How are igneous rocks classified?

Mainly by two axes: chemical composition, judged by silica content from felsic (silica-rich) to ultramafic (silica-poor), and texture, judged by grain size. Coarse grains mean slow cooling at depth, fine grains mean rapid cooling at the surface.

What is the difference between intrusive and extrusive?

Intrusive (or plutonic) rocks crystallise slowly underground, giving large interlocking crystals you can see, like granite. Extrusive (or volcanic) rocks cool quickly at the surface, giving fine or glassy textures, like basalt and rhyolite.

What does felsic and mafic mean?

Felsic rocks are rich in silica and light-coloured minerals such as quartz and feldspar, so they are pale and low density. Mafic rocks are silica-poor and rich in magnesium and iron minerals, so they are dark and denser. Ultramafic rocks are the most magnesium- and iron-rich of all.

What are granite and basalt's extrusive and intrusive twins?

Granite, a coarse felsic rock, has the fine-grained rhyolite as its extrusive twin. Basalt, a fine mafic rock, has the coarse-grained gabbro as its intrusive twin. Each composition pairs a slow-cooled and a fast-cooled rock of matching chemistry.

Why is basalt so common?

Basalt is the most abundant volcanic rock because it makes up the bulk of the oceanic crust and erupts from mid-ocean ridges, hotspots and flood-basalt provinces. Its low-silica magma is runny and erupts readily across the planet.

What is the Igneous Rock Classification Reference?

Reference table of common igneous rocks classified by silica content (felsic to ultramafic) and grain size (intrusive vs extrusive), with mineralogy and a composition-plus-texture classifier. It runs free in your browser on Gera Tools, with nothing uploaded.

Igneous Rock Classification Reference

Name: Igneous Rock Classification Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Sorting volcanic and plutonic rocks

Igneous rocks form when molten magma or lava cools and crystallises. They are classified along two axes: chemical composition, from silica-rich felsic to silica-poor ultramafic, and texture, set by how fast the melt cooled. This reference pairs each composition with its coarse intrusive and fine extrusive forms, and a classifier returns the matching rock for any combination you pick.

How it works

Composition is judged by silica (SiO₂) content, and texture by grain size:

Composition   Silica       Coarse (intrusive)   Fine (extrusive)
Felsic        > 65%        granite              rhyolite
Intermediate  52 – 65%     diorite              andesite
Mafic         45 – 52%     gabbro               basalt
Ultramafic    < 45%        peridotite           komatiite

Slow cooling deep underground grows large, visible crystals (a phaneritic, coarse texture). Rapid cooling at the surface freezes the melt into fine or glassy grains (an aphanitic texture). Each composition therefore has a coarse intrusive rock and a fine extrusive rock of the same chemistry, and the classifier returns the cell where your two choices meet.

Tips for field identification

Lighter, paler rocks are usually felsic; darker, denser rocks are mafic.
Visible interlocking crystals point to slow, deep, intrusive cooling.
Fine or glassy texture points to fast, surface, extrusive cooling.
Granite/rhyolite and gabbro/basalt are the classic intrusive/extrusive twins.
Basalt is the most abundant volcanic rock; it forms the oceanic crust.

More detail on each major type

Granite and rhyolite (felsic)

Granite is the most familiar igneous rock — coarse, speckled with visible quartz, feldspar, and mica crystals, formed from silica-rich magma that cooled slowly kilometers underground. Quarried granite’s durability and aesthetic make it a kitchen countertop staple, but geologically it is the building block of continental crust. Rhyolite is granite’s extrusive twin: the same high-silica composition but fine-grained or glassy because it erupted or intruded near the surface and cooled rapidly. High-silica magmas are viscous and gas-rich, which is why felsic eruptions tend to be explosive.

Diorite and andesite (intermediate)

Diorite is a coarse, salt-and-pepper rock with roughly equal amounts of plagioclase feldspar and dark minerals (hornblende, biotite), and little or no quartz. Its fine-grained extrusive equivalent, andesite, is one of the most common volcanic rocks globally — the Andes are named after it. Intermediate composition magmas form at subduction zones where oceanic crust melts under continental margins.

Gabbro and basalt (mafic)

Gabbro is the deep equivalent of basalt: dark, dense, with large crystals of plagioclase and pyroxene, forming in oceanic crust at mid-ocean ridges and in layered intrusions on land. Basalt covers enormous areas of Earth’s surface — the ocean floors are almost entirely basalt — and forms from low-silica, low-viscosity magmas that flow readily during eruptions. The Columbia River Plateau in the Pacific Northwest and Iceland are dominated by flood basalts covering thousands of square kilometers.

Peridotite and komatiite (ultramafic)

Peridotite is the dominant rock of Earth’s mantle beneath the crust — most people never encounter it at the surface, but kimberlite pipes (which carry diamonds upward) are essentially peridotite. Komatiite is its ancient extrusive equivalent, mostly found in Archaean-age greenstone belts and rare today because the mantle has cooled enough that ultramafic magmas no longer erupt in most settings.

Intermediate textures

Real igneous rocks do not always fall neatly into “coarse” or “fine.” Porphyritic rocks have large crystals (phenocrysts) set in a fine-grained groundmass, reflecting a two-stage cooling history: slow at depth, then rapid after eruption. Vesicular basalt has gas-bubble holes from dissolved volatiles released during eruption. Obsidian is a volcanic glass — so rapidly quenched it has no crystalline structure at all. These intermediate and special textures are not modeled in the classifier but are worth knowing for field identification.