A rounded specimen of augen gneiss with pale mineral eyes enclosed in dark, folded bands.

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Model scale

Rock and material examples. positions and specimen sizes are schematic.

Cooling and crystallisation: Magma → Igneous. Select another path; the cycle has branches and loops, with no compulsory order.

How to read these results

Selected conditions: 500 °C and 0.60 GPa. That pressure equals roughly 21.8 km of a uniform 2800 kg/m³ overburden. Temperature and pressure alone do not identify a rock or its melt fraction; composition, fluids, time and path matter. The moving specimen changes at the symbolic process midpoint; this is a route illustration, not a reaction-rate or melt-fraction model.

Rock cycle

Trace several possible routes through the rock cycle and explain how melting, cooling, pressure and weathering transform rock.

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Why it happens

Rocks can change through melting, crystallisation, weathering, deposition, burial and metamorphism. These processes form a network of possible routes rather than a compulsory journey through every rock type.

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Augen gneiss: deformation and metamorphism organise minerals into bands around larger crystals. Rock-cycle pathways need not follow one fixed loop.

Try a comparison

Choose several paths, including a solid-state and a melting path.

Reading the result

The pathways connect processes at different depths and timescales; the animation does not assign one duration to the whole cycle.

Film

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What Are Rocks and How Do They Form? Crash Course Geography #18 — Crash Course.

A common misunderstanding

Metamorphic rock forms when an existing rock melts completely.

Metamorphism changes a rock’s minerals or texture while it remains largely solid, under changed temperature, pressure or fluid conditions. Complete melting produces magma; subsequent crystallisation forms igneous rock. The rock cycle includes different possible pathways rather than one compulsory sequence.

Cases

Exam language

Igneous

Describing rock formed when magma or lava cools and crystallises. Slow cooling underground commonly produces larger crystals than rapid cooling at the surface. Texture therefore offers evidence of cooling history, although composition also affects the minerals present.

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NOAA, Galápagos Rift, 21 July 2011. Lava quenched in seawater forms pillows close to a spreading centre, where new oceanic crust is built.

Sedimentary

Describing rock formed from accumulated sediment or from chemical or biological precipitation. Clastic sediments commonly become rock through compaction and cementation. Bedding, grains and fossils can preserve evidence of the depositional environment.

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Grand Canyon from Hopi Point, Arizona. Resistant beds form steps and cliffs above valleys cut by running water.

Metamorphic

Describing rock changed by heat, pressure or chemically active fluids while remaining mostly solid. New minerals or fabrics may develop. If the material melts and then crystallises, the resulting rock is igneous instead.

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Augen gneiss: deformation and metamorphism organise minerals into bands around larger crystals. Rock-cycle pathways need not follow one fixed loop.

Lithification

The conversion of loose sediment into solid sedimentary rock, commonly through compaction and cementation. Pressure reduces pore space, while minerals can precipitate between grains. Lithification does not normally require the sediment to melt.

Foliation

A planar fabric in metamorphic rock, formed by aligned minerals or compositional bands. It commonly develops under directed stress. Foliation differs from sedimentary bedding, although both may appear as layers in an outcrop.

Magma

Partly or wholly molten rock beneath the surface, often containing crystals and dissolved gases. Molten rock erupted at the surface is called lava.

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Kīlauea, 25 February 1983. USGS photographed a lava fountain approximately 10 metres high; molten rock is at the surface, not throughout the mantle.

Weathering (to sediment)

The breakdown or alteration of rock in place, supplying particles and dissolved material that may enter sedimentary pathways. Erosion then removes material, transport carries it, and deposition stores it elsewhere. These are connected processes rather than interchangeable names.

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Weathered granite documented by GeoDIL. Mineral breakdown and disintegration progressively turn intact rock into weathered material.

Process0.40Material pathwaySelected temperature500 °CSelected pressure0.60 GPa