Igneous Rocks: Meaning, Types and Formation

2 years ago

Atiqur Rahman

9 minutes

The term ‘igneous’ is derived from the Latin word ‘ignis’ means fire. Igneous rocks are formed through cooling, solidification and crystallisation of molten materials i.e. magma. They are sometimes called primary rocks. These rocks were organised first all the rocks at the time of origin of the earth and especially during the formation of the lithosphere.

In other words, igneous rocks represent the rocks from which all other rocks directly or indirectly have been derived. That is why these rocks are also called parent rocks. Igneous rocks are characterised as the hardest rock as resistant to weathering, with fine to coarse-grained texture with the absence of fossils and no strata like sedimentary rocks.

Types of Igneous Rocks

Classification on the Basis of Mode of Occurrence

Igneous rocks can be divided into groups depending on the conditions under which they solidify. One group is termed extrusive rocks, and another one is intrusive rocks. These groups of igneous rocks are discussed as follows:

1. Extrusive Rocks

Extrusive rocks are those rocks which have been ejected from a volcano or some other vent and are accumulated and solidified on the surface of the earth. These rocks may be further subdivided into two parts on the basis of the way of eruption.

Explosive Type- Explosive type of volcanic eruption ejects accumulated gases and lavas, which are thrown violently into the air. Volcanic materials include bombs which are big fragmented rocks; those about the size of walnut are lapilli, and very fine materials are called ash or volcanic dust. Fine volcanic materials, when deposited in aquatic conditions, are called tuffs. ‘Breccia’ or ‘agglomerates’ are formed after the deposition of coarse and fine materials.

Quiet Type- The molten materials come out through minor cracks on the earth’s surface, which are called lava flows. It may happen that the successive flows give rise to layers of lavas after being piled one on another. These lavas, after being cooled and solidified, form basaltic igneous rocks. Such kind of flood basalts is formed by several episodes of lava flow during the fissure flow of volcanic eruption that further forms lava plateaus and lava plains.

2. Intrusive Rocks

The rising magma solidifies below the earth’s surface during volcanic activity and remains surrounded by older, pre-existing rocks, which are called intrusive rocks. They further subdivided into two major groups on the basis of depth and place of cooling:

Plutonic Igneous Rocks- They are formed due to the cooling of magma at deep inside the earth where surrounding rocks totally cover the hot magma and consequently slow down the rate of cooling. Due to the situation at the greater depth, magma may require many thousands of years for complete cooling, and the mineral crystals in the rocks can grow to a relatively larger size that can be seen with the naked eye.

Hypabyssal Igneous Rocks- When the rising magmas come just below the earth’s surface from the interior with the cracks, pores, crevices and hollow places during the volcanic eruption, the resultant cooled and solidified rocks are known as hypabyssal igneous rocks or volcanic rocks. The mineral crystals in volcanic rocks are so small as to be invisible without microscopic inspection.

Black or dark grey coloured and fine-grained basalt rocks are suitable examples of hypabyssal igneous rocks. There are some important forms of these rocks according to the solidification depending on hollow places, such as batholiths, laccoliths, phacoliths, lopoliths, sills, dykes, necks etc.

Intrusive Igneous Rocks on the Basis of Mode of Occurrence

Classification on the Basis of Texture

On this basis, igneous rocks can be divided into five parts such as:

Glassy Igneous Rocks

They are characterised by a general absence of grains and are produced by very fast cooling of magma on the earth’s surface. Obsidians, pitch stones, pumice perlite etc., are the most common example.

Aphanitic Igneous Rocks

The word ‘aphanitic’ has been derived from the Greek word ‘phaneres’, meaning thereby visible. These rocks are characterised by small grains that can be visible without a microscopic vision. Basalt, felsite and the rocks of sills and dykes are examples of this category.

Phaneritic Igneous Rocks

These rocks contain grains that are large enough to see it without a microscope. As the equal size and form of grains, the equigranular texture represents a uniform cooling rate and the crystals’ large size. For example, coarse-grained and plutonic igneous rocks are such as granite, diorite etc. (See Figure).

