What shapes do igneous bodies take underground and at the surface, and how do we recognise them?
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.
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What this dot point is asking
OCR wants you to describe the forms of intrusive igneous bodies (batholiths, dykes, sills and laccoliths) and whether they are concordant or discordant with the country rock, to recognise chilled margins, baked margins and contact aureoles as evidence of intrusion, and to use the cross-cutting relationships of intrusions and lava flows to establish relative age.
The answer
Intrusive forms
Magma that solidifies underground forms intrusive (plutonic) bodies, classified by shape and by their relationship to the bedding of the surrounding country rock:
- Batholith. A very large, discordant mass of (usually acid) intrusive rock, often granite, formed deep in the crust at the roots of mountain belts. Coarse-grained because it cooled slowly.
- Dyke. A sheet-like body that cuts across the bedding, so it is discordant. Often near-vertical.
- Sill. A sheet-like body that runs parallel to the bedding, so it is concordant. Often near-horizontal where beds are flat.
- Laccoliths. A concordant body with a flat floor and a domed roof, formed where viscous magma is injected along bedding and arches the overlying strata upwards.
Evidence of intrusion: margins and aureoles
You recognise an intrusion (rather than, say, a buried lava flow) from the way it has affected itself and its surroundings:
- Chilled margin. Inside the igneous body: the edges are finer-grained than the centre, because the magma cooled rapidly against the cold country rock, freezing small crystals, while the interior cooled slowly. A chilled margin shows the body was intruded hot.
- Baked margin. Inside the country rock: the rock immediately adjacent is hardened, reddened or recrystallised, because heat from the magma caused contact metamorphism. A baked margin shows the country rock was already there when the magma arrived.
- Contact metamorphic aureole. A zone of altered country rock around a large intrusion, with the highest grade nearest the contact, decreasing outwards.
The logic is symmetrical and very examinable: a chilled margin is in the intrusion, a baked margin is in the country rock, and the presence of a baked margin proves the intrusion is younger than the rock it baked.
Using cross-cutting relationships for relative age
Because an igneous body must intrude rock that already exists, any intrusion that cuts another rock or feature is younger than it (the principle of cross-cutting relationships). A dyke cutting a sill is younger than the sill; a dyke cutting a lava flow is younger than the flow. A lava flow, being extrusive, bakes the rocks below it (not above) and is then buried by younger beds, which is how you tell a buried flow from a sill.
Examples in context
Example 1. A granite batholith and its aureole. A large discordant granite batholith is surrounded by a metamorphic aureole, with high-grade rocks (for example hornfels) near the contact grading to unaltered country rock outwards, recording the heat of the cooling pluton.
Example 2. Sills and dykes in a volcanic complex. Around a volcano, magma is fed through discordant dykes and spreads along bedding as concordant sills; both show chilled margins, and their cross-cutting order records the sequence of magma pulses.
Try this
Q1. State whether a sill is concordant or discordant, and explain why. [2 marks]
- Cue. Concordant; it is intruded parallel to (along) the bedding of the country rock.
Q2. Explain what a chilled margin shows about an intrusion. [2 marks]
- Cue. The magma cooled rapidly against the cold country rock, giving finer crystals at the edge than in the centre, so the body was intruded hot.
Q3. A dyke cuts a sandstone and bakes it at the contact. State which is younger and why. [2 marks]
- Cue. The dyke is younger, because it cuts and bakes the sandstone, which must already have existed.
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 20204 marksA sheet-like igneous body cuts across the bedding of a sandstone. It has fine-grained margins and the sandstone immediately next to it is hardened and recrystallised. Identify the type of intrusion and explain what the two margin features tell you.Show worked answer →
Classify by orientation, then read each margin feature.
- Type of intrusion: a dyke
- A sheet-like body that cuts across (is discordant to) the bedding is a dyke. (A sill would run parallel to, or concordant with, the bedding.)
- The chilled margin (in the igneous rock)
- The fine-grained edges are a chilled margin: the magma cooled rapidly against the cold country rock, freezing small crystals at the contact, while the centre cooled more slowly and is coarser. This shows the body was intruded hot into cooler rock.
- The baked margin and aureole (in the country rock)
- The hardened, recrystallised sandstone next to the dyke is a baked margin (a thin contact aureole): heat from the magma caused contact metamorphism of the adjacent rock. Because the country rock is altered, the dyke must be younger than the sandstone.
Markers reward the discordant dyke identification plus the correct interpretation of chilled (in the intrusion) and baked (in the country rock) margins.
OCR H414/01 20184 marksExplain how the cross-cutting relationships of two dykes and a lava flow can be used to place them in order of formation, oldest first.Show worked answer →
State the principle, then apply it.
The principle of cross-cutting relationships. Any igneous body or feature that cuts across another must be younger than the rock it cuts, because the rock had to exist first to be cut.
Applying it. If dyke A is cut by dyke B, then A is older than B. A lava flow, by contrast, is extrusive: it forms at the surface and is then buried, so it can be cut by later dykes but will itself bake the rocks below it, not above. A dyke that cuts the lava flow is younger than the flow; a dyke baked or truncated by the flow's base is older. Combining these gives a full order, oldest first.
Markers want explicit use of cross-cutting (the cutting feature is younger) and recognition that an extrusive flow bakes the rocks below it.
Related dot points
- 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.
- 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.
- Metamorphic rocks: the agents of metamorphism (heat, pressure and chemically active fluids); the types of metamorphism (regional, contact and dynamic) and their settings; the development of foliation under directed pressure; metamorphic grade and the prograde sequence from mudstone (slate, phyllite, schist, gneiss); the use of index minerals (chlorite, garnet, kyanite, sillimanite) to indicate grade.
A focused answer to the OCR H414 dot point on metamorphism. Covers the agents (heat, pressure and fluids), regional, contact and dynamic metamorphism, the development of foliation, metamorphic grade and the mudstone prograde sequence (slate, phyllite, schist, gneiss), and the use of index minerals to indicate grade.
- Relative dating: the principles used to order geological events (superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession); the recognition of way-up evidence; the application of these principles to construct the geological history of a cross-section, including faults, intrusions and unconformities.
A focused answer to the OCR H414 dot point on relative dating. Covers superposition, original horizontality, cross-cutting relationships, included fragments and faunal succession, way-up evidence, and how to apply these principles to reconstruct the geological history of a cross-section with faults, intrusions and unconformities.
- Geological structures: the response of rocks to stress (folds and faults); fold elements and types (anticline and syncline, limb, hinge and axial plane); fault types and the stress regime they record (normal from tension, reverse and thrust from compression, strike-slip from shear); joints; dip and strike; the recognition and significance of unconformities (angular unconformity, disconformity and nonconformity).
A focused answer to the OCR H414 dot point on geological structures. Covers folds (anticline, syncline, limb, hinge, axial plane), fault types and the stress they record (normal, reverse, thrust, strike-slip), joints, dip and strike, and the recognition and significance of angular unconformities, disconformities and nonconformities.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)