How do heat and pressure transform existing rocks, and how do we read metamorphic grade?
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.
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What this dot point is asking
OCR wants you to identify the agents of metamorphism (heat, pressure and chemically active fluids), distinguish regional, contact and dynamic metamorphism and their settings, explain how directed pressure produces foliation, describe metamorphic grade and the prograde sequence from mudstone (slate, phyllite, schist, gneiss), and use index minerals to indicate grade.
The answer
What metamorphism is and its agents
Metamorphism changes a pre-existing rock in the solid state (without melting) into a new rock with new minerals or textures. The three agents are:
- Heat, from burial or a nearby intrusion, which drives recrystallisation and new mineral growth.
- Pressure, either confining (equal from all directions, from deep burial) or directed (greater in one direction, from tectonic compression).
- Chemically active fluids, which speed reactions and can add or remove ions.
Types of metamorphism and their settings
- Regional metamorphism. High temperature and high directed pressure (plus fluids) over a large area, at convergent (destructive) plate margins during mountain building. Produces foliated rocks across whole regions.
- Contact (thermal) metamorphism. Mainly heat from an igneous intrusion, with little directed pressure, in a metamorphic aureole around the intrusion (highest grade at the contact). Produces non-foliated rocks (marble from limestone, hornfels from mudstone).
- Dynamic (cataclastic) metamorphism. High directed stress along a fault zone, which grinds and deforms the rock, producing rocks such as mylonite. Localised to the fault.
Foliation
Foliation is the parallel alignment of platy or elongate minerals (for example mica), produced by directed pressure, which rotates and grows the minerals at right angles to the maximum stress. Foliation is therefore the signature of regional (and dynamic) metamorphism and is absent in contact metamorphism, where pressure is not directed.
Grade and the prograde sequence
Metamorphic grade is the intensity of metamorphism, mainly controlled by temperature. As a mudstone is taken to higher grade, it passes through a recognisable sequence:
- Mudstone (parent) to slate (low grade, fine, splits into flat sheets, a strong slaty cleavage).
- to phyllite (slightly higher grade, a sheen from larger micas).
- to schist (medium grade, visible aligned micas, a wavy schistosity).
- to gneiss (high grade, coarse, with the minerals segregated into light and dark bands).
Index minerals
Index minerals are stable only within particular temperature and pressure ranges, so their presence marks the grade and lets geologists map metamorphic zones:
- Chlorite indicates low grade.
- Garnet indicates medium grade.
- Kyanite and then sillimanite indicate high grade.
A change from chlorite, through garnet, to sillimanite across a region records grade increasing towards the most deeply buried, most strongly deformed core of a mountain belt.
Examples in context
Example 1. Marble in a contact aureole. A limestone next to a granite intrusion recrystallises under heat alone into marble, a non-foliated rock, with the highest grade at the contact: the hallmark of contact metamorphism.
Example 2. Schist and gneiss in an orogen. In the core of a mountain belt, mudstones taken to medium and high grade under regional metamorphism become schists and gneisses, their strong foliation recording the directed pressure of continental collision.
Try this
Q1. State the three agents of metamorphism. [2 marks]
- Cue. Heat, pressure (confining or directed) and chemically active fluids.
Q2. Explain why regional metamorphism produces foliated rocks but contact metamorphism does not. [2 marks]
- Cue. Regional metamorphism involves directed pressure, which aligns platy minerals into foliation; contact metamorphism is driven mainly by heat with no directed pressure, so the rocks are non-foliated.
Q3. Place these rocks in order of increasing metamorphic grade: gneiss, slate, schist, phyllite. [2 marks]
- Cue. Slate, phyllite, schist, gneiss.
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 H414/01 20206 marksCompare regional and contact metamorphism, referring to the agents involved, the tectonic setting, and the textures produced.Show worked answer →
A level-of-response comparison; address agents, setting and texture for each.
- Regional metamorphism
- Driven by both high temperature and high directed pressure (and fluids) over a large area. Setting: convergent (destructive) plate margins, during mountain building, affecting large volumes of crust. Texture: directed pressure aligns platy minerals, producing a foliated texture (slate, schist, gneiss) and a sequence of increasing grade across the region.
- Contact metamorphism
- Driven mainly by high temperature (heat) from a nearby igneous intrusion, with little directed pressure. Setting: a metamorphic aureole around an intrusion, highest grade nearest the contact. Texture: because there is no significant directed pressure, the rocks are non-foliated (for example marble from limestone, hornfels from mudstone).
- The key contrast
- Regional involves pressure and produces foliation over a wide area; contact involves mainly heat and produces non-foliated rocks in a local aureole.
Top-band answers explicitly link directed pressure to foliation (regional) and its absence to the non-foliated contact rocks.
OCR H414/03 20194 marksA region shows metamorphosed mudstones containing chlorite in one zone and garnet and then sillimanite in zones closer to the centre of a mountain belt. Explain what the index minerals reveal about how metamorphic grade changes across the region.Show worked answer →
Use the index minerals as a grade thermometer.
- What index minerals are
- Certain minerals are stable only within particular temperature and pressure ranges, so their presence indicates the grade of metamorphism.
- Reading the zones
- Chlorite is a low-grade index mineral, so the outer zone experienced low grade (lower temperature and pressure). Garnet indicates medium grade, and sillimanite indicates high grade, so grade increases towards the centre of the mountain belt.
- Interpretation
- The increase from chlorite, through garnet, to sillimanite shows that temperature and pressure rose towards the core of the orogen, where the crust was most deeply buried and most strongly deformed. This lets geologists map metamorphic zones (isograds).
Markers reward the idea that each index mineral marks a grade and that the sequence records increasing temperature and pressure towards the centre.
Related dot points
- Sedimentary rocks: the stages from sediment to rock (deposition, compaction and cementation as lithification); the classification of sedimentary rocks into clastic (by grain size, from conglomerate to mudstone), chemical (precipitates such as evaporites) and biogenic or biochemical (limestone and coal); the description of clastic texture using grain size, sorting and roundness.
A focused answer to the OCR H414 dot point on sedimentary rocks. Covers lithification (deposition, compaction and cementation), the clastic, chemical and biogenic or biochemical classes, the grain-size scale from conglomerate to mudstone, and how clastic texture is described using grain size, sorting and roundness.
- 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.
- 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.
- Plate margins: the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-continent, ocean-ocean and continent-continent collision); the Benioff zone and subduction; the characteristic rocks, structures, earthquakes and volcanoes produced at each margin type.
A focused answer to the OCR H414 dot point on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-continent, ocean-ocean and continent-continent sub-types, the Benioff zone and subduction, and the characteristic rocks, structures, earthquakes and volcanoes of each.
- 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)