Weathering occurs in situ (in place, with no movement). It is not erosion. Erosion is where rocks and minerals are moved downhill (usually towards the sea) by water, ice and wind.
Dynamic movement is always taking place on Earth. One type of change raises the land surface upwards to form (at the highest) huge mountain ranges. This orogeny is formed as a result of plate tectonics, such as the collision of continental plates.
At the same time, all land surfaces are being ground down gradually all the time. Material from the land ends up in the seas or lakes, and forms the sediment which finally ends up as sedimentary rock. Rocks and minerals break up under the influence of water, ice, wind and gravity, and other causes.
The two main types of weathering are mechanical and chemical.
Mechanical/physical weathering [change]
Wind processes are called 'aeolian'. Wind erodes the Earth's surface by removing loose, fine-grained particles, called 'deflation'.
Wind carrying sand wears grinds down surfaces with its windbourne particles.
Regions which have intense and sustained erosion are called deflation zones. Most aeolian deflation zones are composed of desert pavement, a sheet-like surface of rock fragments that remains after wind and water have removed the fine particles. Almost half of Earth's desert surfaces are stony deflation zones. The rock mantle in desert pavements protects the underlying material from deflation.
Rain is another force that works slowly. The force of raindrops on some rocks make them wear down. Rain also can make a chemical change in some rocks. The water mixes with the minerals in the rock to break it down.
Changing temperature can make a rock crack. Every day when the sun shines on the rock, its surface is heated. Heat causes the surface to expand (get bigger) a little. The inside of the rock, though, does not heat up as fast as the outside of the rock. The inside of the rock stays cooler. At night, the surface cools down and contracts. The expanding and contracting makes some places on the surface weak, and a crack is made.
Also, if water gets into a crack in a rock and the temperature goes below the freezing point, the water will freeze and expand. After some time, the rock may be weak enough to break into pieces.
Ice, which can be miles thick, grinds the surface of the rock below it. The particles are carried with the ice, and if a glacier ends up in the sea, so does all the material carried with it.
Chemical weathering [change]
Chemical weathering is the direct effect of atmospheric or biological chemicals in the breakdown of rocks, soils and minerals.
Carbon dioxide cycle [change]
While it is in the air, CO2 can dissolve in water droplets to form dilute carbonic acid.
- "Weathering is a large consumer of the atmospheric carbon dioxide essential for dissolving rocks".
When rain hits rock it does so with mechanical energy and dilute acid. The acid dissolves many types of minerals and rocks though, of course, very slowly. When a mineral like feldspar is dissolved, it lets sodium ions into the sea; chlorine ions come from other minerals. The sea tastes salty because of the elements which have been dissolved out of rocks.
Other pages [change]
- Van Andel T.H. 1994. New views on an old planet:a history of global change. Cambridge University Press. p233
- Drury S. 1999. Stepping stones: the making of our home world. Oxford University Press. p57