Igneous Petrology

Igneous Rock Formation

Igneous rocks are formed by the solidification of magma or lava.
This process can occur beneath the Earth’s surface (intrusive) or on the surface (extrusive).
The cooling rate of magma or lava affects the crystal size. Slow cooling results in large crystals (phenocrysts), while rapid cooling leads to small or no visible crystals (aphanitic texture).
Common types of igneous rocks include granite, basalt, obsidian, and pumice.

Igneous rocks are classified based on their mineralogical content and texture.
Rocks are classified as felsic, intermediate, mafic, or ultramafic, depending on the proportion of light or dark minerals.
Felsic rocks contain a higher proportion of silica, aluminium, sodium, and potassium, whereas mafic and ultramafic rocks contain a greater proportion of magnesium and iron.
Textural classifications include phaneritic (coarse-grained), aphanitic (fine-grained), porphyritic, and glassy.

Magma is generated by various processes, including partial melting of the mantle, crustal heating, and water-induced melting.
Magmatic differentiation is the process where a single batch of magma can produce various types of igneous rocks such as mafic and felsic rocks.
The Bowen’s reaction series explains how the cooling of magma leads to the crystallisation of different minerals at different rates. It’s divided into a continuous series (plagioclase feldspar) and a discontinuous series (olivine to quartz).

Most igneous activity is associated with plate boundaries, including divergent boundaries (mid-ocean ridges), convergent boundaries (subduction zones), and hotspots.
Divergent boundaries often produce mafic rocks like basalt, while subduction zones generate intermediate rocks like andesite and felsic rocks like rhyolite.
Hotspots, such as the Hawaiian Islands, are known for their basaltic composition, producing shield volcanoes.

Common igneous structures include dykes (vertical intrusions), sills (horizontal intrusions), plutons (large, deep intrusions), and batholiths (the largest intrusions).
Pyroclastic deposits, volcanic bombs, and pillow lavas are some recognised features related to extrusive igneous activity.

Many valuable resources are found in igneous rocks. For instance, diamonds are typically found in kimberlite, an ultramafic rock.
Precious metals such as gold, silver and copper are often associated with porphyry copper deposits in felsic igneous rocks.
Granites and basalts are frequently used in the construction industry due to their durability and aesthetic appeal.

Detailed study of thin sections under a microscope can reveal mineralogical details, grain sizes, and other textural features of igneous rocks, offering clues about their cooling history, magma composition, and crystallisation sequence.

Certain igneous rocks can be dated using radiometric dating methods, allowing us to determine their absolute age and understand Earth’s geological history.
For example, the uranium-lead method can be used to date zircon crystals in granite or the potassium-argon method can be used to date volcanic rocks.