How are igneous rocks classified by composition and texture, and what does texture tell us about how a magma cooled?
Igneous rock classification and textures: the classification of igneous rocks by silica content and composition (ultramafic peridotite, mafic basalt and gabbro, intermediate andesite and diorite, felsic rhyolite and granite) and by grain size and cooling rate (glassy, aphanitic, phaneritic, porphyritic, vesicular and pyroclastic textures); and the relationship between cooling rate and crystal size.
A focused answer to the Eduqas Geology statement on igneous rock classification. Covers the compositional series from ultramafic peridotite through mafic basalt and gabbro and intermediate andesite and diorite to felsic rhyolite and granite, the link between cooling rate and crystal size, and the named textures (glassy, aphanitic, phaneritic, porphyritic, vesicular and pyroclastic).
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What this dot point is asking
Eduqas wants you to classify igneous rocks two independent ways: by silica content and composition (ultramafic, mafic, intermediate, felsic) and by grain size and texture (which record cooling rate). You must link cooling rate to crystal size, describe the named textures (glassy, aphanitic, phaneritic, porphyritic, vesicular and pyroclastic), and name the common igneous rocks of each box in the table (peridotite, basalt and gabbro, andesite and diorite, rhyolite and granite). This underpins Bowen's reaction series, magma differentiation and the recognition of intrusions in the field.
The answer
Classifying by silica content and composition
The chemistry of the magma sets the composition, which controls colour, density and the minerals present. The silica content rises across the series while iron and magnesium fall, so the rocks lighten and become less dense:
- Ultramafic (ultrabasic). Below about silica; very dark and dense, dominated by olivine and pyroxene. Example: peridotite (coarse, the rock of the mantle).
- Mafic (basic). About to silica; dark, rich in pyroxene and calcium-rich plagioclase. Examples: basalt (fine, extrusive) and gabbro (coarse, intrusive).
- Intermediate. About to silica; medium grey. Examples: andesite (fine, extrusive) and diorite (coarse, intrusive).
- Felsic (acid). Over about silica; pale, quartz-rich, with potassium feldspar and sodium plagioclase. Examples: rhyolite (fine, extrusive) and granite (coarse, intrusive).
As silica falls from felsic to ultramafic, the colour index (the percentage of dark ferromagnesian minerals) rises, so colour is a quick first guide to composition.
Classifying by grain size and cooling rate
Grain size records how fast the magma cooled. Slow cooling gives ions time to migrate to growing crystals, building a few large ones; fast cooling freezes many tiny crystals at once:
- Coarse-grained (phaneritic), crystals over about . Slow cooling deep underground: intrusive (plutonic) rocks (granite, gabbro, peridotite).
- Fine-grained (aphanitic), crystals too small to see with the naked eye. Fast cooling at or near the surface: extrusive (volcanic) rocks (basalt, andesite, rhyolite).
The rule is simple and very examinable: slow cooling gives large crystals; fast cooling gives small crystals; near-instant cooling (a quench) gives glass with no crystals at all. Grain size and composition are independent axes, so basalt and gabbro share a composition but differ in cooling.
The named textures
The texture (the size, shape and arrangement of crystals) is the evidence you read in the exam:
- Glassy. No crystals (for example obsidian); a quench, cooled too fast for any crystals to grow.
- Aphanitic (fine-grained). Crystals too small to see; fast, surface cooling.
- Phaneritic (coarse-grained). All crystals large and visible; slow, deep cooling.
- Porphyritic. Large crystals (phenocrysts) set in a finer groundmass; two-stage cooling, slow at depth then fast near the surface.
- Vesicular. Full of frozen gas-bubble holes (vesicles), as in pumice and scoria; gas escaping from a degassing lava as it erupts.
- Pyroclastic. Made of fragments (pyroclasts) blasted out by explosive eruption and welded or compacted together, as in tuff and ignimbrite.
Examples in context
Example 1. Basalt and gabbro at a spreading ridge. New oceanic crust forms basaltic lava that cools fast on the seafloor (fine-grained), while magma that crystallises more slowly in chambers below forms gabbro (coarse-grained): the same mafic composition, different cooling, hence different grain size.
Example 2. Pumice from explosive eruptions. Highly vesicular, glassy pumice records a gas-rich felsic magma that froze almost instantly while degassing violently, so it is full of bubble holes and can float on water. The fragmented ash welds into pyroclastic tuff.
Try this
Q1. State the approximate silica content that defines a mafic igneous rock and name one example. [2 marks]
- Cue. About to silica; for example basalt (extrusive) or gabbro (intrusive).
Q2. Explain why an intrusive rock is coarse-grained. [2 marks]
- Cue. It cooled slowly deep underground, giving ions time to migrate and grow a few large crystals.
Q3. Name the texture of a rock with large phenocrysts set in a fine groundmass, and state what it shows about cooling. [2 marks]
- Cue. Porphyritic; it records two-stage cooling, slow at depth then fast near the surface.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20184 marksA coarse-grained igneous rock is dark in colour and consists almost entirely of pyroxene and calcium-rich plagioclase. Name the rock, classify it by composition, and explain how it formed, using its texture as evidence.Show worked answer →
Name the rock, classify it, then read the texture as a cooling history.
