Freezing water cracking rocks turns out to be the lesser culprit
Textbooks long taught that rocks split because water expands by about 9 per cent as it freezes in cracks. Newer experiments point elsewhere. In ice segregation, a thin liquid-like film on ice grains sucks in water from warmer rock, growing ice lenses that push up to ten times harder.
Weathering is the breakdown of rock, soil, minerals and even wood or concrete through contact with water, air, sunlight and living things. Unlike erosion, it happens in place, without the material being carried off. Its leftovers mix with organic matter to become soil, and sedimentary rock made from weathered debris covers about 66 per cent of the continents and much of the sea floor.
Chemists and physicists split it into two families. Chemical weathering involves reactions with water, oxygen, carbon dioxide and chemicals released by organisms. Physical weathering shatters rock without altering its chemistry and is usually the less important of the two, except in alpine and subarctic settings. The two cooperate: every crack opened mechanically exposes fresh surface for chemistry to attack. After frost, the next strongest physical agent is plant roots prying joints apart, helped by burrowing animals and lichens.
The numbers behind frost wedging look convincing. Freezing water could in theory exert over 200 megapascals, and a realistic 14 still dwarfs granite's tensile strength of about 4. But ice simply squeezes out of straight open cracks, and unsaturated rock gives it room to expand harmlessly, so wedging needs narrow twisting fractures in soaked rock with daily freeze-thaw cycles. Ice segregation works best in rock hovering between minus 4 and minus 15 degrees Celsius, where ice needles and lenses gradually lever the rock apart.
Heat does its own damage. Thermal shock cracks rock instantly but is rare; thermal fatigue, repeated expansion and contraction, is more typical and can split rock into rectangular blocks. It matters in deserts with scorching days and cold nights, yet calling it insolation weathering is misleading, because any large temperature swing works, including in cold climates and during wildfires. Geologists long dismissed it after flawed early 20th-century experiments suggested it was trivial.
Source: Weathering
