[Geography Notes] on Igneous Rocks Pdf for Exam

We have all seen the eruption of volcanoes on television or computers. With the eruption of the volcanoes, lava starts flowing on the surface. Magma is usually a molten rock liquid that is found below the surface when the crust of the earth melts. The formation of igneous rocks starts taking place when the molten lava or magma begins to cool down and solidify. Igneous rock meaning relates to the solidification process that leads to the creation of rocks. The features of the igneous rock can be identified by its texture, mineral composition, density, and colour. These rocks are formed either with or without crystallization. 

Cooling Process of Igneous Rock

In the intrusive igneous rock, the process of cooling is usually slow that allows the growth of large mineral crystals within the rocks. The feature of igneous rocks having coarse minerals is due to the crystals of the intrusive rocks. Examples of igneous rocks include granite, peridotite, diorite, and gabbro. 

Next comes the extrusive type of igneous rocks that don’t allow crystallisation to take place. Thus, the final appearance is the fine-grained, glassy and vesicular rock formation. Examples of igneous rocks in the extrusive category include basalt, andesite, and rhyolite. 

Types of Igneous Rocks

The igneous rocks definition includes two categories of rock formation. These two types are discussed below in detail. 

Intrusive Igneous Rocks: When the molten lava cools slowly below the earth’s surface, the crystallization results in the formation of large crystals. These typically have a large amount of silica content within them and based on it they are known as diorite, gabbro, pegmatite, and granite. Actually, most of the magma available in the crust is never able to reach the earth’s surface. 

Extrusive Igneous Rocks: The extrusive igneous rocks are also known as volcanic rocks due to their formation from volcanoes. During the volcanic eruption when the magma reaches the earth’s surface, these are known as lava or volcanic rocks. The features of the igneous rocks in the extrusive category also have a silica content in a higher amount. Some of these rocks cool down so instantly that these form an amorphous glass. Examples of extrusive igneous rocks are basalt, pumice, tuff, etc. 

Types of Magma and Formation of Igneous Rocks

There are types of magma and these igneous rocks meaning comes from the type of magma they originate from. We will see the classification and properties of igneous rocks based on the magma. 

Intermediate Igneous Rocks: The composition of magma between felsic and mafic leads to the formation of intermediate igneous rocks. These are typically formed in the subduction zones that also include the oceanic plates. The structure of the rock includes examples like feldspar, pyroxene, biotite, quartz, and amphibole. 

Ultramafic Igneous Rocks: The characteristics of igneous rocks are mainly ferromagnesian and olivine in nature. For example, a slow cooking rock peridotite is a perfect example in this category. These igneous rocks cool down very slowly and are rare in nature. 

Mafic Igneous Rocks: When magma cools down, the ferromagnetic minerals dominate this type of rock formation. Typically, it is found prevalent in oceanic divergent zones. It contains minerals such as magnesium and iron silicate. Moreover, these rocks also have other minerals like olivine, pyroxene and others.

Felsic Igneous Rocks: This rock formation by magma contains aluminium and silicon. The formation takes place in the continental crust having high gas content. Besides, it also has mineral contents like biotite, quartz, potassium, and more. Examples of rock in this category include rhyolite and granite. 

Identification Process of Igneous Rocks

What we generally understand by the meaning of igneous rocks is that it is formed from the cooling and solidification of molten rocks or magma. When the molten lava starts to cool a new layer of minerals and textures are formed with the addition of new chemical components surrounding the rocks. The pressure and temperature at which the lava begins to cool and the time is taken depends on several factors. Due to this, we can see every igneous rock vary in its texture and composition. However, it is the textural and compositional properties that help us to identify the igneous rocks and determine the cooling process and magma formation. 

Did You Know

The first type of rocks formed on the earth surface was igneous rocks. These are also known as the primary rocks.