Igneous Rocks

Igneous rocks form when molten rock cools and solidifies, recording the conditions of melting deep within the Earth and the eruption or intrusion of magma.

Definition

An igneous rock is a rock that forms by the cooling and crystallization of magma or lava, classified broadly by where it solidifies (intrusive versus extrusive) and by its silica content and mineral makeup.

Scope

This topic introduces igneous rocks from a general-geology standpoint: how magma forms and where, the contrast between intrusive and extrusive rocks, the basic compositional and textural distinctions used in the field, and the link to volcanism and plate setting. Detailed petrologic classification and phase relations are treated in mineralogy-and-petrology.

Core questions

• How and where does magma form in the Earth?
• What distinguishes intrusive from extrusive igneous rocks?
• How do cooling rate and composition control rock texture and type?

Key theories

Bowen's reaction series

Bowen showed experimentally that minerals crystallize from a cooling magma in a predictable sequence, so that fractional crystallization can produce a range of igneous rock compositions from a single parent magma.

Origins of magma

Magma forms by decompression melting beneath ridges and hotspots, flux melting where water lowers the melting point in subduction zones, and heat-induced melting of the crust, linking igneous rock types to plate-tectonic setting.

Mechanisms

Rock melts where temperature rises, pressure falls, or volatiles are added. Magma that cools slowly at depth grows large crystals and forms coarse-grained intrusive rock, whereas magma that erupts and cools quickly forms fine-grained or glassy extrusive rock. As magma cools, minerals crystallize in the order of Bowen's series, and removal of early crystals can drive the remaining melt toward more silica-rich compositions.

Clinical relevance

Igneous rocks supply important building and dimension stone, host many metallic ore deposits, and their character records past volcanism that informs volcanic-hazard assessment and reconstruction of plate-tectonic history.

History

The igneous origin of many rocks was contested in the late eighteenth century between Plutonists, following Hutton, and Neptunists, who held all rock precipitated from water. Bowen's early-twentieth-century experimental work established how magmas crystallize and differentiate, founding modern understanding of igneous rocks.

Key figures

• Norman L. Bowen
• Reginald Daly
• James Hutton

Seminal works

• bowen1928

Frequently asked questions

What is the difference between intrusive and extrusive igneous rocks?

Intrusive rocks crystallize slowly underground and have large, visible crystals, like granite, while extrusive rocks form when lava cools quickly at the surface and have fine grains or glass, like basalt.