Porphyritic Igneous Rocks

Porphyritic rocks contain two types of grains of different sizes. These are found in two stages of cooling. In the initial stage, the cooling rate is slow, resulting in larger crystals, but in the next stage, the cooling rate is faster than earlier, forming smaller crystals. Porphyritic texture occurs in both aphanitic and phaneritic rocks. For example, basalt, granite, felsite, diorite etc.

Pyroclastic Igneous Rocks

The word ‘pyroclastic’ has also been derived from the Greek word ‘klastos’, meaning thereby broken. These rocks are characterised by broken and fragmented rather than interlocking or interconnected crystals. Such type of igneous rock constitutes bombs, lapilli, breccia, volcanic dusts, ashes, tuffs etc.

Classification of Igneous Rocks on the Basis of Chemical Composition

The dominant chemical present in igneous rock is silica (SiO2). Igneous rocks are divided into four types on the basis of silica content such as:

Acid Igneous rock

In these rocks, silica content ranges between 65 to 85 per cent, and average density ranges between 2.75 to 2.8. Acid igneous rocks are composed of Quartz, white and pink feldspar. The most predominant example of such type of rock is granite (See Figure 4).

Basic Igneous Rocks

In these rocks, silica content ranges between 45 to 60 per cent and the average density varies between 2.8 to 3.0. These rocks are heavy and dark-coloured because of the dominance of iron content. Significant examples are basalt, dolerite, gabbro etc. (See Figure).

Intermediate Igneous Rocks

Silica content in these rocks is less than the amount present in the acid igneous rocks but more than basic igneous rocks. The most dominant rocks of this group are diorite and andesite (See Figure).

Ultra- Basic Igneous Rocks

These rocks contain less than 45 per cent of silica, but their average density differs from 2.8 to 3.8, which means the highest average density is found in this group of igneous rocks. Peridotite is the representative of this group of rocks.

Some Important Igneous Rocks and Their Distribution

Granite- The granite is a coarse-grained plutonic intrusive rock which is formed deep within the earth. Since the rate of cooling and solidification of magmas inside the earth is very slow because of very high temperatures prevailing underground and hence, granites become coarse-grained. According to chemical composition, granites are acidic rocks, whereas silica content ranges between 65 to 85 per cent. These rocks are generally light in weight as their density varies from 2.7 to 2.8.

Table: Mineral Composition of Granite

Sl. No.	Minerals	Percentage
1	Feldspar	52.3
2	Quartz	31.3
3	Mica	11.5
4	Hornblende	2.4
5	Iron	2.0
6	Others	0.5
7	Total	100

Source: Physical Geography by Savindra Singh

Rhyolite- It is the extrusive equivalent of granite and, likewise, is composed essentially of light-coloured silicates. This rock is fine-grained and frequently contains fragmented glass and voids, indicating rapid cooling in a surface environment.

Basalt- It is a very fine-grained dark extrusive igneous rock formed due to the cooling and solidification of molten lavas at the earth’s surface. Basaltic rocks contain a high percentage of dark silicate minerals, so geologists also refer to them as mafic. If basalt has small grains rather than a minute, it is called aphanitic basalt. Chemically basalt contains 45 to 65 per cent of silica content. Basaltic rocks are typically darker and denser than granitic rocks.

Some Important Igneous Rocks on the Basis of Texture and Composition

Table: Mineral Composition of Basalt

Sl. No.	Minerals	Percentage
1	Feldspar	46.2
2	Augite	36.9
3	Olivine	7.6
4	Mineral Iron	9.3
5	Total	100

Source: Physical Geography by Savindra Singh

Due to the cooling of lavas, polygonal cracks are developed in basalts, which give birth to peculiar uneven landforms. Many volcanic Islands, such as the Hawaiian Islands and Iceland, mainly comprise basalt.

Gabbro- The coarse-grained intrusive equivalent of basalt is gabbro. Although gabbro is not commonly exposed at the surface, it makes up a significant percentage oceanic crust. Olivine may occur considerably, but biotite and hornblende are rarely present in gabbro.

Andesite- It contains a mixture of both light and dark-coloured minerals, mainly amphibole and plagioclase. They are typically confined to continental margins.

Diorite- When magma of intermediate composition is crystallised at depth, it forms the coarse-grained rock called as diorite. Diorites are composed of hornblende and various feldspars.

Igneous Rocks: Meaning, Types and Formation

Types of Igneous Rocks