By Kevin Beck | Updated Aug 30, 2022
Abrasion is the physical wearing down of rocks by friction or impact. When a rock falls or rolls downhill, it can fracture upon impact and simultaneously abrade surrounding surfaces. Wind‑blown sand grains or pebbles, themselves fragments of larger rocks, slowly grind away at exposed rock faces over time. Frost action is a common form of abrasion: when water infiltrates fissures, freezes, and expands by roughly 9 %, it exerts pressure that exceeds the rock’s tensile strength, leading to cracking and eventual breakdown.
Deep‑buried rocks normally endure immense confining pressure from overlying strata. When erosion removes surface material, the reduced pressure can cause differential stresses within the rock. These stresses often act parallel to bedding planes, producing shearing fractures that may propagate upward, sometimes forming vertical columns or ledges visible at the surface.
Rocks experience cyclical expansion and contraction as temperatures rise and fall. Unlike liquids, solid rocks do not change phase, but the repeated stress can generate micro‑fractures, especially in composite rocks like granite where mineral grains respond differently to heat. In environments with extreme diurnal temperature swings—such as fire‑prone regions—these cycles accelerate rock disintegration.
When dissolved minerals precipitate as salts within rock pores and fractures, they grow and exert directed pressure. Common salts like sodium chloride build up perpendicular to cavity walls, eventually causing fissures to widen and the rock to crumble. This process, often called solution weathering, is a key driver of mechanical breakdown in arid and semi‑arid landscapes.
Once rocks are fragmented into pebbles and smaller particles, they become susceptible to erosion. Wind, water, or ice transports these fragments, shaping the landscape over geological time scales.
