The rock cycle — Earth science, 14–17 years
Rocks do not belong permanently to one type. Heat, pressure, melting, cooling, weathering and burial can transform the same material over geological time.
Rocks can change identity
The three broad rock groups are igneous, sedimentary and metamorphic. Igneous rocks crystallise from melted rock, sedimentary rocks form from deposited material, and metamorphic rocks change under heat and pressure without fully melting. One rock can pass through several groups.
Why a cycle is better than a list
A simple list of rock types cannot explain why the same minerals appear in very different places. Earth’s surface is worn down while its interior supplies heat and pressure. The cycle connects these processes, so it explains both the variety of rocks and their possible histories.
From mountain to sedimentary rock
Start with granite exposed in a mountain. Rain and ice crack it, and rivers carry grains downhill: weathering and erosion. The grains settle in layers on a sea floor, then burial and cement bind them into sandstone. If later heated and squeezed, the sandstone becomes metamorphic rock.
The cycle has no single start
A diagram often begins with magma, so it is tempting to think every rock starts there. That is only a choice for reading the diagram. A rock may begin as sediment, be changed by pressure, melt, or be uplifted and weathered; the cycle has many entry points.
Reading landscapes and resources
Geologists use the rock cycle to interpret cliffs, volcanoes, river valleys and layers exposed by construction. It also helps locate useful materials: ores may form with magma, while some fuels and groundwater occur in sedimentary rocks. The cycle does not tell the exact history without evidence from samples.
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