When rock is exposed to weather conditions it becomes subject to processes, such as freeze-thaw weathering, that cause it to change and break down.

Freeze-thaw weathering

The freeze-thaw weathering process is also known as frost shattering.

Water - eg from rainfall or melting snow and ice - becomes trapped in a crack or joint in the rock.
If the air temperature drops below freezing, the water will freeze and expand by 9-10 per cent putting pressure on the rock.
The ice will melt when the temperature rises above freezing.
If this process happens repeatedly, the rock will weaken and eventually shatter into angular fragments.
The fragments may then be deposited as scree at the foot of a slope.
It is most effective where the temperature fluctuates around 0°C, eg on north-facing high altitude slopes in Snowdonia.

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Geography - Rocks and Landscapes