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Erosion features in Navajo Sandstone near Moab, Utah. Erosion by water and wind
A rock in Abisko, Sweden fractured along existing joints possibly by frost weathering or thermal stress
Geological exfoliation of granite dome rock in the Enchanted Rock State Natural Area, Texas
Toklat River, Denali National Park, Alaska. This river, like other braided streams, rapidly changes the positions of its channels through processes of erosion, sediment transport, and deposition
Comparison of unweathered (left) and weathered (right) limestone

Weathering is the breaking down of rocks, soil and their minerals through direct contact with the Earth's atmosphere, waters, or living things.

Weathering occurs in situ (in place, with no movement). It is a type of erosion. Erosion is where rocks and minerals are moved downhill (usually towards the sea) by water, ice and wind.

Context[change | change source]

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.

What does the weathering?[change | change source]

The two main types of weathering are physical and chemical. Sometimes there are also aspects of biology.

Physical weathering is important in very cold or very dry environments. Chemical reactions are most intense where the climate is wet and hot. However, both types of weathering occur together, and each tends to accelerate the other. For example, physical abrasion (rubbing together) decreases the size of particles and therefore increases their surface area, making them more susceptible to rapid chemical reactions.

The effect is to convert primary minerals, such as feldspars and micas, to secondary minerals (clays and carbonates) and release plant nutrient elements in soluble forms. The materials left after the rock breaks down combined with organic material creates soil.

Mechanical weathering[change | change source]

Mechanical or physical weathering means the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure.

Wind[change | change source]

Wind processes are called 'aeolian'. Wind erodes the Earth's surface by removing loose, fine-grained particles, called 'deflation'. Sand carried by wind wears down surfaces.

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[change | change source]

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, because it is usually slightly acidic. The water mixes with the minerals in the rock to break it down.

Temperature[change | change source]

Freeze-thaw weathering of a rock in southern Iceland

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[change | change source]

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.

Lava or magma[change | change source]

Lava or magma can cause weathering as when the molten rock touches rock (either intrusive rock extrusive) it causes the rock to change form to add to the quantity of molten rock causing the rock to have changed form. So the rocks has formed a different crystal structure to before it came in direct contact with the lava or magma.

Chemical weathering[change | change source]

Chemical weathering is the direct effect of atmospheric or biological chemicals in the breakdown of rocks, soils and minerals.[1]

Carbon dioxide cycle[change | change source]

The carbon dioxide cycle is the most important for weathering. CO2 is put into the atmosphere mostly by volcanoes, and it is taken out of the atmosphere by photosynthesis, and by one other process.

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".[2]

In fact, since volcanoes have not been very active in the Cainozoic, the supply of CO2 has been run down somewhat. It is thought this may have led to the Pleistocene ice ages.[2]

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.[3]

Other pages[change | change source]

References[change | change source]

  1. Gore, Pamela J. W. Weathering. Georgia Perimeter College
  2. 2.0 2.1 Van Andel T.H. 1994. New views on an old planet:a history of global change. Cambridge University Press. p233
  3. Drury S. 1999. Stepping stones: the making of our home world. Oxford University Press. p57