How are igneous rocks classified by composition and texture, and what do textures reveal about cooling?
Igneous rocks: classification by silica content (acid, intermediate, basic and ultrabasic) and by grain size (coarse-grained intrusive, fine-grained extrusive); the relationship between cooling rate and crystal size; igneous textures (phaneritic, aphanitic, porphyritic, glassy and vesicular) and what they show about the cooling history; naming common igneous rocks such as granite, gabbro, basalt and rhyolite.
A focused answer to the OCR H414 dot point on igneous rock classification. Covers acid, intermediate, basic and ultrabasic compositions, coarse versus fine grain size, the link between cooling rate and crystal size, the main textures (phaneritic, aphanitic, porphyritic, glassy, vesicular), and naming granite, gabbro, basalt and rhyolite.
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What this dot point is asking
OCR wants you to classify igneous rocks two ways, by silica content (acid, intermediate, basic, ultrabasic) and by grain size (coarse intrusive, fine extrusive), to explain how cooling rate controls crystal size, to describe the main textures and what they reveal about the cooling history, and to name common igneous rocks such as granite, gabbro, basalt and rhyolite.
The answer
Classifying by silica content
The chemistry of the magma sets the composition, which controls colour and density:
- Acid (felsic). Over about silica, light-coloured, quartz-rich. Example: granite (coarse), rhyolite (fine).
- Intermediate. About to silica. Example: diorite (coarse), andesite (fine).
- Basic (mafic). About to silica, dark, rich in pyroxene and calcium plagioclase. Example: gabbro (coarse), basalt (fine).
- Ultrabasic (ultramafic). Below about silica, very dark and dense, rich in olivine and pyroxene. Example: peridotite.
As silica falls, iron and magnesium rise, the rock darkens, and the density increases.
Classifying by grain size and cooling rate
Grain size records how fast the magma cooled, because slow cooling gives ions time to migrate and build a few large crystals, while fast cooling freezes many tiny crystals.
- Coarse-grained (phaneritic), crystals over about . Slow cooling, deep underground: intrusive (plutonic) rocks (granite, gabbro).
- Fine-grained (aphanitic), crystals too small to see. Fast cooling, at or near the surface: extrusive (volcanic) rocks (basalt, rhyolite).
Textures
The texture (the size, shape and arrangement of crystals) is the evidence you read in the exam:
- Phaneritic. All crystals large and visible; slow, deep cooling.
- Aphanitic. Crystals too small to see; fast, surface cooling.
- Porphyritic. Large crystals (phenocrysts) set in a finer groundmass; two-stage cooling (slow then fast).
- Glassy. No crystals at all (for example obsidian); extremely rapid cooling, a quench.
- Vesicular. Full of frozen gas-bubble holes (for example pumice and scoria); rapid degassing as a gas-rich lava erupts.
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 the magma that crystallises more slowly in chambers below forms gabbro (coarse-grained); same composition, different cooling.
Example 2. Pumice from explosive eruptions. Highly vesicular, glassy pumice records a gas-rich acid magma that froze almost instantly while degassing violently, so it can even float on water.
Try this
Q1. State the approximate silica content that defines a basic 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 form a few large crystals.
Q3. Name the texture of a rock with large phenocrysts in a fine groundmass, and state what it shows. [2 marks]
- Cue. Porphyritic; it shows two-stage cooling, slow at depth then fast near the surface.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/03 20194 marksA coarse-grained igneous rock is dark in colour and made almost entirely of pyroxene and calcium-rich plagioclase. Name the rock, state its composition (acid, intermediate, basic or ultrabasic) and explain how it formed, using the texture as evidence.Show worked answer →
Name, classify, then read the texture as a cooling history.
- Name: gabbro
- A coarse-grained, dark rock of pyroxene and calcium plagioclase is gabbro.
- Composition: basic
- It is dark and rich in iron-magnesium minerals with calcium plagioclase, so its silica content is roughly to percent, which is basic.
