuff, etc.

Igneous Rock Properties

1. These types of rocks are known as primary or basic rocks as this type was considered to be the first form of rock and the supply materials for other types of rocks.
2. No fossils contain found in these types of Rock.
3. These types of rock forms include more than one mineral deposit and these may be either glassy or coarse in nature.
4. Based on the rate of cooling of lava, the granular size of the rocks was dependent, for very fast cooling the size of the granular becomes smaller, whereas for slow cooling the size of the granular becomes larger.
5. Based on the chemical and mineral composition of Igneous rocks, some different types of rocks are formed.
   • Mafic or Basic Rocks: Rocks contain 44-52% of silica in nature e.g., Gabbro basalt, Dolerite, etc. This type of rock is denser and darker in color.
   • Ultrabasic or Ultramafic Rocks: Rocks contain less than 44% of silica in nature e.g., Peridotite, etc.
   • Silicic rocks: Rocks contain more than 66% of silica in nature e.g., Granite, Granodiorite, etc. This type of rock is less dense and lighter in color.
   • Intermediate Rocks: Rocks contain 52-66% of silica in nature e.g., Andesite, Diorite, etc. This type of rock is less dense and lighter in color.

Igneous Rock Examples:

• Intrusive igneous rock – diorite, granite, etc.
• Extrusive igneous rock – basalt, tuff, etc.

Based on the origin of rocks, they are classified into three types – Igneous rocks, Sedimentary rocks, and Metamorphic rocks. There are also a few rock substances of organic origin which sometimes contain fossils, for example, coal, peat, etc.

Rock Formation takes place by naturally occurring accumulation of mineral particles. The solid part of Earth is made up of various types of rocks differing from one another in texture, structure, permeability, mode of occurrence, and degree of resistance to denudation.