Igneous Rocks – Definition, Classification and Examples

Igneous Rocks are formed by cooling or solidification of molten or partly molten material, i.e. from a magma or lava. Igneous rocks are first formed rocks which had made up the primodial earth’s crust. For this reason, they are called primary rocks.

Origin of Igneous Rocks

The igneous rocks are formed at high temperature directly as a result of solidification or crystalization of magma or lava.  Based on the mode of occurrence igneous rocks are classified as extrusive and intrusive igneous rocks. Extrusive igneous rocks are formed by the solidification of lava over the earth’s surface whereas intrusive igneous rocks are formed by solidification of hot molten material below the earth’s surface.

The principal elemental composition of igneous rock are oxygen (O), silicon (Si), aluminum (Al), calcium (Ca), sodium (Na), potassium (K), iron (Fe), and magnesium (Mg). Two constituents—silica (SiO₂) and water (H₂O)—largely control the physical properties of magma, such as its density, viscosity (the tendency for a material to resist flow), and the manner in which it is extruded.

Classification based on depth of formation

Igneous rocks can be classified into three categories based on depth of formation; Volcanic, hypabyssal and plutonic rock.

Volcanic rocks are generally finely crystalline or glassy igneous rock resulting from volcanic eruption at or near the Earth’s surface, either ejected explosively or extruded as lava; e.g. basalt. The term includes near-surface intrusions that form a part of the volcanic structure.

Hypabyssal rocks are igneous rock or intrusive body formed at a shallow depth.

Plutonic rocks are formed at considerable depth; it is characteristically medium- to coarse-grained and of granitoid texture.

Classification based on Silica Percentage

Ingneous rocks are also classified based on silica content. When silica content icrease more than 66%, the igneous rock called acidic igneous rock, eg. Granite. When silica content is 52-66% it is called as intermediate igneous rock, e.g. Syenite. If silica content is 45-52% it is termed as Basic Igneous rock, Dolerite and Gabbro. Rocks containing less than 45% silica content are called Ultrabasic rocks, e.g., peridotite.

The following table is a simple subdivision of igneous rocks according to both their composition and mode of occurrence.