There are three basic types of rocks. The igneous rocks, the metamorphic, and the sedimentary rocks. This classification has been made by earth scientists on the basis of their formation, properties, and composition. Read this classification in detail. Types of rocks.
The term igneous has been derived from the Latin word “Ignis”, which means fire. Thus the name denotes that the rocks, which originates through fire. Scientifically speaking, these rocks were formed as a result of the cooling of molten magma erupted in the form of Lava, either inside the crust or on the surface. According to the definition of H.J.de Blij,
“Magma is not only a complex melt of many minerals, it also means gases including water vapor. It is a surging, swelling mass that pushes outward and upward. Sometimes facing itself into and though existing layers of rocks in the crust. If its upward thrust ceases before it reaches the surface, the resulting rock formed from the cooled magma are called intrusive igneous rocks. If it penetrates all the way to the surface and spills out as lava or ash, the rocks formed from these materials are called extrusive igneous rocks.”
Types of Igneous Rocks:
(1). Types on the basis of Location:
There are three types of the Igneous Rocks which are categorized on the basis of the their mode of formation:
- Plutonic Rocks:
These rocks are found at great depths because the molten magma cools down at places situated deep in Earth. The plutonic rocks are crystalline such as granite. The crystals of these rocks are large in size.
- Dyke Rocks:
These rocks are formed in dykes of vertical wall-like structure because the molten material gushing from the interior of Earth does not reach the surface of Earth though volcano’s mouth. The structure of these rocks is semi- crystalline.
- Volcanic Rocks (Lava):
The rocks which are formed as result of the volcanic eruptions are called volcanic rocks. These are glassy and belong to the extrusive category of igneous rocks.
(2). Types of igneous rocks on the basis of Elements:
The rocks are composed of different elements. These are divided into four types on the basis of the proportion of element:
These are composed mostly of the alkalis.
In these rocks, in the percentage of silicon is more than the percentage of irons, magnesium and lime. e.g. Dacite, Tonalite, Porphyrite and Quartz.
These rocks are composed of oxides, silicates and ferro magnesium.
Basalt and Tachylite Rocks:
These rocks are composed of Iron oxides, ferro magnesium silicates and lime.
(3). Types on Basis of composition:
The rocks are divided into Acid Rocks and Basic Rocks on the basis of their composition:
(1). Acid Rocks:
These Rocks are the pale of color but very hard and weather resisting. These are composed on silica (80%) and other elements. These rocks are located in the region of Earth called Sial.
(2). Basic Rocks:
These Rocks are composed of Silica (40%), magnesia (40%) and lime (20%) iron oxide and aluminum etc. These are found in region of Earth is called Sima.
(4). Types on Basis of Magma Position:
(1). Intrusive Igneous Rocks
(2). Extrusive Igneous Rocks
(1). Intrusive Igneous Rocks:
To the upward, thrust of magma (Lava) stops and cools to from rocks before reaching the surface of Earth. Then such Rocks are called Intrusive igneous rocks. In the formation of these rocks, the cooling process is slow thus very large crystals are formed in them. These are also called Plutonic rocks as per the Roman god, Pluto—the gold of underworld.
(2). Extrusive Igneous Rocks:
These rocks are formed on the surface of Earth when magma succeeds in coming out of the crust. Therefore, these are also called volcanic rocks. In such rocks, the process of cooling is very fast. Extrusive igneous rocks are fine grained due to small crystals.
(5). Types on Basis of Mineralogical Ratio:
On the basis of Mineralogical compositions, the rocks are divided into following categories:
- Essential Minerals (Product of Magmatic Crystallization)
- Accessory Minerals (Product of Magmatic Crystallization)
- Secondary Minerals ( Formed Through Change of Primary Minerals)
Economic importance of Igneous Rocks:
Igneous rocks are economically very important due to the following seasons:
- These rocks are the source of important minerals such as platinum, nickel, iron, tin, lead and gold, etc.
- Bright coloured stone called granite is also obtained from these rocks.
- These rocks are the source of many stones which serve as building materials.
- The element contained in these rocks add to the fertility of the land.
- Many igneous rocks are extremely hard and weather resistant. Due to these qualities, these are used for the construction of metalled roads.
- Some rocks are used for grave stone and monuments.