By Doug Bennett – Updated Aug 30, 2022

What Are Igneous Rocks?

Igneous rocks, also called volcanic rocks, form when molten material from the Earth's interior—magma underground or lava on the surface—cools and solidifies. Their characteristics are shaped by the cooling rate and the chemistry of the original magma, which determines mineral content, grain size, texture, and color.

Classification by Cooling Environment

Igneous rocks are divided into two primary categories based on where they solidify:

• Intrusive (Plutonic) – Slow cooling beneath the surface produces coarse‑grained, crystalline textures. Common examples include granite, diorite, gabbro, and peridotite.
• Extrusive (Volcanic) – Rapid cooling at or above the surface yields fine‑grained, vesicular, or glassy rocks. Typical types are rhyolite, andesite, basalt, scoria, pumice, and obsidian.

Classification by Magma Composition

The silica content of the magma dictates whether a rock is felsic, intermediate, mafic, or ultramafic. Each group has distinct mineral assemblages and typical color ranges.

Felsic Igneous Rocks

High‑silica, silica‑rich magma dominated by silicon and aluminum. It is viscous, cools slowly or quickly, and is rich in gases. Key minerals: potassium feldspar, sodium‑plagioclase feldspar, quartz, biotite. Typical colors: light gray to pink. Examples:

• Granite – slow‑cooling intrusive felsic rock.
• Rhyolite – fast‑cooling extrusive felsic rock.
• Pumice, obsidian – very rapid cooling, forming glass or highly vesicular textures.

Intermediate Igneous Rocks

Composed of silica levels between felsic and mafic magmas. Often generated in subduction zones where oceanic plates descend. Mineral mix: feldspar, amphibole, pyroxene, biotite, quartz. Typical colors: medium gray to brown. Examples:

• Diorite – slow‑cooling intrusive intermediate rock.
• Andesite – fast‑cooling extrusive intermediate rock.
• Scoria – very rapid cooling of intermediate lava, producing a vesicular, black rock.

Mafic Igneous Rocks

Dominated by ferromagnesian minerals, often found in oceanic divergent zones. These magmas are hot, low‑density, and rich in magnesium and iron. Key minerals: calcium‑plagioclase feldspar, pyroxene, olivine, amphibole. Typical colors: dark gray or black (basaltic). Examples:

• Gabbro – slow‑cooling intrusive mafic rock.
• Basalt – fast‑cooling extrusive mafic rock.
• Scoria – can also form from mafic lava during rapid cooling.

Ultramafic Igneous Rocks

Almost entirely ferromagnesian minerals, frequently containing olivine. These rocks are called “gabbroic” or “basaltic” when they form at the Earth's surface. Peridotite is a classic ultramafic example and is typically only found in mantle‑crustal settings. Because of its near‑complete mineral composition, there are no common fast‑cooling ultramafic variants found at the surface.

Key Properties Influenced by Cooling Time

• Crystal Size – Slow cooling allows crystals to grow, resulting in a coarse texture (e.g., granite).
• Texture – Rapid cooling prevents crystal growth, creating a “magma‑or‑rock‑forming” (magma) texture that can be fine‑grained, vesicular, or glassy.
• Color and Composition – These factors define the rock’s classification and influence its aesthetic and structural characteristics.

Understanding these properties helps geologists interpret Earth's geological history, assess volcanic hazards, and locate valuable mineral resources.