How do cooling rate and emplacement control igneous textures and forms?
The control of cooling rate on crystal size and texture (glassy, fine, coarse, porphyritic), and the recognition of intrusive forms (dykes, sills, batholiths, laccoliths) and extrusive forms (lava flows, pyroclastic deposits) with their contact relationships.
A focused WJEC and Eduqas A-Level Geology G1 answer on how cooling rate controls crystal size and igneous texture, the meaning of porphyritic and glassy textures, and how to recognise intrusive bodies (dykes, sills, batholiths, laccoliths) and extrusive forms by their geometry, grain size and contact relationships such as chilled margins and baked contacts.
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What this dot point is asking
Having explained how magma forms and evolves, G1 now asks you to read the rock. WJEC wants you to use texture to deduce cooling history, and to recognise the shapes of igneous bodies and their contact relationships, both in hand specimen and on a map or cross-section. These skills feed straight into Component 1 and into map work in T2.
The answer
Cooling rate controls crystal size
The single most useful idea is that crystal size records cooling rate. Slow cooling gives atoms time to migrate and build large crystals; fast cooling freezes many small crystals; instant quenching gives a glass.
A porphyritic texture is the examiner's favourite because it encodes a two-stage history: phenocrysts grew slowly at depth, then the magma rose and the rest cooled quickly, locking the phenocrysts into a fine matrix. Vesicles (gas bubble holes) and amygdales (vesicles later filled by minerals) record gas escape near the surface.
Intrusive forms and their geometry
Intrusions cool underground, so they are usually coarser, and they are classified by shape and by their relationship to the surrounding rock (the country rock).
- Dyke: a sheet that cuts discordantly across bedding, often near-vertical.
- Sill: a sheet intruded concordantly, parallel to bedding.
- Batholith: a very large, deep-seated mass of (usually) granite, with a discordant margin.
- Laccolith: a concordant intrusion that has domed up the overlying strata.
Contact relationships: reading the evidence
Contacts tell you whether a rock was intruded or erupted, and its relative age.
The decisive test between a sill and a lava flow: a sill bakes the rock above and below and has chilled margins on both sides; a lava flow bakes only the rock beneath it (the rock above is younger and unbaked) and may have a weathered, vesicular or rubbly top.
Examples in context
The Whin Sill in northern England is a concordant dolerite sheet that bakes Carboniferous limestone above and below it, a classic teaching sill that Hadrian's Wall partly follows. Dartmoor and the Cornubian batholith show coarse granite emplaced deep in the crust, now exposed by erosion, with a metamorphic aureole around it. Giant's Causeway basalts show fine-grained flows with columnar cooling joints and weathered (bole) horizons between successive flows, distinguishing them from sills.
Try this
Q1. State the cooling history recorded by a porphyritic texture. [2 marks]
- Cue. Slow cooling at depth grew the large phenocrysts, then rapid cooling froze the fine groundmass around them.
Q2. Distinguish a dyke from a sill in terms of their relationship to bedding. [2 marks]
- Cue. A dyke cuts discordantly across bedding; a sill is concordant, intruded parallel to bedding.
Q3. Explain how a chilled margin and a baked contact establish the relative age of an intrusion. [2 marks]
- Cue. The intrusion chilled against cold country rock and baked it, so the intrusion was hotter and therefore younger than the rock it cuts.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC Eduqas 20185 marksExplain how the texture of an igneous rock can be used to deduce its cooling history, referring to a porphyritic texture.Show worked answer →
Link crystal size to cooling rate, because that is the principle being tested.
Slow cooling at depth gives crystals time to grow large, producing a coarse-grained (phaneritic) rock such as granite. Fast cooling at or near the surface gives little time, producing a fine-grained (aphanitic) rock such as basalt, and quenching can give a glass such as obsidian with no crystals at all.
A porphyritic texture records two stages of cooling. Large crystals (phenocrysts) grew slowly while the magma was still at depth; the magma then rose and cooled quickly, freezing the remaining liquid into a fine-grained groundmass around the phenocrysts.
So a porphyritic rock tells you the magma cooled slowly first and then rapidly, a two-stage history of storage at depth followed by ascent and eruption or shallow emplacement.
Markers reward large crystals meaning slow cooling, fine groundmass meaning fast cooling, and the two-stage interpretation of phenocrysts in a fine matrix.
WJEC Eduqas 20224 marksDescribe two features that would allow you to distinguish a sill from a lava flow in the field.Show worked answer →
Give features that test whether the rock was intruded or erupted, because that is the distinction.
A sill is intrusive and concordant, so it bakes the rock both above and below it and may have a chilled (fine-grained) margin against both contacts. A lava flow is extrusive, so only the rock beneath it is baked; the rock above is younger and unbaked, and the top of the flow may be weathered, vesicular or show a baked soil (bole) beneath the next flow.
A sill also lacks the surface features of a flow, such as a vesicular or rubbly top, ropy or blocky surfaces, and it may include fragments (xenoliths) of the rock it intruded.
Markers reward baking on both sides plus chilled margins for the sill, baking only below plus a weathered or vesicular top for the flow.
Related dot points
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- The interpretation of geological maps: reading dip and strike from outcrop patterns, the rule of Vs for outcrops crossing valleys, recognising horizontal, dipping, folded and faulted strata and unconformities, and using the pattern to deduce the underlying structure.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)