Igneous Rocks

Defining Igneous Rocks

  • Igneous rocks are formed from the solidification and cooling of magma or lava.

Formation of Igneous Rocks

  • Intrusive igneous rocks form when magma cools slowly beneath the Earth’s surface. Due to the slow cooling rate, larger crystals have time to form. Examples include granite and diorite.
  • Extrusive igneous rocks form when lava cools rapidly at the Earth’s surface. This rapid cooling allows only small crystals to form. Examples include basalt and obsidian.

Classification of Igneous Rocks

  • Igneous rocks are classified on the basis of their mineral content (mineralogy) and the size of the crystals in their texture. The mineral content is often grouped into felsic, intermediate, mafic and ultramafic based on the proportions of silica and other elements.
  • Felsic rocks (like granite) are rich in silica and aluminium, are lighter in colour and less dense than mafic rocks.
  • Mafic rocks (like basalt) have less silica and more iron, magnesium and calcium, are typically darker and more dense than felsic rocks.
  • Intermediate rocks (like andesite and diorite) have compositions between felsic and mafic ends.
  • Ultramafic rocks like peridotite are very rich in iron and magnesium, and contain very little silica.

Features of Igneous Rocks

  • Igneous rocks often have a granular structure, where individual mineral grains are visible to the naked eye.
  • Some igneous rocks may display phenocrysts, which are distinctly larger crystals within a smaller-grained groundmass.
  • Vesicular texture may be seen in rocks like pumice, which is a characteristic texture formed due to gas bubbles trapped in the cooling lava.

Igneous Rock Structures

  • Common structures include dykes (vertical intrusions of magma), sills (horizontal intrusions), batholiths (large blob-shaped intrusions) and volcanoes (extrusive structures).

Igneous Rocks and the Rock Cycle

  • Igneous rocks can weather and erode to become sediments, which can undergo lithification to form sedimentary rocks. They can also be subjected to heat and pressure to become metamorphic rocks.

Economic Importance of Igneous Rocks

  • Igneous rocks are important because they are a major source of minerals like copper, gold, silver and diamonds.
  • They are also used in construction for building material, like granite for countertops and basalt for road pavement.