3.1.2.2 - Weathering and mass movement in drylands

Thermal stress and physical disintegration

Rock surfaces heat and cool as energy inputs change. Uneven expansion and contraction between a hot surface and cooler interior, or between minerals with different responses, can stress rock. Repeated stress may extend existing cracks; moisture, salt and rock structure can act alongside thermal changes. A large daily temperature range alone does not establish that thermal fracture caused every broken block.

Block disintegration produces blocks along joints or larger fractures. Granular disintegration separates individual mineral grains, often where minerals expand differently or their bonds have weakened. They describe different scales of breakdown; neither is an exclusive signature of one mechanism.

Exfoliation produces curved sheets or shells peeling from rock surfaces. Near-surface thermal stresses can contribute, while unloading as overlying material is removed can create sheet joints. Weathering can widen those joints. Explain the relevant mechanism and evidence rather than treating every exfoliated dome as proof of daily heating alone. Weathering occurs in place: movement of a detached block down a slope is a separate process.

Chemical weathering still requires water

Chemical weathering changes mineral composition. Water enables reactions such as hydrolysis in silicate minerals; oxygen and moisture allow oxidation, while weakly acidic water can dissolve carbonate rock. Limited moisture often slows these reactions in desert interiors, but intermittent rain, dew, fog, groundwater and water retained in fractures provide local opportunities. Warm temperatures can speed reactions when water is present.

Chemical weakening can make grains or blocks easier to detach physically. Salt crystallisation, by contrast, is physical disruption: dissolved salts enter pores with water, evaporation concentrates the solution, and crystal growth can exert stress. It is useful in explaining some desert breakdown, but it should not replace the specified thermal and chemical mechanisms.