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Igneous Rock

igneous rock

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The term “Igneous rock” is gotten from a Latin word “Ignis” which means “of fire”. Due to the occasional movement of the earth crust such as the pulling apart or combination of plates which occur deep into the ocean, volcanic eruptions occur. This can be above the ground, forming lava or underground in which the resultant liquid is called magma. These eruptions get trapped in small pockets, within the earth, and the resultant body is what is referred to as the igneous rock. An igneous rock can be intrusive or extrusive, depending on its formation. Intrusive igneous rocks are formed below the earth surface (plutonic rocks) while the extrusive are formed above the earth surface (volcanic rocks).

According to research, there are over 700 types of igneous rocks and each one differs from the other depending on the mode of formation and composition. The melting and solidifying of lava is brought about by processes such as a decrease in pressure, an increase in environmental temperature or a change in composition due to external influence. Igneous rocks are more widespread than the other two types of rocks (sedimentary and metamorphic rocks) because they occupy about 16 kilometers of the upper part of the Earth’s crust.

Classification of Igneous Rocks

Igneous rocks can be classified based on different factors such as their texture, chemical composition, mode of occurrence, and physical geometry (such as particle size). Out of all these factors, the present minerals and size of particles in the igneous rock are first used in igneous rock classification. Some of the most important minerals contained in igneous rocks include the quartz, olivine, feldspars, pyroxenes, micas and amphiboles. All other minerals present in such rocks are classified as nonessential.

On the basis of mineral classification, igneous rocks are grouped based on the absence or presence of quartz, type of feldspar they contain and the type of magnesium or iron minerals present. On the basis of particle size, igneous rocks can either be phaneritic (having visible crystals), aphanitic (having extremely small crystals) or porphyritic (having extremely large crystals formed before the complete crystallization of the magma). On the basis of chemical composition, they are classified based on their alkali-silica content or percentage of silicon.

The two types of igneous rocks are the Basalt which is rich in magnesium and the granite rock which is plutonic and has contributed immensely to building construction and interior decoration.

Additional Resources

Hornblende   Indian Stones   Iron Oxide

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