How do heat and pressure transform existing rocks, and how do we read metamorphic grade and facies?
Metamorphism, grade and facies: contact (thermal) metamorphism producing hornfels within an aureole versus regional metamorphism producing foliated rocks; the agents of metamorphism (heat, pressure and chemically active fluids); metamorphic grade and the prograde sequence from mudstone (slate, phyllite, schist, gneiss) with index minerals (chlorite, biotite, garnet, kyanite, sillimanite); foliated rocks (slate, schist, gneiss) versus non-foliated rocks (marble from limestone, quartzite from sandstone); protoliths; and metamorphic facies in outline.
A focused answer to the Eduqas Geology statement on metamorphism. Covers contact versus regional metamorphism, the agents (heat, pressure, fluids), metamorphic grade and the mudstone prograde sequence (slate, phyllite, schist, gneiss) with index minerals, foliated versus non-foliated rocks (marble, quartzite), protoliths, and metamorphic facies in outline.
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What this dot point is asking
Eduqas wants you to distinguish contact (thermal) metamorphism (hornfels in an aureole) from regional metamorphism (foliated rocks), to identify the agents (heat, pressure, fluids), to describe grade and the prograde sequence from mudstone (slate, phyllite, schist, gneiss) with its index minerals, to contrast foliated rocks with non-foliated rocks (marble, quartzite) and name their protoliths, and to outline metamorphic facies. It follows the sedimentary statement and links to plate margins.
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 starting rock is the protolith. The three agents are:
- Heat, from deep burial or a nearby intrusion, driving recrystallisation and new mineral growth.
- Pressure, either confining (equal from all directions, from burial) or directed (greater in one direction, from tectonic compression).
- Chemically active fluids, mainly water, which speed reactions and can add or remove ions.
Contact (thermal) versus regional metamorphism
- Contact (thermal) metamorphism. Mainly heat from an igneous intrusion, with little directed pressure, in a metamorphic aureole (highest grade at the contact, decreasing outwards). Because the pressure is not directed, the rocks are non-foliated: mudstone bakes to hornfels, limestone to marble.
- Regional metamorphism. High temperature and directed pressure (plus fluids) over a large area, at convergent plate margins during mountain building. Directed pressure aligns platy minerals, so the rocks are foliated (slate, schist, gneiss) with increasing grade.
Foliation
Foliation is the parallel alignment of platy or elongate minerals (such as mica), produced by directed pressure, which grows and rotates the minerals at right angles to the maximum stress. It is therefore the signature of regional metamorphism and is absent in contact metamorphism, where the pressure is not directed.
Grade and the prograde sequence
Metamorphic grade is the intensity of metamorphism, controlled mainly by temperature. A mudstone taken to higher grade passes through a sequence, with grain size and foliation both increasing:
- Mudstone (protolith) to slate (low grade; very fine, a strong slaty cleavage).
- to phyllite (a little higher; a silky sheen from larger micas).
- to schist (medium grade; visible aligned micas, a wavy schistosity).
- to gneiss (high grade; coarse, light and dark minerals in bands).
Index minerals
Index minerals are stable only within particular temperature and pressure ranges, so their first appearance marks a grade and lets geologists map metamorphic zones. In a metamorphosed mudstone, in order of increasing grade: chlorite (low), then biotite, then garnet (medium), then kyanite and sillimanite (high). A change from chlorite, through garnet, to sillimanite records grade rising towards the most deeply buried, most strongly deformed core of a mountain belt.
Foliated versus non-foliated rocks, and their protoliths
- Foliated rocks (formed under directed pressure): slate, schist and gneiss, all commonly from a mudstone/shale protolith.
- Non-foliated rocks (formed under heat with little directed pressure): marble (from limestone) and quartzite (from quartz sandstone); they are non-foliated because calcite and quartz are equidimensional and do not align into platy fabrics.
Metamorphic facies in outline
A metamorphic facies is a set of mineral assemblages that form under a particular range of temperature and pressure, regardless of the protolith. Named facies (for example greenschist at low grade and amphibolite at higher grade) correspond to fields on a pressure-temperature diagram, so a rock's mineral assemblage records the conditions it experienced. You only need the idea that facies map conditions, not the full diagram.
Examples in context
Example 1. Marble in a contact aureole. A limestone next to a granite intrusion recrystallises under heat alone into non-foliated marble, the highest grade at the contact: the hallmark of contact metamorphism.
