How to Classify Igneous Rocks: Felsic to Ultramafic

Classify igneous rocks by silica content—from felsic to ultramafic—and name coarse and fine equivalents like granite and rhyolite.

Uni Year 1Earth science
⏱️ About 20 min
How to Classify Igneous Rocks: Felsic to Ultramafic — illustration
Illustrative image (AI-generated).

Geologists have named thousands of igneous rocks, but almost every one you will meet in an introductory course fits into a simple two-axis system: composition (how much silica) and texture (crystal size). Learn those two axes and you can name the common rocks cold — and predict their colour, density, and even where on Earth they are likely to form.

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The big idea: Igneous rocks are classified by <strong>silica content</strong> (chemical composition) and <strong>texture</strong> (crystal size). Silica content runs from felsic (high silica, light-coloured, low density) through intermediate and mafic to ultramafic (low silica, dark, dense). Texture runs from phaneritic (coarse, intrusive) to aphanitic (fine, extrusive). Crossing the two axes gives the standard names: granite–rhyolite, diorite–andesite, gabbro–basalt, peridotite.
🎯 By the end, you'll be able to
  • Classify igneous rocks by silica content into felsic, intermediate, mafic, and ultramafic categories
  • Name the coarse (phaneritic) and fine (aphanitic) equivalents for each composition class
  • Relate silica content to colour, density, and the dominant minerals in each class
  • Estimate igneous composition from mineralogy (quartz and feldspars for felsic–intermediate; olivine and pyroxene for mafic–ultramafic)
📎 Helpful to know first

The two axes: composition and texture

Every igneous rock can be placed on a grid with two axes:

  • Composition — dominated by silica (SiO₂) content. This controls the minerals that crystallise and the rock's colour and density.
  • Texture — crystal size, which tells you whether the rock cooled intrusively or extrusively.

Crossing composition with texture gives the standard rock names. A felsic, coarse-grained rock is granite; the same composition cooled quickly is rhyolite. A mafic, coarse-grained rock is gabbro; its fine-grained twin is basalt.

The silica scale

Geologists divide igneous compositions into four classes based on weight percent SiO₂:

  • Felsic — > 65 % SiO₂. Light-coloured (pink, grey, white). Dominated by quartz and feldspars. Low density (~2.65 g/cm³). Example: granite / rhyolite.
  • Intermediate — 55–65 % SiO₂. Medium colour (salt-and-pepper). Mix of plagioclase, amphibole, and biotite. Moderate density (~2.8 g/cm³). Example: diorite / andesite.
  • Mafic — 45–55 % SiO₂. Dark (black, dark grey). Dominated by pyroxene and calcium-rich plagioclase. Higher density (~3.0 g/cm³). Example: gabbro / basalt.
  • Ultramafic — < 45 % SiO₂. Very dark (greenish-black). Almost entirely olivine and pyroxene. Very high density (~3.3 g/cm³). Example: peridotite. Ultramafic rocks are rare at the surface; most are fragments of the upper mantle.
🔑 Silica content drives colour and density
High-silica rocks are light and relatively low-density because quartz and feldspar are silica-rich and light. Low-silica rocks are dark and dense because iron- and magnesium-rich minerals (olivine, pyroxene) are heavy. You can often guess composition from colour alone: light = felsic, dark = mafic/ultramafic.
Igneous rock classification by silica content and texture Igneous rocks: silica content × texture Coarse — phaneritic (intrusive) Fine — aphanitic (extrusive) QAPF: felsic–intermediate Felsic >65% SiO₂ Granite Rhyolite Intermediate 55–65% SiO₂ Diorite Andesite Mafic 45–55% SiO₂ Gabbro Basalt Ultramafic <45% SiO₂ Peridotite Komatiite (rare) Silica rises → colour lightens and density falls; silica falls → colour darkens and density rises. The QAPF diagram classifies only the felsic–intermediate field; mafic–ultramafic rocks use olivine–pyroxene–plagioclase diagrams. Each coarse–fine pair shares the same chemistry — only cooling rate (texture) differs.
A silica-versus-texture overview of the common igneous rocks. Coarse-grained (phaneritic) names sit on the left; fine-grained (aphanitic) names on the right. Colour lightens as silica rises. The dashed field marks the felsic–intermediate range the QAPF diagram classifies — mafic and ultramafic rocks need separate diagrams.

