How do heat and pressure change one rock into a metamorphic rock?
Metamorphic rocks form by recrystallisation of existing rocks in the solid state under heat and pressure, without melting; contact metamorphism (heat from an intrusion) produces non-foliated rocks such as metaquartzite and marble; regional metamorphism (heat and directed pressure over a wide area) produces foliated rocks such as slate and schist; protolith and conditions determine the product.
A focused answer to the Eduqas GCSE Geology statement on metamorphic rocks. Covers solid-state recrystallisation under heat and pressure, the difference between contact metamorphism (non-foliated metaquartzite and marble) and regional metamorphism (foliated slate and schist), foliation, and how the protolith and conditions set the product.
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What this dot point is asking
Eduqas wants you to explain that metamorphic rocks form by recrystallisation in the solid state under heat and pressure, without melting, and to distinguish the two settings: contact metamorphism (heat from an intrusion, producing non-foliated metaquartzite and marble) and regional metamorphism (heat and directed pressure over a wide area, producing foliated slate and schist). You also need to know that the starting rock (the protolith) and the conditions together determine the metamorphic product.
The answer
What metamorphism is
Metamorphism is the changing of an existing rock into a new rock in the solid state, by heat, pressure or both, without the rock melting. The minerals recrystallise (grow into new crystals or larger, interlocking ones), and new minerals can grow, but the rock stays solid throughout. This is the crucial distinction: as soon as a rock melts it becomes magma and the product is igneous, not metamorphic. The starting rock is called the protolith (or parent rock).
Contact metamorphism: heat only, local
Contact metamorphism is caused by heat alone from a nearby igneous intrusion. The country rock is "baked" in a zone around the intrusion called the metamorphic aureole, which is widest next to the hottest, largest intrusions. Because there is little or no directed pressure, the minerals recrystallise into an interlocking mosaic with no preferred alignment, giving non-foliated (massive) rocks:
- Limestone recrystallises to marble (a sugary, interlocking calcite rock that still fizzes in acid).
- Sandstone recrystallises to metaquartzite (a hard, interlocking quartz rock).
Regional metamorphism: heat plus directed pressure, widespread
Regional metamorphism acts over a wide area and involves both high heat and high directed pressure, typically where plates collide and mountain belts form. The directed pressure aligns platy minerals, producing foliated rocks (rocks with a layered or banded, easily split fabric). As the intensity (the metamorphic grade) increases, a mudstone passes through:
- Slate (low grade): very fine, splits into thin flat sheets (slaty cleavage), used for roofing.
- Schist (higher grade): coarser, with visible aligned micas that give a shiny, wavy foliation.
Foliation: why some metamorphic rocks split
Foliation is the parallel alignment of platy minerals (such as mica) produced by directed pressure. The minerals grow with their flat faces perpendicular to the direction of greatest pressure, creating planes of weakness along which the rock splits. Non-foliated rocks (marble, metaquartzite) lack foliation because they formed under heat without strong directed pressure, or from minerals (calcite, quartz) that are not platy.
Protolith plus conditions sets the product
The metamorphic rock you get depends on what you start with and the conditions:
| Protolith | Contact (heat only) | Regional (heat + pressure) |
|---|---|---|
| Mudstone | hornfels (baked, non-foliated) | slate to schist (foliated) |
| Sandstone | metaquartzite | metaquartzite |
| Limestone | marble | marble |
Quartz and calcite are not platy, so sandstone and limestone give non-foliated metaquartzite and marble in both settings; clay-rich mudstone gives strongly foliated rocks under regional pressure.
Examples in context
Example 1. Welsh slate. The slate of north Wales formed by low-grade regional metamorphism of mudstone, and its perfect slaty cleavage made it the roofing material of the industrial era.
Example 2. Marble around a granite. Where a granite intrudes limestone, a halo of marble forms in the aureole, recording contact metamorphism by the heat of the intrusion.
Try this
Q1. State what happens to the minerals in a rock during metamorphism, and what must not happen. [2 marks]
- Cue. They recrystallise in the solid state (new or larger interlocking crystals); the rock must not melt.
Q2. Name the metamorphic rocks produced from (a) limestone and (b) mudstone under regional metamorphism. [2 marks]
- Cue. (a) Marble; (b) slate (low grade) to schist (higher grade).
Q3. Explain why marble is not foliated even though it is a metamorphic rock. [2 marks]
- Cue. It is made of calcite, which is not a platy mineral, so there is nothing to align into a foliation.
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 20184 marksExplain the difference between contact metamorphism and regional metamorphism, giving one rock produced by each.Show worked answer →
The mark is for distinguishing the two settings and naming the right products.
