How rocks change — Science, 7–10 years
Rocks are not permanent. Weathering, pressure, heat and melting can turn one kind of rock into another over very long periods.
The idea
A rock can change without disappearing forever. Rain and ice can break it into smaller pieces; layers can press those pieces together; heat can change rock deep underground; and melted rock can cool into a new solid rock. This repeating set of changes is called the rock cycle.
Why this idea is needed
At first, a pebble, a hard cliff and black lava seem like completely separate things. The rock cycle solves the puzzle of how they can be connected. It shows that Earth's materials are moved and changed by weather, burial, heat, pressure and volcanoes, rather than being made once and staying the same.
A worked example
Follow one piece of granite. Step 1: rain, roots and ice crack it, making gravel. Step 2: a river carries the gravel and lays it down in a layer. Step 3: more layers press it for millions of years, and minerals glue it into sedimentary rock. Step 4: deeper heat and pressure may change it again into a different rock.
The common trap
The word cycle may make it seem that every rock follows the same neat route and returns on a fixed timetable. That is a reasonable picture because cycles often repeat regularly. In reality, a rock can take several paths, skip stages, or wait millions of years; the cycle describes possible changes, not a clock.
Where it is useful
The rock cycle helps geologists read clues in landscapes. Layers may tell them about ancient rivers or seas, while volcanic rock can show where melted material cooled. It also matters when people use stone, gravel and sand: these materials are part of a slow Earth system, so taking or protecting them can change local landscapes.
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