Coarse and fine equivalents

Here are the standard pairs. Memorising these four rows covers most introductory geology:

CompositionSiO₂ rangeCoarse (intrusive)Fine (extrusive)
Felsic> 65 %GraniteRhyolite
Intermediate55–65 %DioriteAndesite
Mafic45–55 %GabbroBasalt
Ultramafic< 45 %PeridotiteKomatiite (rare, ancient)
✨ Colour index is a quick field guide
Geologists use the colour index — the percentage of dark minerals in a rock — as a fast estimate of composition. Felsic rocks have a low colour index (< 30 % dark minerals). Mafic rocks have a high colour index (> 50 % dark minerals). It is not exact, but in the field it is often the first clue you get.

Reading mineralogy to find composition

If you can identify the major minerals, you can place a rock on the composition scale:

  • Quartz + alkali feldspar dominate → felsic.
  • Plagioclase + amphibole ± biotite dominate → intermediate.
  • Calcic plagioclase + pyroxene dominate → mafic.
  • Olivine + pyroxene dominate → ultramafic.

This mineral-reading logic underlies the QAPF diagram (Quartz–Alkali feldspar–Plagioclase–Feldspathoid) used by petrologists. The QAPF diagram classifies only felsic to intermediate rocks — those defined by quartz, alkali feldspar, and plagioclase. Mafic and ultramafic rocks (gabbro, basalt, peridotite) are dominated by olivine and pyroxene, which are not axes on the QAPF diagram, so petrologists classify them with separate olivine–pyroxene–plagioclase diagrams instead. For introductory work a simplified mineral-assemblage reading is enough.

📝 Worked example: A coarse-grained igneous rock contains 30 % quartz, 40 % pink orthoclase feldspar, 25 % plagioclase feldspar, and 5 % biotite. Classify it by composition and name the rock.
  1. Quartz + orthoclase = 70 % of the rock. These are the hallmark felsic minerals.
  2. Plagioclase is 25 % and biotite is 5 % — subordinate mafic components.
  3. Total felsic minerals > 65 % → felsic composition.
  4. The rock is coarse-grained (phaneritic) and felsic → granite.
✓ Felsic composition; the rock is granite.
📝 Worked example: A fine-grained, dark-grey volcanic rock contains 55 % calcium-rich plagioclase, 30 % pyroxene, 10 % olivine, and 5 % iron oxide. Name the rock and state its composition class.
  1. Plagioclase + pyroxene + olivine are the dominant minerals — no quartz or alkali feldspar.
  2. Dark minerals (pyroxene + olivine + oxide) make up 45 %, giving a moderate-to-high colour index.
  3. Silica content is roughly 48–52 % based on the mineral assemblage → mafic.
  4. Fine-grained (aphanitic) and mafic → basalt.
✓ Mafic composition; the rock is basalt.

Check your understanding

1. Which igneous composition class has the highest silica content?
Felsic rocks contain > 65 % SiO₂, the highest of the four classes. Ultramafic rocks contain < 45 % SiO₂, the lowest.
2. What is the coarse-grained (intrusive) equivalent of basalt?
Basalt is the fine-grained (extrusive) equivalent of gabbro. Both are mafic in composition; only the texture (and therefore cooling rate) differs.
3. A rock is dominated by olivine and pyroxene with almost no quartz or feldspar. What is its composition?
Olivine and pyroxene are the dominant minerals of ultramafic rocks, which contain < 45 % SiO₂ and very little quartz or feldspar.
🔎 Related lessons
Previous: Intrusive vs Extrusive Next: Bowen's Reaction Series Magma vs Lava: Composition & Origin
✅ Key takeaways
  • Igneous rocks are classified by composition (silica content) and texture (crystal size).
  • Four composition classes: felsic (&gt;65 % SiO₂, light), intermediate (55–65 %), mafic (45–55 %, dark), ultramafic (&lt;45 %, very dark/dense).
  • Standard coarse/fine pairs: granite–rhyolite (felsic), diorite–andesite (intermediate), gabbro–basalt (mafic), peridotite–komatiite (ultramafic).
  • Mineralogy reveals composition: quartz + K-feldspar = felsic; plagioclase + amphibole = intermediate; pyroxene + olivine = mafic/ultramafic.
  • Colour index (percentage of dark minerals) is a quick field estimate of composition.
➡️ We now know how to name rocks by composition and texture. But <em>why</em> do minerals crystallise in a predictable order as magma cools? That order — Bowen's reaction series — is the key to understanding fractional crystallisation and why magmas evolve.
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🎓 Go deeper: university courses & trusted references

Handpicked external material for this module — for when you want the full university treatment of igneous rocks & volcanism.

External sites are listed for reference only. This course is independent and has no affiliation with, or endorsement from, the institutions named.