Igneous Rocks: Composition, Types and Examples of Igneous Rocks

The upper 16 kilometers of the Earth’s crust is made up of 95% Igneous rock, with a thin covering of sedimentary and metamorphic rocks. Igneous rocks are formed when molten rock cools, forming silicate mineral crystals. Felsic minerals are light colored and less dense, and mafic minerals are dark colored and more dense. The igneous rocks are generally hard and water percolates in them not so easily.

The most important characteristics of Igneous rocks are as follows:

  • They usually do not occur in distinct beds or strata like sedimentary rocks.
  • Igneous rocks are generally not having any fossils
  • They are generally granular and crystalline.
  • They are less affected by chemical weathering as the water does not percolate in them easily.

Magma as source of Igneous Rocks

The mixture of the Molten Rocks which makes the Igneous rocks is called Magma. Magma in fact is a mixture of molten rocks, volatiles (gas) and other solids. It originated from the partial melting of the lower crust and the upper mantle, mainly at depths of 15-200 kilometers. Most magma is as hot as 700 °C to 1300 °C and is silicate mixtures mostly.

Most igneous rock consists of silicate minerals. These rocks also contain mostly metallic elements. The mineral grains in igneous rocks are very tightly interlocked, and so the rock is normally very strong. Quartz, which is made of silicon dioxide (SiO2), is the most common mineral of all rock classes. It is quite hard and resists chemical breakdown.

The chambers under a volcano where Magma collects are called magma chambers. The magma chambers feed a volcano. Bulks of the igneous rocks are result of the cooling and solidifying of Magma. There are two processes by which Magma cools and solidifies. These are called “plutonic” and “Volcanic Eruption”. When the Molten Magma goes down deep within the earth and gets solidified, it is called Plutonism. On the contrary, the molten Magma can also come out on the surface of earth via a volcanic eruption.

Intrusive and Extrusive Igneous Rocks

Magma that solidifies below the Earth’s surface and remains surrounded by older, pre-existing rock is called intrusive igneous rock. Because intrusive rocks cool slowly, they develop large mineral crystals that are visible to the eye. They are further classified into Plutonic, Hypabyssal, Batholiths and Laccoliths as follows:

  • Plutonic: Generally very large crystal and they were formed due to cooling of magma very deep inside the Earth
  • Hypbyssal / subvolcanic : Consolidated in a zone above the base of Earth’s crust and hence has distinct structural characteristics.
  • Batholiths: They extend to greater depths and larger areas
  • Laccoliths: A sheet intrusion that has been injected between two layers of sedimentary rock

If the magma reaches the surface and emerges as lava, it forms extrusive igneous rock. Extrusive igneous rocks cool very rapidly on the land surface or ocean bottom and thus show crystals of only microscopic size.

We note here that Granite typically accumulates in batholiths. A single batholith sometimes extends down several kilometers and may occupy an area of several thousand square kilometers.

Felsic Rocks and Mafic Rocks

Whatever may be the process of cooling and solidifying, the magma while converting into a rock, undergoes numerous chemical and physical changes.  Accordingly, there are two major types of Igneous rocks are produced viz. Felsic Rocks and Mafic Rocks. Felsic rocks are rich in silicon, oxygen, aluminium, sodium, and potassium, while the mafic rocks are rich in magnesium and iron. If the rock is highly dominated by Magnesium and Iron, it is called Ultramafic.

Examples of Igneous Rocks

  • Granite: Intrusive (batholith generally), Felsic, igneous rock. Worldwide average chemical composition of Igneous Rocks has SiO2 — 72.04% & Al2O3 — 14.42%
  • Diorite: intermediate intrusive igneous rock
  • Gabbro: Mafic igneous rocks equivalent to basalt.
  • Peridotite, Rhyolite, Andesite, Basalt, Komatiite, Diabase etc.