Rocks & the Rock Cycle
The three rock families — igneous, sedimentary, metamorphic — how to tell them apart, and how the rock cycle recycles them.
Three families of rock
- Igneous — formed when molten rock (magma underground, lava at the surface) cools and crystallizes. Slow underground cooling → large crystals (granite); fast surface cooling → small crystals or glass (basalt, obsidian).
- Sedimentary — formed when sediments are compacted and cemented, or when minerals precipitate from water. These are the layered rocks that can contain fossils (sandstone, shale, limestone).
- Metamorphic — existing rock changed by heat and pressure without fully melting. Often shows banding or aligned minerals (shale → slate, limestone → marble, granite → gneiss).
Quick check #1
In a rock-cycle diagram, an arrow points FROM metamorphic rock TO magma. What process does it represent?
Magma forms when rock melts. So an arrow from any rock (including metamorphic) to magma represents melting.
The rock cycle
The rock cycle describes how rock material moves between the three families. The key processes — and these are exactly the arrows you'll be asked to label:
- Melting turns any rock into magma.
- Cooling and crystallization turns magma/lava into igneous rock.
- Weathering and erosion break rock into sediment.
- Compaction and cementation turn sediment into sedimentary rock.
- Heat and pressure turn any rock into metamorphic rock.
There's no fixed order — a sedimentary rock can be buried and metamorphosed, melted into magma, or weathered back into sediment. Energy for the cycle comes from Earth's internal heat (driving melting and uplift) and the Sun (driving weathering and erosion).
Reading clues in a rock
- Layers (strata) and fossils → sedimentary.
- Interlocking crystals, no layering → igneous (large crystals = slow cooling).
- Banding or aligned crystals → metamorphic.
Quick check #2
A rock shows distinct flat layers and contains the fossil of a shell. Which rock family is it?
Layering and fossils are signatures of sedimentary rock, which forms from sediments deposited in layers — the only family that commonly preserves fossils.