Weathering describes the breaking down or dissolving of rocks and minerals on the surface of the Earth.
Water, ice, acids, salts, plants, animals, and temperature changes are all agents of weathering.
Once a rock has been broken down, a process called erosion transports the bits of rock and mineral away.
No rock on Earth is hard enough to resist the forces of weathering and erosion.
2. Classification
Weathering is often divided into the processes of mechanical weathering and chemical weathering.
Biological weathering, in which living or once-living organisms contribute to weathering, can be a part of both processes.
2.1 Mechanical weathering
Mechanical weathering, also called physical weathering and disaggregation, causes rocks to crumble.
Water, in either liquid or solid form, is often a key agent of mechanical weathering.
For instance, liquid water can seep into cracks and crevices in rock.
If temperatures drop low enough, the water will freeze. When water freezes, it expands.
The ice then works as a wedge.
It slowly widens the cracks and splits the rock.
Temperature changes can also contribute to mechanical weathering in a process called thermal stress.
Changes in temperature cause rock to expand (with heat) and contract (with cold).
As this happens over and over again, the structure of the rock weakens. Over time, it crumbles.
2.2 Chemical Weathering
Chemical weathering changes the molecular structure of rocks and soil.
For instance, carbon dioxide from the air or soil sometimes combines with water in a process called carbonation.
This produces a weak acid, called carbonic acid, that can dissolve rock.
Carbonic acid is especially effective at dissolving limestone.
When carbonic acid seeps through the limestone underground, it can open up huge cracks or hollow out vast networks of caves.
2.3 Anthropogenic Weathering
Weathering is a natural process, but human activities can speed it up.
For example, certain kinds of air pollution increase the rate of weathering. Burning coal, natural gas, and petroleum releases chemicals such as nitrogen oxide and sulfur dioxide into the atmosphere.
When these chemicals combine with sunlight and moisture, they change into acids. They then fall back to Earth as acid rain.
