Chemical weathering refers to the decomposition of rocks at or near the earth’s surface and under relatively low temperatures.
During chemical weathering, water and chemically active water solutions as well as oxygen and carbon dioxide from the atmosphere attack the minerals comprising rocks.
The susceptible parent minerals are frequently altered to softer secondary minerals, as when feldspars are converted to clay minerals and lose certain ions to solution. Chemical weathering is an exceedingly important process, for it leads to the formation of soils and in some parts of the world has resulted in the concentration of iron, aluminum, uranium, gold, and tin.
Relation of Disintegration to Decomposition
ADVERTISEMENTS:
The processes of chemical weathering and mechanical weathering are not truly independent of one another. Because of climatic factors, especially the amount and frequency of rainfall, one or the other process may be dominant in a particular location.
For the most part, however decomposition and disintegration are interacting processes. For example, disintegration greatly promotes decomposition by reducing large intact rock masses to accumulations of smaller fragments.
This fragmentation provides more total surface area for chemical attack than was originally present in the larger mass of rock. One can see what happens readily by imagining a block of rock measuring 2 meters on each edge.
Such a block would have a total of 24 square meters of surface area. If the block is split once along each of three perpendicular planes, each resulting block would have 6 square meters of surface area or a total of 48 square meters for all eight pieces. Chemically reactive solutions are only able to decompose surfaces they can reach, and so the rate of chemical attack is enhanced by increasing surface area.
ADVERTISEMENTS:
One can demonstrate this fact by applying a few drops of dilute hydrochloric acid first to an unfragmented piece of calcite and then to a similar piece of calcite that has been pulverized. Because of the greater amount of total surface area, the pulverized rock will display the more vigorous effective scence.