How do heat, pressure and fluids transform rocks in the solid state?
Contact, regional and dynamic metamorphism, the controls of temperature, pressure and fluids, the increase of grade and the use of index minerals and metamorphic facies, and the textures (slate, schist, gneiss, hornfels, marble, quartzite) they produce.
A focused WJEC and Eduqas A-Level Geology G1 answer on contact, regional and dynamic metamorphism, the controls of temperature, pressure and fluids, how grade increases, the use of index minerals and metamorphic facies to read pressure-temperature conditions, and the foliated and non-foliated textures (slate to gneiss, hornfels, marble, quartzite) that result.
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What this dot point is asking
G1 completes the rock-forming picture by asking how rocks change in the solid state. WJEC wants the three types of metamorphism, the controls (temperature, pressure, fluids), how increasing grade builds a predictable sequence of textures, and how index minerals and metamorphic facies are read to recover the pressure and temperature a rock experienced.
The answer
Types of metamorphism
Metamorphism alters a rock in the solid state by heat, pressure and chemically active fluids, without melting it. The type depends on the setting:
- Contact (thermal) metamorphism: heat from an intrusion, low confining pressure, no strong directed stress. Produces a baked aureole of non-foliated rock such as hornfels.
- Regional metamorphism: high temperature and high directed pressure over large areas during mountain building. Produces foliated rocks (slate, schist, gneiss).
- Dynamic metamorphism: intense directed stress along a fault zone, producing crushed and recrystallised rocks (mylonite).
Grade, index minerals and facies
Grade is the intensity of metamorphism, increasing with temperature and pressure. As grade rises, new minerals grow that are stable at the new conditions.
A metamorphic facies is a set of minerals that forms together under a particular field of pressure and temperature (for example greenschist, amphibolite and granulite facies at increasing grade, and blueschist and eclogite at high pressure). Reading the facies lets you recover the conditions a rock experienced, which is powerful evidence for the tectonic setting.
Textures: foliated and non-foliated
Directed pressure is what produces foliation, the parallel alignment of platy minerals. With rising regional grade the foliated sequence is:
Examples in context
The Scottish Highlands preserve a classic regional sequence, with Barrow's zones (chlorite through to sillimanite) mapped across Dalradian rocks, the original basis for index-mineral mapping. Slate belts in Wales and the Lake District were mudstones metamorphosed at low grade, then quarried for roofing. Skiddaw in the Lake District shows a contact aureole of hornfels and spotted slate around a granite, illustrating heat-dominated, non-foliated contact metamorphism.
Try this
Q1. Name the texture produced by directed pressure and state the rock formed from mudstone at low grade. [2 marks]
- Cue. Foliation (here slaty cleavage); the rock is slate.
Q2. Place chlorite, garnet, biotite and sillimanite in order of increasing metamorphic grade. [2 marks]
- Cue. Chlorite, biotite, garnet, sillimanite.
Q3. Explain why a contact aureole produces non-foliated hornfels rather than schist. [2 marks]
- Cue. Contact metamorphism has heat but no directed pressure, so new minerals grow without alignment, giving a non-foliated texture.
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 20196 marksDescribe how a mudstone changes as it is subjected to increasing grade of regional metamorphism.Show worked answer →
Track the rock through increasing temperature and pressure, because grade is the variable.
At low grade the mudstone becomes slate. New platy minerals (clay-derived micas) grow perpendicular to the maximum stress, giving a slaty cleavage along which the rock splits.
At medium grade the slate becomes phyllite and then schist. The mica crystals grow larger and visibly aligned, giving a lustrous, wavy schistosity; minerals such as garnet may appear as porphyroblasts.
At high grade the schist becomes gneiss. Minerals segregate into light (quartz and feldspar) and dark (mica and amphibole) bands, giving a coarse gneissic banding. Near the highest grade the rock may begin to partially melt (migmatite).
So the sequence is slate, phyllite, schist, gneiss, with grain size and mineral alignment increasing throughout, and index minerals appearing in order.
Markers reward the named sequence slate to gneiss, the growth and alignment of micas, the appearance of index minerals such as garnet, and segregation banding at high grade.
WJEC Eduqas 20214 marksExplain the difference between contact and regional metamorphism in terms of the conditions and the textures produced.Show worked answer →
Contrast the controlling conditions, because they explain the textures.
Contact metamorphism is driven mainly by heat from a nearby intrusion, with low confining pressure and no strong directed stress. Because there is no directed pressure, new minerals grow without preferred orientation, so the rock is non-foliated, for example hornfels around an aureole.
Regional metamorphism acts over large areas during mountain building, with high temperature and high directed (differential) pressure. The directed stress aligns platy minerals, so the rock is foliated, giving slate, schist or gneiss according to grade.
Markers reward heat-dominated low-pressure non-foliated hornfels for contact metamorphism, and high temperature plus directed pressure giving foliated slate-schist-gneiss for regional metamorphism.
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Sources & how we know this
- WJEC Eduqas A-level Geology specification — WJEC Eduqas (2017)