What is metamorphism?
The word comes from Greek: meta = change, morph = form. Metamorphism is the change of an existing rock into a new rock by heat, pressure and fluids, while the rock stays solid.
The original rock is called the parent rock (or protolith). It can be igneous, sedimentary or even another metamorphic rock.
Metamorphism happens between about 200 °C (below that, it is just burial and cementing of sediments) and about 700–850 °C (above that, many rocks start to melt and form magma, which makes igneous rock).
During metamorphism:
- Minerals recrystallise: small grains grow into bigger ones.
- New minerals form that are stable at the new conditions (like mica, garnet).
- Grains may line up, giving the rock a new texture.
The agents of metamorphism
1. Heat
Heat comes from deep burial (the Earth gets hotter about 25–30 °C per km in the crust) or from nearby magma. Heat gives atoms energy to move and rebuild crystals.
2. Pressure
- Confining pressure: equal from all sides, like water pressure on a diver. It makes the rock more compact.
- Directed (differential) pressure: stronger in one direction, as when two plates push together. It flattens and lines up grains.
3. Chemically active fluids
Hot water with dissolved substances moves through rock, carries ions in and out, and speeds up reactions.
4. Time
Most metamorphism takes millions of years.
Foliated and non-foliated rocks
Foliated rocks
Foliation is a layered or banded look made when flat or long minerals (like mica) line up at right angles to the directed pressure. From low to high grade (amount of heat and pressure), shale changes like this:
- Slate: very fine grains; splits into flat sheets (slaty cleavage). Used for roofs.
- Phyllite: fine mica gives a silky shine.
- Schist: large, visible mica flakes; often has garnet crystals; sparkles.
- Gneiss: coarse, with light bands (quartz, feldspar) and dark bands (mica, hornblende).
Non-foliated rocks
These have no layering. They form when pressure is equal from all sides or when the parent rock has mainly one mineral.
- Marble: from limestone (calcite). Fizzes with dilute acid. Used for statues and buildings.
- Quartzite: from sandstone (quartz). Very hard; breaks through the grains, not around them.
- Hornfels: from shale baked by magma, fine and hard.
Other common pairs: granite → gneiss, coal → anthracite (and at very high grade, graphite).
Types of metamorphism and where they happen
Regional metamorphism
Happens over huge areas (thousands of km²) where tectonic plates collide and mountains form. Rocks are pushed deep and squeezed, so they are usually foliated. Belts of slate, schist and gneiss are found in the cores of old and young mountain ranges, such as the Himalaya, the Alps and the Appalachians.
Contact (thermal) metamorphism
Happens next to a hot magma body. The rock is mainly heated, not squeezed, so it is usually non-foliated. The changed zone around the magma is called a contact aureole; it may be only metres to a few kilometres wide. The rock nearest the magma changes most.
Other types
- Subduction zone (high-pressure, low-temperature): a cold ocean plate sinks fast, so pressure rises faster than temperature, making rare rocks like blueschist and eclogite.
- Dynamic (fault) metamorphism: grinding along faults crushes rock into mylonite.
- Burial metamorphism: slow change of deep sediment layers in basins.
Grade and index minerals
Geologists read the grade from index minerals that form at certain conditions: chlorite (low), biotite, garnet (medium), sillimanite (high). Mapping these shows how hot and deep rocks once were.
Metamorphic rocks in the rock cycle
Any rock can become metamorphic. If heated further it melts into magma (igneous). If lifted up and worn down, its pieces become sediment (sedimentary). Metamorphic rocks also preserve clues about old plate collisions in a region's history.
Try it: a practical
In the 3D: in the last step, keep the parent rock as shale. Predict which rock forms at 280 °C with strong squeeze, then at 650 °C. Now switch the squeeze off: what forms instead? Then try limestone and sandstone.
At home (foliation model): mix a handful of short pieces of uncooked spaghetti or rice grains into a ball of play dough so they point every way. Press the ball hard between two books. Cut it open: the pieces now lie flat, at right angles to the push. That is how foliation forms.
Key formulas and definitions
- Metamorphism: solid-state change by heat, pressure and fluids (about 200 °C to melting)
- Shale → slate → phyllite → schist → gneiss (increasing grade, foliated)
- Limestone → marble; sandstone → quartzite; shale + magma heat → hornfels (non-foliated)
- Granite → gneiss; coal → anthracite
- Regional: collision, heat + directed pressure, foliated; Contact: near magma, heat, non-foliated
- Foliation forms at right angles to directed pressure
Worked examples
1. A rock splits into thin flat sheets and has very fine grains. Name it and its parent rock.
Slate, formed from shale by low-grade regional metamorphism.
2. A white rock fizzes when a drop of dilute acid is put on it and has interlocking crystals but no layers. Name it.
Marble. It is made of calcite (which reacts with acid) and formed from limestone; non-foliated.
3. Why is gneiss banded but marble is not?
Gneiss forms under strong directed pressure and has several minerals that separate into light and dark bands. Marble usually forms from almost pure calcite, often without directed pressure, so there is nothing to line up in bands.
4. Magma rises into a layer of sandstone. Describe the changes in the rock around it.
Heat from the magma bakes the sandstone into quartzite in a contact aureole. The change is strongest next to the magma and fades farther away. The quartzite is non-foliated.
5. At a depth of 15 km with a temperature increase of 30 °C per km from 15 °C at the surface, what is the temperature? Is metamorphism likely?
15 + 30 × 15 = 465 °C. Yes, this is within the metamorphic range; with directed pressure shale would become schist.
6. Schist from a mountain belt contains garnet. What does this tell a geologist?
Garnet is an index mineral of medium grade, so the rock reached fairly high temperature and pressure, probably during plate collision.
Common mistakes
- Saying metamorphic rocks form by melting. They change while solid; melting makes igneous rock.
- Calling any striped rock 'sedimentary'. Bands in gneiss come from metamorphism, not from layers of sediment.
- Mixing up the order slate → phyllite → schist → gneiss.
- Thinking contact metamorphism happens over whole regions. It is limited to a zone (aureole) around the magma.