- Contact metamorphism
- This is caused by heat alone from a nearby igneous intrusion. It affects a narrow zone (a metamorphic aureole) around the intrusion, where the country rock is baked. Because there is no strong directed pressure, the product is non-foliated: a limestone is recrystallised to marble, and a sandstone to metaquartzite.
- Regional metamorphism
- This is caused by both high heat and high directed pressure over a wide area, usually where plates collide and mountains form. The directed pressure aligns platy minerals, so the product is foliated: a mudstone is changed to slate at low grade and to schist at higher grade.
- The key difference
- Contact is heat-only and local, giving non-foliated rocks; regional is heat plus directed pressure over a large area, giving foliated rocks.
Markers reward heat-only and local for contact, heat plus directed pressure over a wide area for regional, and one correct product for each.
Eduqas 20223 marksSlate splits easily into thin flat sheets. Explain how this foliation forms during regional metamorphism.Show worked answer →
A short explain question on foliation.
- Directed pressure during regional metamorphism
- Regional metamorphism applies strong pressure from one direction (for example during a continental collision).
- Platy minerals grow at right angles to the pressure
- New platy minerals such as mica recrystallise and grow with their flat faces lined up perpendicular to the direction of greatest pressure.
- This alignment is foliation
- Because the platy minerals are all parallel, the rock has planes of weakness and splits easily into thin flat sheets along them (slaty cleavage), which is why slate is used for roofing.
Top answers link directed pressure, the alignment of platy minerals perpendicular to it, and the resulting planes of weakness.
Related dot points
- Igneous rocks form by the crystallisation of magma or lava; cooling rate controls crystal size (slow cooling at depth gives coarse-grained intrusive rocks such as granite, fast cooling at the surface gives fine-grained extrusive rocks such as basalt); rocks are classified by crystal size and by silica content (felsic, intermediate, mafic); minerals also crystallise from hydrothermal fluids to form veins.
A focused answer to the Eduqas GCSE Geology statement on igneous rocks. Covers how magma and lava crystallise, how cooling rate controls crystal size (intrusive granite versus extrusive basalt), classification by silica content (felsic to mafic), and the crystallisation of minerals from hydrothermal fluids in veins.
- Sedimentary rocks form by weathering, erosion, transport, deposition, and lithification (compaction and cementation); they are classified as clastic (conglomerate, breccia, sandstone, shale), biological (limestone) or chemical (evaporites); grain size, shape, sorting, sedimentary structures and fossil content are used to interpret the depositional environment; fossils form by preservation of hard parts and record past life.
A focused answer to the Eduqas GCSE Geology statement on sedimentary rocks. Covers weathering, transport, deposition and lithification, the clastic, biological and chemical classes (conglomerate, sandstone, shale, limestone, evaporites), reading the depositional environment from grain size, sorting and structures, and how fossils form and what they record.
- Minerals are identified using physical properties: colour, crystal size, hardness (tested against fingernail, copper coin, steel and glass), cleavage and fracture, lustre, streak, and the reaction of carbonates with dilute hydrochloric acid; common minerals include quartz, feldspar, mica, calcite, halite, galena and haematite.
A focused answer to the Eduqas GCSE Geology statement on identifying minerals. Covers the physical properties used (colour, crystal size, hardness, cleavage and fracture, lustre, streak and the acid test) and the diagnostic features of quartz, feldspar, mica, calcite, halite, galena and haematite.
- The rock cycle links igneous, sedimentary and metamorphic rocks through the processes of weathering, erosion, transport, deposition, burial and lithification, melting and crystallisation, and metamorphism; the cycle is driven by energy from the Sun (at the surface) and from the Earth's interior (at depth), and any rock can be changed into any other given time and the right conditions.
A focused answer to the Eduqas GCSE Geology statement on the rock cycle. Covers the three rock families and the processes that connect them (weathering, erosion, transport, deposition, lithification, melting, crystallisation and metamorphism), the two energy sources that drive the cycle, and how any rock can become any other.
- Rocks deform when stressed: compression produces folds (anticlines arch upwards, synclines sag downwards) and reverse faults, while tension produces normal faults; the type and orientation of folds and faults are evidence of the direction of past Earth movements and are shown on geological maps and cross-sections.
A focused answer to the Eduqas GCSE Geology statement on folds and faults. Covers how compression produces folds (anticlines and synclines) and reverse faults, how tension produces normal faults, the parts of a fold and fault, and how these structures record the direction of past Earth movements.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Geology specification (teaching from 2017) — WJEC Eduqas (2017)