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.

Igneous Rock Classification Reference

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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 and notes

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.