Igneous rocks originate from the cooling and solidification of molten rock material. This fundamental geological process forms one of the three primary rock types, alongside sedimentary and metamorphic rocks. Igneous rocks are prevalent, constituting 90–95% of the Earth’s upper 16 kilometers of crust by volume, and they cover approximately 15% of the Earth’s current land surface.
Origin of Molten Rock
Molten rock, known as magma when located beneath the Earth’s surface and lava once it erupts, begins deep within the planet. High temperatures are necessary to melt rock. This melting occurs in specific geological settings such as mid-ocean ridges, subduction zones, and hot spots. The process involves partial melting, where only some minerals within a rock melt due to varying melting points. This can be triggered by a decrease in pressure as rock rises towards the surface (decompression melting) or by the addition of water, which lowers the rock’s melting temperature (flux melting).
The Cooling Process
The transformation of molten rock into solid igneous rock is a process called crystallization. As magma or lava cools, mineral crystals begin to form and grow from the liquid melt. The rate at which this cooling occurs profoundly influences the size of the crystals that develop within the rock.
When molten material cools slowly, mineral crystals have ample time to grow larger. Conversely, rapid cooling restricts crystal growth, resulting in very small crystals or even a glassy texture where no crystals form at all. For example, magma trapped deep underground can take thousands or even millions of years to solidify, allowing for substantial crystal development. If magma cools quickly, such as during a volcanic eruption, crystals do not have sufficient time to enlarge.
Environments of Formation
Igneous rocks form in two primary environments, each dictating the cooling rate and subsequent rock characteristics. Intrusive igneous rocks, also known as plutonic rocks, develop when magma cools and solidifies deep within the Earth’s crust. The surrounding rock acts as an insulator, leading to a slow cooling process. This prolonged cooling allows mineral crystals to grow to visible sizes.
In contrast, extrusive igneous rocks, or volcanic rocks, form when molten material reaches the Earth’s surface as lava and cools rapidly. This rapid cooling occurs due to exposure to the atmosphere or water. Extrusive rocks also form from fragmented material ejected during explosive volcanic eruptions, such as ash and volcanic bombs.
Rock Characteristics from Formation
The cooling rate and environment directly determine the texture of an igneous rock, which refers to the size and arrangement of its mineral crystals. Intrusive rocks, formed from slow cooling underground, typically exhibit a phaneritic texture, characterized by large, visible mineral crystals. Granite is a common example of an intrusive rock with this coarse-grained texture.
Extrusive rocks, which cool rapidly, often display an aphanitic texture, where crystals are too small to be seen with the naked eye. Basalt is a common fine-grained extrusive rock. In cases of extremely rapid cooling, such as when lava is quenched by water, a glassy texture forms, meaning the rock contains few or no crystals at all, similar to obsidian. Additionally, if gas bubbles are trapped in rapidly cooling lava, the resulting rock can have a vesicular texture, appearing pitted with small cavities.