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Rocks and the rock cycle
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Rocks shape our landscapes. Whether a rock is hard or soft, and how it formed, explains the hills, cliffs and valleys we see.
Igneous rocks
Magma that cools slowly underground forms large crystals, as in granite (for example, Dartmoor).
Lava that cools quickly at the surface forms small crystals, as in basalt (for example, the Giant's Causeway in Northern Ireland).
Igneous rocks are usually very hard.
Sedimentary rocks
They often contain fossils and show layers (strata).
Examples: sandstone, limestone, chalk (the White Cliffs of Dover) and clay.
Metamorphic rocks and the rock cycle
Limestone becomes marble; mudstone or shale becomes slate.
The rock cycle links them: rocks are weathered and eroded into sediment, buried and changed, melted into magma, and cooled into new rock, over millions of years.
Why does granite have bigger crystals than basalt?
- Granite forms from magma that cools slowly underground.
- Slow cooling gives crystals a long time to grow.
- Basalt forms from lava that cools quickly at the surface, so its crystals stay small.
Answer: Granite cooled slowly underground, so its crystals had time to grow; basalt cooled quickly.
Igneous: cooled magma or lava (granite: big crystals; basalt: small). Sedimentary: compressed layers, often with fossils (sandstone, limestone, chalk). Metamorphic: changed by heat and pressure (limestone to marble, mudstone to slate). The rock cycle links them.
The interactive lesson includes the diagrams for this topic.
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