Processes and landforms in arid environments

A barchan is a crescent- (or horseshoe-) shaped dune when seen in plan.

In cross-section it is asymmetrical: it has a gentle, convex slope on the windward (upwind) side and a much steeper slope, the slip-face, on the leeward (downwind) side.

The two ends of the crescent are drawn out into pointed horns, and these horns point downwind, away from the wind.

Deflation is the removal of loose, fine material from the desert surface by the wind. The wind lifts and carries away sand, silt and dust, lowering the surface and sometimes leaving a coarse lag of pebbles (a desert pavement) or scooping out a deflation hollow.

Abrasion (corrasion) is the wearing away of solid rock by wind-blown sand. The wind hurls sand grains against exposed rock, sand-blasting, smoothing and pitting it.

The key difference is what each acts on: deflation removes loose material that is already there, whereas abrasion cuts into solid rock. A further difference is that abrasion is concentrated within about a metre of the ground, because the wind cannot carry heavy sand grains high, so it undercuts rocks at their base.

A yardang is a streamlined ridge of rock aligned roughly parallel to the dominant wind, and it forms by wind erosion acting on rock of varying resistance.

The starting point is an area of softer, less resistant rock or sediment that the wind can attack. A strong, consistent prevailing wind carries sand close to the ground, and this sand sand-blasts the rock by abrasion (corrasion). Because abrasion is strongest within about a metre of the surface, the lower parts of the rock are cut away fastest.

The wind exploits lines of weakness, so it picks out and erodes the softer rock along grooves parallel to the wind direction, while deflation removes the loosened debris. Over time this leaves more resistant rock standing up as streamlined ridges separated by wind-cut troughs.

The ridge becomes streamlined to the wind, with a steeper, blunt windward end where erosion is concentrated and a lower, tapering tail on the leeward side, so that the whole landform is aligned with the dominant wind (as in the Lut Desert of Iran).

An alluvial fan is a cone- or fan-shaped mass of sediment deposited by water where an ephemeral channel leaves the highlands, and it forms because the flow suddenly loses energy.

After an intense desert storm, a flash flood rushes down a steep mountain channel carrying a large, poorly sorted load of sediment, including coarse debris, because the flow has high energy and there is little vegetation to hold sediment back.

Where the channel reaches the break of slope at the foot of the mountain front, the water is no longer confined and spreads out across the plain. As it spreads and the gradient falls, the water loses velocity and therefore energy, so it can no longer carry its load and deposits it.

The coarsest, heaviest material is dropped first, near the apex at the mountain front, while finer material is carried further and spread towards the toe. Repeated floods build up the cone-shaped fan, which slopes away from the apex. Where neighbouring fans grow and merge along a mountain front, they coalesce into a continuous apron called a bajada.

Arid landscapes are shaped by three agents — weathering, wind (aeolian) and water (fluvial) — working together. Because deserts are dry and largely unvegetated, it is tempting to assume that wind is the dominant agent, and the spectacular sand seas of the Sahara and Namib seem to confirm this. This essay assesses the relative importance of wind and water and argues that, while wind dominates the desert surface and builds the great depositional sand landforms, it is water — through rare but violent flash and sheet floods — that does most of the large-scale erosional shaping; so the statement is only partly true.

The case for wind is strong. Aeolian processes operate constantly across vast areas wherever the surface is dry, loose and unprotected. Wind erodes by abrasion (corrasion), sand-blasting rock close to the ground to cut ventifacts, rock pedestals undercut at their base, yardangs streamlined parallel to the wind, and zeugen where soft rock is undercut beneath a resistant caprock; deflation lifts and removes fine material, lowering surfaces and scooping out deflation hollows such as the Qattara Depression in Egypt. Above all, wind deposition builds the enormous sand seas (ergs): barchan dunes under a single constant wind, seif (linear) dunes under two prevailing winds, and star dunes under variable winds, as in the Grand Erg Oriental of the Sahara. Because dune fields can cover hundreds of thousands of square kilometres, wind is clearly the dominant agent of surface and depositional landforms.

