Weathering

Weathering of rocks means that the rocks are broken down into small fragments which are then capable of being taken away by the agents of erosion (e.g. moving ice, rivers, the wind, the sea).

There are two basic types of weathering:

• Mechanical (physical) weathering
• Chemical weathering.

Mechanical weathering

The two most important types of mechanical weathering are

• Freeze-thaw (also called frost action); and,
• Onion weathering (also called exfoliation)

Freeze-thaw action occurs in mountain ranges.

During the day water seeps into cracks in rocks and at night the water freezes. When water freezes it expands by about 9% of its volume. This constant expansion and contraction makes small fragments of rock break off. These little pieces accumulate at the foot of the mountain and are known as scree.

Onion weathering occurs in the hot desert regions of the world. These areas experience a large diurnal temperature range i.e. it can be very hot during the day but quite cold at night.

Rocks expand during the heat of the day and contract with the coldness of the night. This constant expansion and contraction causes the outer layer of the rock to peel or break away. This type of weathering works best if there is a small amount of moisture present in the atmosphere.

Chemical Weathering

There are many forms of chemical weathering including oxidation (rusting )which affects iron compounds in rocks. However, by far the most significant type of chemical weathering is carbonation.

Carbonation is a particular type of chemical weathering that occurs in limestone regions. It happens when rain falling through the air takes on carbon dioxide (CO2). This turns the rainwater into a weak form of carbonic acid. This rainwater then turns the calcium carbonate in limestone into calcium  biconbonate which is soluble. In other words, the rainwater dissolves the limestone.