- Formation from the texture
- It is coarse-grained (phaneritic), with crystals over about . Large crystals form when a magma cools slowly, because ions have time to migrate and grow few, large crystals. Slow cooling means the magma solidified deep underground, so gabbro is an intrusive (plutonic) rock. Basalt is its fine-grained extrusive equivalent.
Markers want the name, the basic classification, and the slow-cooling, deep, intrusive origin justified by the coarse grain size.
OCR H414/01 20213 marksExplain why a porphyritic igneous rock contains both large crystals (phenocrysts) and a fine-grained groundmass.Show worked answer →
A porphyritic texture records two cooling stages.
Stage 1, slow cooling at depth. The magma first cooled slowly underground, so a few minerals (the early-crystallising ones, for example olivine or feldspar) grew into large, well-formed crystals called phenocrysts.
Stage 2, fast cooling at or near the surface. The remaining magma, with the phenocrysts suspended in it, was then erupted or intruded to a shallow level and cooled quickly, so the rest crystallised as a fine-grained groundmass around the phenocrysts.
The contrast in crystal size therefore records a change from slow to fast cooling. Markers reward both stages and the link between cooling rate and crystal size.
Related dot points
- Minerals and rocks: the structure of the silicate minerals based on the silica tetrahedron; the silicate groups (isolated, chain, sheet and framework silicates) and how the degree of polymerisation links to composition; the physical properties (colour, lustre, hardness, cleavage, fracture, streak, density and habit) used to identify the common rock-forming minerals quartz, feldspar, mica, olivine, pyroxene and amphibole in hand specimen.
A focused answer to the OCR H414 dot point on rock-forming minerals. Covers the silica tetrahedron, the isolated, chain, sheet and framework silicate groups, how polymerisation links to composition, and how to identify quartz, feldspar, mica, olivine, pyroxene and amphibole from physical properties in hand specimen.
- Igneous processes: Bowen's reaction series as the order in which silicate minerals crystallise from a cooling magma; the discontinuous (olivine to biotite) and continuous (calcium-rich to sodium-rich plagioclase) branches; the use of the series to explain fractional crystallisation, magma differentiation and the resistance of minerals to weathering.
A focused answer to the OCR H414 dot point on Bowen's reaction series. Covers the discontinuous and continuous branches, the crystallisation order of silicate minerals, how fractional crystallisation drives magma differentiation from basic to acid compositions, and how the series predicts weathering resistance.
- Igneous bodies: the forms of intrusive igneous bodies (batholiths, dykes, sills and laccoliths) and their relationship to the country rock (concordant versus discordant); chilled margins, baked margins and contact metamorphic aureoles as evidence of intrusion; the recognition of extrusive forms (lava flows and their cross-cutting relationships) and the use of these relationships to establish relative age.
A focused answer to the OCR H414 dot point on igneous bodies. Covers batholiths, dykes, sills and laccoliths, concordant versus discordant intrusions, chilled and baked margins and contact aureoles as evidence of intrusion, and how cross-cutting relationships of dykes, sills and lava flows establish relative age.
- Volcanism: the control of magma composition (silica content), viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions and andesitic or rhyolitic explosive eruptions; volcanic products (lava, tephra, pyroclastic flows and gases); volcanic landforms (shield volcanoes, stratovolcanoes, calderas and fissures); the link between volcanism and plate setting.
A focused answer to the OCR H414 dot point on volcanism. Covers how silica content, viscosity and gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, volcanic products (lava, tephra, pyroclastic flows, gases), the landforms (shield, stratovolcano, caldera, fissure), and the link to plate setting.
- The rock cycle: the continuous transformation between igneous, sedimentary and metamorphic rocks; the processes that link them (crystallisation, weathering, erosion, transport, deposition, lithification, metamorphism, melting, uplift and exposure); the role of plate tectonics in driving the cycle; recognising that any rock type can be converted into any other.
A focused answer to the OCR H414 dot point on the rock cycle. Covers the continuous transformation between igneous, sedimentary and metamorphic rocks, the processes that link them (crystallisation, weathering, transport, lithification, metamorphism, melting and uplift), the role of plate tectonics in driving the cycle, and how any rock type can become any other.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)