Weathering: Weathering is the Physical, Chemical or Biological break down of rock or minerals into smaller particles near to or at the surface of the rock. The rocks are broken down "in situ" which means there is no movement (unlike erosion which is caused by the movement of water, ice and wind)
Biological weathering: is the breakdown of rocks by animals and plants.
Plants grow in cracks in the rocks. As plants grow their roots put a lot of pressure on the rock and the rock breaks apart. Plant roots also produce weak acids which can break the rock down. Animals burrow into the soil and have the same effects as plant roots.
Chemical weathering: The break up of rocks by processes such as solution causing changes in the minerals that form the rock
The chemical break down of rocks is mainly due to rocks being dissolved in rain water. This absorbs carbon dioxide in the air to form weak carbonic acid. Futhermore air pollution adds to the acidity of the water, increasing it's ability to attack rocks. This is called solution weathering.
The chemical formular for limestone solution is
CaCO3 +H2O + CO2 ----> Ca(HC03)2
calcium carbonate+ water+ carbon dioxide -----> calcium bicarbonate
This process happens very slowly it can take carbonic acid 100,000 years to weather 45cm of Limestone. Chemical weathering is most effective in hot, humid conditions wear reactions happen faster.
Physical Weathering: This is the mechanical breakdown of rocks without any change in the minerals which make up the rock. This usually occurs because of extreme temperature changes. For example;
Threeze thaw weathering: This occurs when water enters cracks in the rock, when temperatures drops, the water freezes and the water expands by about 9%. When the temperature warms up the ice melts. This alternating expanding and contracting eventually splits the rock futher creating a widened space where the water can trickle hence starting the process all over again.
Exfoliation (onion peel weathering): This happens in places with extreme temperature changes for example the desert. Hot temperatures make the particles in the rock spread out and so the rock exands slightly, when temperatures drops the rock contracts. This alternate contracting and exapanding puts great strain on the rock eventually causing it to crack across the surface and start to peel away.