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Crystallization Weathering
When chemicals within rocks solidify and form crystals, they often change in volume, placing stress on the surrounding rock structure, which can lead to particles of rock breaking away over time. The most common example of this type of weathering is called freeze-thaw weathering. Freeze-thaw weathering occurs mainly in porous rocks. Water seeps into cracks and holes in the rock, and if it becomes cold enough to freeze, the water expands. This action forces the crack or hole to become bigger. Repeated freezing and thawing can eventually result in the rock breaking apart. This type of weathering is common in temperate and polar regions, where the temperature frequently drops below freezing.
Salt weathering may also occur, but is more likely in hot arid regions. Salt crystals form when water evaporates. The volume change when salt crystals form is smaller than for water, but still large enough to exert significant pressure on surrounding rocks.
Insolation Weathering
Many rocks are made of crystals consisting of different types of mineral. Rocks do not conduct heat well, and each mineral expands and contracts by a different amount in response to temperature change. Adjacent minerals repeatedly expanding by different amounts places strain on the rock structure, and eventually rupture may occur. This type of weathering is most common in areas subject to large daily fluctuations in temperature.
Slaking
Slaking occurs as a result of the alternate wetting and drying of a rock. This pattern can lead to the gradual accumulation of a layer of water molecules between rock grains. The water layer becomes gradually thicker with each phase of wetting, and eventually the rock grains break apart. This process is accelerated in the presence of dissolved sodium sulfate.
Unloading
Unloading is most common in igneous rocks formed beneath the Earth's surface in conditions of high pressure. As these rocks are pushed towards the surface by tectonic activity and erosion, the pressure gradually decreases. This unloading of pressure results in the appearance of horizontal fractures in the rock.
Abrasion
Abrasion occurs when rocks are worn away at their surface by repeated physical contact with another hard surface. Common causes of abrasion include the effect of moving water, which causes small rocks and stones to repeatedly grind at the surfaces of larger rocks. The wind can cause abrasion by blowing particles of sand across the surface of exposed rocks. Abrasion also occurs due to the movement of glaciers when they convey rock particles from place to place, wearing away underlying rock surfaces.