The rock cycle — Earth science, 11–13 years
Rocks can be melted, cooled, broken, pressed and changed, so the three main rock groups are parts of a cycle rather than permanent boxes.
Rocks can change identity
A rock is not locked forever into one kind. Magma cools into igneous rock; layers of sediment can become sedimentary rock; heat and pressure can change either kind into metamorphic rock. Any rock may later melt, break down or be buried, so the cycle has many possible paths rather than one fixed circle.
Why classify changing rocks
At first, naming rocks may seem like sorting objects into tidy boxes. The problem is that rocks found at Earth’s surface record different histories, and scientists need clues about how they formed and what happened to them. The cycle connects visible features, such as layers or crystals, to processes like cooling, erosion, heat and pressure.
From mountain to new rock
Take a piece of granite in a mountain. First, rain, ice and plant roots break it into small grains. Next, rivers carry the grains and lay them down in a lake. Burial presses the layers together, and minerals help cement them into sandstone, a sedimentary rock. If it is later heated and squeezed, it may become metamorphic rock.
The three-box mistake
It is reasonable to think that every rock belongs permanently to one of three types, because textbooks often show three clear labels. Those labels describe how a rock formed at a particular time, not an unchangeable identity. A granite pebble can later become sediment, sandstone or a metamorphic rock if its conditions change.
Rocks in buildings and clues
Builders choose rocks for different jobs: granite can make a hard worktop, while sandstone may be cut for walls. Geologists also read rocks when looking for groundwater, useful minerals or signs of ancient environments. A layer of limestone with sea fossils, far inland, can reveal that the area was once under a shallow sea.
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