Weathering occurs when the appearance or texture of an object (generally rock) is worn down by exposure to the atmosphere. This can occur due to either chemical decomposition or physical disintegration. While weathering usually occurs on the earth’s surface, it can also happen far beneath, where for example, groundwater percolates through fractures in the bedrock. It is important to note that for weathering rather than erosion to have occurred, the object being acted upon must remain stationary. While there are many causes of weathering, there are four that are by far the most common.
Frost weathering occurs in the presence of water, particularly in areas where the temperature is near the freezing point of water. Water freezes at 32 degrees Fahrenheit, or 0 degrees Celsius. This is particularly common in Alpine areas and around the edges of glaciers. When water freezes, it expands, so when liquid water seeps into a crevice in the rock or soil and freezes, its expansion can cause deeper cracks in the rock and eventually break pieces off.
Thermal stress occurs when heat absorbed from the surrounding air causes a rock to expand. This expansion, and the subsequent contraction when the rock eventually cools, can cause thin sheets of the rock's outer layer to peel off. While changes in temperature are the principal driver of thermal stress weathering, moisture can play a part here as well. This process is often found in desert areas, where temperatures vary greatly between day and night.
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Like frost weathering, salt weathering is caused by water. Water can get into the rock in a number of ways. Common ways are up from a groundwater supply, through the action of seawater waves along a rocky coast, or downward through traditional rainfall. Unlike frost weathering, in this case the water evaporates, leaving behind salt, which eventually forms into crystals. The growing crystals can exert a pressure on the rock that eventually breaks it.
When plants and animals weather rocks, the process is called biological weathering. Biological weathering occurs when plants break up rocks with roots, prying the rock apart. When burrowing animals, such as badgers, moles and rabbits, burrow into rocks in search of shelter or food, this is considered biological weathering as well.