Igneous rocks, named from the Latin word for fire, are fundamental components of the Earth’s crust. They originate from the cooling and solidification of molten rock material (magma or lava). They are the initial building blocks from which other rock types are derived through processes like weathering and metamorphism.
How Igneous Rocks Form
Igneous rock formation begins deep within the Earth, where high temperatures and pressures generate molten material called magma. Magma is less dense than the surrounding rock, causing it to rise toward the surface. If this material solidifies beneath the surface, it forms one type of igneous rock.
If magma reaches the Earth’s surface through a volcanic vent, it is referred to as lava. The rate at which this material cools is the primary factor influencing the final rock texture. Slow cooling allows atoms to organize, resulting in the growth of large, visible mineral crystals. Rapid cooling, conversely, prevents the formation of large crystals, leading to a fine-grained or glassy texture.
The Two Main Categories
Igneous rocks are classified into two groups based on where the molten material solidifies.
Intrusive igneous rocks, also known as plutonic rocks, form when magma cools and crystallizes far beneath the Earth’s surface. The insulating effect of the surrounding rock causes this cooling process to take place over thousands or even millions of years, resulting in a coarse-grained texture where individual mineral crystals are easily visible.
Extrusive igneous rocks, also termed volcanic rocks, form when lava cools rapidly after erupting onto the surface or into the atmosphere. This quick chilling yields rocks with a fine-grained (aphanitic) texture or a non-crystalline, glassy structure.
Examples of Intrusive Rocks
Intrusive rocks are characterized by their interlocking, coarse-grained texture.
Granite
Granite is the most common intrusive rock and a major component of continental crust. It is classified as felsic, meaning it is rich in silica and typically light in color. Composed primarily of quartz and various feldspar minerals, granite is extensively used as a building material and for countertops due to its durability.
Diorite
Diorite has an intermediate composition and is often described as having a “salt and pepper” appearance. It is made up of roughly equal amounts of light-colored plagioclase feldspar and dark-colored minerals like hornblende and pyroxene. Diorite is frequently found in large intrusive bodies alongside granite.
Gabbro
The mafic end of the intrusive spectrum is Gabbro, which is rich in iron and magnesium. Gabbro contains minerals such as pyroxene, olivine, and calcium-rich plagioclase feldspar. It is a major constituent of the lower oceanic crust and can be deep black or dark green.
Examples of Extrusive Rocks
Extrusive rocks are defined by their fine-grained or non-crystalline nature, a consequence of rapid cooling on the surface.
Basalt
Basalt is the most abundant extrusive igneous rock and forms the majority of the Earth’s oceanic crust. This mafic rock is typically dark gray to black and has a fine-grained (aphanitic) texture. Basalt can sometimes exhibit a vesicular texture, containing small holes formed by gas bubbles trapped during solidification.
Obsidian
Obsidian is a natural volcanic glass that cools so quickly no crystals form at all. This rock has a dark, glassy appearance and fractures with sharp, curved surfaces (conchoidal fracture). Obsidian’s rapid chilling often occurs when silica-rich lava encounters water or air.
Pumice
Pumice forms from highly viscous, gas-rich lava. The violent expansion of gases creates a frothy, highly porous texture, making it one of the few rocks that can float on water. Pumice is often light in color and is used commercially as a lightweight abrasive material.