Example 2. Schist and gneiss in an orogen. In the core of a collision belt, mudstones taken to medium and high grade by regional metamorphism become schists and gneisses, their 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. [3 marks]
- Cue. Regional metamorphism involves directed pressure, which aligns platy minerals such as mica into foliation; contact metamorphism is driven mainly by heat with no directed pressure, so the rocks (hornfels, marble) are non-foliated.
Q3. Name the protolith of marble and of quartzite. [2 marks]
- Cue. Marble forms from limestone; quartzite forms from quartz sandstone.
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 20186 marksCompare contact and regional metamorphism, referring to the agents involved, the tectonic setting, and the textures produced. Use named rocks in your answer.Show worked answer →
A levels-of-response comparison; address agents, setting and texture for each, then state the key contrast.
- Contact (thermal) metamorphism
- Driven mainly by heat from a nearby igneous intrusion, with little directed pressure (fluids may assist). Setting: a metamorphic aureole around the intrusion, with the highest grade nearest the contact, decreasing outwards. Texture: with no significant directed pressure, the rocks are non-foliated, for example hornfels from mudstone and marble from limestone.
- Regional metamorphism
- Driven by high temperature and high directed pressure (plus fluids) over a large area, at convergent (destructive) plate margins during mountain building. Texture: directed pressure aligns platy minerals to produce a foliated texture, giving slate, schist and gneiss, and a sequence of increasing grade across the region.
- The key contrast
- Contact metamorphism is mainly heat and produces non-foliated rocks in a local aureole; regional metamorphism adds directed pressure and produces foliated rocks over a wide area.
Top-band answers explicitly link directed pressure to foliation in the regional case and its absence to the non-foliated contact rocks, with named rocks for each.
Eduqas 20214 marksA metamorphosed mudstone contains chlorite in one zone, garnet in a zone closer to the centre of a mountain belt, and sillimanite in the innermost zone. Explain what these 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 marks the grade of metamorphism.
- Reading the zones
- Chlorite is a low-grade index mineral, so the outer zone reached only low grade (lower temperature and pressure). Garnet indicates medium grade, and sillimanite indicates high grade, so grade increases towards the centre of the belt.
- Interpretation
- The change 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. Geologists map the boundaries (isograds) to chart metamorphic zones.
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 and depositional environments: the classification of clastic rocks by grain size (conglomerate and breccia, sandstone including arkose, greywacke and orthoquartzite, siltstone, mudstone and shale) and of chemical and biogenic rocks (limestone including oolitic, micritic and fossiliferous, chalk, the evaporites rock salt and gypsum, and coal); sedimentary structures (cross-bedding, graded bedding, ripple marks, desiccation cracks) as way-up and environment indicators; depositional environments (fluvial, deltaic, shallow marine, deep marine, desert); and diagenesis and lithification.
A focused answer to the Eduqas Geology statement on sedimentary rocks. Covers clastic classification (conglomerate to mudstone, with arkose, greywacke and orthoquartzite), chemical and biogenic rocks (limestones, chalk, evaporites, coal), sedimentary structures as way-up and environment indicators, depositional environments, and diagenesis and lithification.
- 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).
- 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.
- Weathering, erosion and sediment transport: physical weathering (freeze-thaw and exfoliation), chemical weathering (hydrolysis of feldspar to clay, carbonation of limestone, oxidation) and biological weathering; the distinction between weathering and erosion; transport by traction, saltation, suspension and solution, and how transport rounds and sorts grains to determine the maturity of a sediment.
A focused answer to the Eduqas Geology statement on surface processes. Covers physical weathering (freeze-thaw, exfoliation), chemical weathering (hydrolysis of feldspar to clay, carbonation, oxidation), biological weathering, the weathering versus erosion distinction, and transport (traction, saltation, suspension, solution) with rounding, sorting and maturity.
- Plate margins and their features: the processes and characteristic features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-ocean island arcs, ocean-continent margins and continent-continent collision); the Benioff zone, subduction and decompression melting; the diagnostic rocks, structures, earthquakes and volcanoes of each margin type.
A focused answer to the Eduqas Geology statement on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-ocean, ocean-continent and continent-continent sub-types, the Benioff zone, subduction and decompression melting, and the diagnostic rocks, structures, earthquakes and volcanoes that identify each margin in the exam.
Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)