However, water is arguably the more important agent of large-scale erosion, despite the low rainfall. Desert rain is episodic, but it is often intense and falls on bare, baked surfaces with a low infiltration capacity, so it runs off almost at once with great energy. Flash floods surge through channels, deepening and widening wadis (arroyos) and transporting coarse debris, while sheet floods plane and smooth gently sloping surfaces to form pediments. Over long periods this episodic water, with weathering and slope undercutting, drives the parallel retreat of caprock-protected slopes, reducing plateaux through the mesa → butte → inselberg sequence seen in Monument Valley in the USA. Water also builds major depositional landforms — alluvial fans where channels leave the highlands, coalescing into bajadas, and playas and salt lakes where floodwater evaporates in enclosed basins. Much of the largest and most dramatic desert relief is therefore water-made, not wind-made.

The most accurate assessment is that the two agents operate at different scales and times, and that aridity itself gives water its disproportionate power. Wind does steady, widespread work, reshaping the surface and moving sand more or less continuously, so it dominates the appearance of sandy deserts. Water acts rarely but with enormous energy per event, so a single storm can do more erosional work than years of wind, cutting the large-scale relief. Crucially, the two interact: wind reworks the fine sediment that floods leave behind, and the bare, sealed surfaces that make wind effective are the same surfaces that make floods so erosive — so it is the desert climate that empowers both. Importance therefore depends on which landform and which desert: in a sandy erg, wind dominates; in a rocky, dissected desert such as the Colorado Plateau, water dominates.

In conclusion, I only partly agree that wind is more important than water. Wind is the more important agent of surface modification and of depositional sand landforms, and dominates the great sand seas. But water, through episodic flash and sheet floods, does most of the large-scale erosional shaping and builds the major fans, bajadas and playas. The two are not really in competition: they operate together, at different scales and times, under the control of the arid climate. Overall, water is the more important shaper of large-scale relief, while wind dominates the surface — so the statement, taken as a blanket claim, oversimplifies a landscape shaped by both.

Weathering is the in-situ breakdown of rock at or near the surface, and in hot deserts the dominant type is undoubtedly physical (mechanical) weathering, driven by the large diurnal temperature range and strong evaporation. This essay assesses the claim that physical weathering is the ONLY significant process. It argues that, while physical weathering does dominate, chemical weathering is limited rather than absent and is locally significant, and that the two often work together — so the word 'only' is an overstatement.

The case for physical weathering as dominant is strong. Clear, cloudless desert skies and dry air give intense daytime insolation and rapid night-time cooling, so rock surfaces experience a very large diurnal temperature range. This drives thermal fracture (insolation weathering), in which different minerals expand and contract unequally each day, cracking the rock, and exfoliation, in which the heated outer skin expands and peels away as curved shells, rounding boulders. Where deserts are cold at night or lie at altitude, freeze–thaw shatters rock as water in cracks freezes and expands by about 9%. Salt crystal growth is especially effective: salty water enters pores and cracks, and as it evaporates the salt crystallises and prises the grains apart. Because all of these depend on the desert climate rather than on abundant water, they operate widely and account for most of the rock breakdown, which strongly supports the view.

However, the claim that physical weathering is the ONLY significant process is too absolute. Chemical weathering needs water, and deserts are dry, so hydrolysis (water attacking feldspar to form clay) and carbonation (weak carbonic acid dissolving carbonate rock) are slow and limited — but they are not absent. Dew, fog (for example the regular coastal fog of the Namib), occasional rain and salty groundwater all supply enough moisture for some chemical reaction, particularly at and just below the surface. Chemical weathering can therefore be locally significant, for instance in producing surface crusts and weakening rock around salts.

The most accurate assessment is that the processes interact rather than acting alone, which itself undermines the word 'only'. Slight chemical attack and salt crystallisation weaken grain boundaries, so that thermal stress and exfoliation then work faster; equally, physical breakdown exposes fresh surfaces and increases the surface area available for the limited chemical weathering to act on. In other words, even where physical weathering dominates, chemical weathering contributes by preparing the rock — so the two are not cleanly separable.

In conclusion, physical weathering is clearly the dominant weathering process in hot deserts, because the large diurnal range, freeze–thaw at altitude and salt crystal growth all thrive in the arid climate. But the view that it is the ONLY significant process is an overstatement: chemical weathering is limited by the scarcity of water rather than absent, is locally significant where dew, fog and groundwater supply moisture, and works together with physical processes to break down the rock. The most defensible judgement is therefore that physical weathering dominates while chemical weathering plays a real, if subordinate, role.