Igneous rocks and processes

Igneous Rocks

Igneous rocks are formed from the solidification and cooling of magma or lava.
They are broadly categorised into two types: intrusive and extrusive rocks, based on where this solidification occurs.
Intrusive igneous rocks, also called plutonic rocks, are formed from magma that cools and solidifies within the earth’s crust.
Extrusive igneous rocks, also known as volcanic rocks, are formed when lava cools and solidifies on the earth’s surface.
Common igneous rocks include granite, gabbro, basalt, and andesite.

The formation of igneous rocks involves crystallisation processes, where the liquid magma or lava transforms into a solid crystalline state.
Two key types of crystallisation are fractional crystallisation and crystal settling.
Fractional crystallisation is when different minerals in the magma solidify at different temperatures, changing the composition of the remaining magma.
Crystal settling happens when earlier-formed minerals are heavier than the liquid magma and settle towards the bottom of the magma chamber.

Igneous rocks display a range of textures which provide insight to their formation processes. These include crystalline, glassy, aphanitic, porphyritic and pyroclastic textures.
Crystalline texture indicates that slow cooling has occurred, allowing large visible crystals to form. This is common in intrusive igneous rocks.
Glassy texture is the result of rapid cooling which prevents crystal formation. This is seen in some extrusive igneous rocks, like obsidian.
Aphanitic texture is when the crystals are too small to be discerned with the naked eye, also indicating rapid cooling.
Porphyritic texture describes rocks that have a mixture of large and small crystals, suggesting more than one stage in the cooling process.
Pyroclastic texture is found in volcanic rocks that are formed from the consolidation of volcanic ash and deposits.

The order of mineral crystallisation during cooling is explained by Bowen’s Reaction Series.
It includes two series: the continuous series, from calcium-rich to sodium-rich plagioclase, and the discontinuous series, from olivine to quartz, demonstrating the sequential formation of different minerals.
Understanding this series helps in determining the mineralogical composition and formation processes of various igneous rocks.

Igneous rocks are of significant economic importance.
They are mined for minerals like gold, silver, copper, nickel, lead, zinc, etc.
Construction industries utilise them as crushed stone for road and building construction. Granite, a type of igneous rock, is in high demand for countertops due to its aesthetic appeal and durability.
Some igneous rocks, like pumice, have industrial uses for abrasive cleaning, grinding, and polishing.