The main types of geological weathering are mechanical and chemical. Sometimes, “biological” is included as a third category. Mechanical weathering can be divided into two types: fracturing and abrasion. Since plants and trees can push rocks apart, biological weathering overlaps with mechanical weathering. Mechanical weathering also exposes more rock surface, therefore increasing chemical weathering.
Frost Wedging (Freeze-Thaw)
Water expands by 9 percent when it freezes. As it expands, it exerts up to 4.3 million pounds per square foot of pressure. Since water seeks low points, repeated freezing and thawing can lead to deep vertical fissures as cracks are extended downward. Cracks that allow entry of roots lead to similar wedging.
Crystal formation cracks rock in a similar way. Most water contains dissolved salts. When water in rock fissures evaporate, the salt concentration increases, which leads to crystallization. This occurs in arid regions where the humidity fluctuates widely, so that the salt can be redissolved and carried into new fissures.
Unloading and Exfoliation
Glacier weight compresses the rock beneath it. When the glacier finally melts, for example, at the start of an interglacial period, the rock expands from the reduction of pressure. This causes fracturing between the layers parallel to the Earth’s surface. The top layer breaks apart in sheets, or exfoliates, having no load above it at all. As the rock below is exposed, it too exfoliates.
Thermal Expansion and Contraction
Heating causes rock to expand. Cooling causes it to contract. The resulting cracking looks similar to frost wedging. Such wear is found where there are extremes in daily temperature. The moon has almost no atmosphere and no tectonic activity to weather the rock, and the temperature variation between day and night is 280 degrees C. Thermal expansion and contraction may therefore be the only form of weathering that occurs.
Abrasion can be intense in both very dry and very wet regions. Dry regions have much abrasion because wind-driven particles tend to be dry and hard–effectively, it is a natural form of sandblasting. Obversely, the waves at the beach slowly grind river deposits against rock to produce sand.
This form of weathering includes landslides and glaciers. Glaciers carry boulders along on their underside. These boulders grind away the earthen surface underneath. The resulting weathering is similar to that of a river.