- Name: gabbro
- A coarse-grained, dark rock of pyroxene and calcium-rich plagioclase is gabbro.
- Composition: mafic (basic)
- It is dark and rich in ferromagnesian minerals with calcium plagioclase, so its silica content is roughly to percent, which is mafic.
- Formation from the texture
- It is coarse-grained (phaneritic), with crystals over about . Large crystals grow when a magma cools slowly, because ions have time to migrate and build a few large crystals. Slow cooling means the magma solidified deep underground, so gabbro is intrusive (plutonic). Basalt is its fine-grained extrusive equivalent.
Markers reward the name, the mafic classification, and the slow-cooling, deep, intrusive origin justified by the coarse grain size.
Eduqas 20214 marksA basaltic lava flow is thick and a geologist estimates its mean cooling rate as per year, while a gabbro body of the same composition cooled at about per year. Calculate how many times faster the basalt cooled, and explain how this difference is recorded in the texture of the two rocks.Show worked answer →
A short calculation, then a texture explanation.
Calculation. Divide the two cooling rates:
So the basalt cooled about times faster than the gabbro.
Texture. Fast cooling freezes many tiny crystals, because ions have no time to migrate far, so the basalt is fine-grained (aphanitic). Slow cooling lets a few crystals grow large, so the gabbro is coarse-grained (phaneritic). The two rocks share a mafic composition but differ in grain size purely because of cooling rate: the lava cooled fast at the surface and the gabbro cooled slowly at depth.
Markers reward the correct factor of , and the link from fast cooling to fine grain (basalt) and slow cooling to coarse grain (gabbro).
Related dot points
- Magma differentiation and Bowen's reaction series: the order of crystallisation of silicate minerals from a cooling magma (the discontinuous ferromagnesian branch olivine to pyroxene to amphibole to biotite, and the continuous plagioclase branch from calcium-rich to sodium-rich, then potassium feldspar, muscovite and quartz); fractional crystallisation and partial melting; and how differentiation evolves a magma from mafic to felsic.
A focused answer to the Eduqas Geology statement on magma differentiation. Covers Bowen's reaction series (the discontinuous ferromagnesian branch and the continuous plagioclase branch), fractional crystallisation and partial melting, the order of crystallisation, and how differentiation evolves a magma from mafic to felsic compositions.
- Igneous intrusions and volcanic forms: concordant intrusions (sills and laccoliths) versus discordant intrusions (dykes, batholiths and stocks); chilled margins, and baked margins and contact aureoles around intrusions, as way-up and relative-age evidence; cross-cutting relationships; and volcanic forms (shield volcanoes, stratovolcanoes or composite cones, cinder cones, calderas and lava plateaux).
A focused answer to the Eduqas Geology statement on igneous bodies. Covers concordant sills and laccoliths versus discordant dykes, batholiths and stocks, chilled and baked margins and aureoles as way-up and relative-age evidence, cross-cutting relationships, and the main volcanic forms (shield, stratovolcano, cinder cone, caldera, lava plateau).
- Silicate minerals and mineral classification: the silicon-oxygen tetrahedron as the building block of silicates; the polymerisation series from isolated tetrahedra (olivine) through chains (pyroxenes, amphiboles) and sheets (micas, clays) to frameworks (quartz, feldspars); and the classification of non-silicate minerals into carbonates, oxides, sulphides, halides and native elements.
A focused answer to the Eduqas Geology statement on silicate structures and mineral groups. Covers the silicon-oxygen tetrahedron, the polymerisation series from isolated tetrahedra to frameworks, the silicate families (olivine, pyroxenes, amphiboles, micas, feldspars, quartz), and the classification of carbonates, oxides, sulphides, halides and native elements.
- The rock cycle and rock classification: the threefold classification of rocks into igneous, sedimentary and metamorphic; the processes that link them (crystallisation, weathering, erosion, transport, deposition, compaction and cementation, burial, metamorphism, melting and uplift); and the role of the surface (external) and internal processes driven by solar energy and the Earth's internal heat.
A focused answer to the Eduqas Geology statement on the rock cycle. Covers the threefold classification of rocks, the surface and internal processes that link them (crystallisation, weathering, transport, deposition, lithification, metamorphism, melting and uplift), and the energy sources that drive the cycle.
- Volcanic activity and eruption styles: the control of silica content, viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions (shield volcanoes and fissures) and andesitic or rhyolitic explosive eruptions (stratovolcanoes, pyroclastic flows and calderas); the volcanic products (lava, tephra and pyroclastic material); the link between eruption style and plate setting.
A focused answer to the Eduqas Geology statement on volcanic activity. Covers how silica content, viscosity and dissolved gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, the products (lava, tephra and pyroclastic material), the landforms (shield, stratovolcano, caldera and fissure), and the link to plate setting.
Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)