Weathering and erosion
People often say some things are “as solid as a rock”, but even rocks don’t stay solid forever.
When rock is exposed to the air, and to sun and rain, it gradually starts to change. When rocks are broken into pieces by the weather, we call it weathering. Over thousands of years, weathering can change the hardest granite into soft, sticky clay.
Sedimentary rocks are formed from worn-away bits of other rocks. Rocks can be broken up by changes of temperature. Rocks ‘expand’ as they get hot and then ‘contract’ (shrink again) as they cool down.
In deserts, where the days are very hot and the nights are cold, the constant expanding and contracting begins to break up the rock. Thin sheets start to peel away from its surface. This is called onion-skin weathering because of the appearance it gives the rock.
Rocks can also be broken by ice. Water ‘expands’ when it freezes. If water gets into a crack in a rock and then turns into ice it pushes the crack open. When the ice melts, water can get further into the rock so that when the water freezes again it opens the crack a bit more. Eventually, the rock breaks open altogether.
After a rock has been broken up, the pieces may be moved to a different place, perhaps by wind, water or ice. This is called ‘erosion’. As the rock fragments are carried along they bump against each other. This breaks them down even further. Desert sand dunes are built up from tiny fragments of rocks that have been weathered by heat and cold and eroded by the wind.
Weathering usually only affects rocks near the surface, but water can carry its effects as far down as 185 metres.
Rocks are also worn away by chemicals. Acid rain is caused by chemicals that are produced by burning fuels in power stations and car engines.
The chemicals get into the atmosphere from chimneys and car exhausts and create acid rain, which attacks rocks. You can often see the damage caused to rocks that have been used in buildings or to make statues.