Breaking down and moving rock
The Earth's surface is constantly reshaped. Weathering is the breaking down of rocks into smaller pieces where they lie. It can be physical, such as water freezing in cracks and splitting the rock, or chemical, such as acid rain slowly dissolving limestone. Erosion is the moving away of these broken pieces by wind, flowing water or ice. Together, weathering and erosion slowly wear down mountains and shape valleys and coastlines. These changes are usually too slow to see in a single day, but over thousands of years they can completely change the look of the land.
Example
A river carries tiny bits of rock downstream. Where the river slows down near the sea, these bits settle to form new layers of sediment.
The water cycle and the rock cycle
The water cycle moves water around the Earth. The Sun heats water in the sea, causing evaporation. The water vapour rises, cools and forms clouds by condensation, then falls as rain or snow, called precipitation. The water flows back to the sea and the cycle repeats.
The rock cycle shows how the three rock types can change from one to another over very long times. Igneous rock can be weathered into sediment, which forms sedimentary rock. Heat and pressure can change it into metamorphic rock, and melting can turn any rock back into magma.
Remember
- Weathering breaks rock in place; erosion moves the pieces.
- Water cycle: evaporation, condensation, precipitation.
- The rock cycle recycles rock over millions of years.