11.2 Desert landforms and dune types

Aeolian Landforms in Deserts

Desert landforms result from wind-driven erosion, transport, and deposition of sediment in arid environments. Because deserts lack widespread vegetation and moisture, wind becomes the dominant force shaping the landscape. This section covers the major erosional features, dune types, and the factors that control how dunes form and migrate.

Wind-Driven Processes and Formations

Aeolian processes act on desert surfaces through three main mechanisms: deflation (removal of loose particles by wind), abrasion (sandblasting of rock surfaces), and deposition (accumulation of wind-carried sediment). The balance between these processes determines what landforms develop in a given area.

• Sand seas (ergs) are vast expanses of sand dunes covering large portions of desert regions. The Sahara's Grand Erg Oriental, for example, spans over 100,000 $\text{km}^2$.
• Desert pavements are flat, stone-covered surfaces that form as wind selectively removes finer particles (silt and sand), leaving a lag deposit of coarser pebbles and cobbles behind.
• Deflation basins are large depressions created when wind scours away loose sediment, sometimes exposing bedrock or the water table. Egypt's Qattara Depression reaches 134 m below sea level and is the largest such feature in the Sahara.

Distinctive Aeolian Erosional Features

Wind erosion sculpts bedrock into a range of recognizable forms, depending on rock type, structure, and wind direction.

• Yardangs are streamlined, elongated ridges aligned parallel to the prevailing wind. They form through differential abrasion of softer rock layers, leaving harder ridges standing. They can range from meters to kilometers in length.
• Ventifacts are individual rocks that develop flat, polished facets and grooves on their windward side from sustained sandblasting. A ventifact with three facets is called a dreikanter.
• Mushroom rocks and pedestal rocks form when abrasion is concentrated near the ground (where sand transport is densest), undercutting the base of a rock column more than the top.
• Zeugen are tabular landforms where a resistant cap rock protects softer underlying material from erosion, creating a flat-topped ridge with eroded flanks.
• Wind-carved arches and natural bridges can develop where jointed or layered rock allows selective erosion to penetrate through a formation.

Dune Types and Formation

Dune morphology is controlled primarily by three factors: wind regime (direction and variability), sand supply, and vegetation cover. Different combinations of these factors produce distinct dune types.

Common Dune Morphologies

• Barchan dunes are among the simplest dune forms. They develop where sand is scarce and wind blows consistently from one direction. The crescent shape results from sand moving faster at the edges (less sand to transport) than at the center.
• Transverse dunes are essentially connected barchans that merge when sand supply is high enough. They form ridges that can stretch for kilometers.
• Linear dunes are common in the Australian and Namib deserts. Their formation under bidirectional winds is still debated, but the prevailing model involves helical wind vortices that concentrate sand along parallel axes.
• Star dunes grow vertically rather than migrating laterally, because winds shift direction frequently. They're the tallest dune type and are prominent in the Grand Erg Oriental and the Namib Sand Sea.
• Parabolic dunes are the morphological inverse of barchans. Vegetation anchors the arms while the central nose advances downwind, common in coastal and semi-arid settings.

Complex Dune Systems

Not all dunes fit neatly into one category. Larger and older dune fields often develop composite forms.

• Compound dunes result from smaller dunes of the same type superimposed on a larger dune (e.g., small barchans riding on a barchanoid ridge).
• Complex dunes involve the interaction of different dune types, such as star dunes forming on top of linear dunes when wind regimes change.
• Draas are very large compound dune forms, often exceeding 100 m in height and spaced kilometers apart. They represent long-term sand accumulation under relatively stable wind conditions.
• Mega-dune fields can cover thousands of $\text{km}^2$. The Rub' al Khali in the Arabian Peninsula is the world's largest continuous sand desert, spanning roughly 650,000 $\text{km}^2$.

Dune Morphology and Migration

Factors Influencing Dune Shape and Movement

The shape a dune takes is not random. It reflects the interplay of several variables:

• Wind regime is the single most important control. The number of dominant wind directions (one, two, or many) largely determines whether barchans, linear dunes, or star dunes form.
• Sand supply affects dune size and spacing. With abundant sand, dunes merge into continuous ridges; with limited sand, isolated dunes form with bare ground between them.
• Grain size matters because finer grains are easier to entrain and transport. Dunes composed of well-sorted medium sand (around 0.25–0.5 mm) are the most mobile.
• Vegetation stabilizes dunes by reducing surface wind speed and trapping sand. Partially vegetated surfaces produce nebkhas (coppice dunes), which are small mounds of sand accumulated around shrubs.
• Topography and bedrock influence where sand accumulates and how wind is channeled across the landscape.

Dune Migration Dynamics

Dunes migrate when sand is transported up the gentle windward slope (stoss side) and avalanches down the steep lee slope (also called the slip face). The slip face maintains an angle near the angle of repose for dry sand, typically around 30–34°.

Here's how the migration process works:

1. Wind carries sand grains up the stoss slope by saltation (bouncing) and creep.
2. Sand accumulates at the dune crest until the slope exceeds the angle of repose.
3. Gravity-driven avalanches (grainflows) cascade down the slip face.
4. Each avalanche deposits a thin layer, building the dune forward in the downwind direction.
5. Over time, these repeated avalanche layers create cross-bedding, the inclined internal stratification preserved in the rock record.

Migration rates vary widely. Small barchans in areas with strong, consistent winds can move 10–30 m/year, while large star dunes are nearly stationary. Seasonal wind reversals can cause dunes to oscillate or even reverse direction. Over longer timescales, climate shifts between wetter and drier periods control whether dune fields are active or become stabilized by vegetation.

Human activity (overgrazing, off-road vehicles, removal of vegetation) can reactivate stabilized dunes, turning them into mobile hazards for infrastructure and agriculture.

Dunes and Other Desert Landforms

Interactions with Surrounding Environment

Dunes don't exist in isolation. They interact with other desert landforms in ways that reshape the broader landscape.

• When migrating dunes encounter bedrock outcrops, they can form echo dunes (dunes that develop on the windward side of an obstacle, separated from it by an eddy zone) or climbing dunes/sand ramps that drape over the obstacle.
• Dune movement can block or redirect ephemeral streams, altering drainage patterns and creating temporary lakes or expanding playas.
• Advancing dune fields can bury alluvial fans, desert pavements, and other pre-existing surfaces.
• Nebkhas form where individual shrubs trap windblown sand, creating small vegetated mounds that dot otherwise bare surfaces.
• Dune fields act as barriers to fine sediment transport. Silt and clay that bypass the dunes can be carried far downwind and deposited as loess, which forms thick, fertile deposits (e.g., the Chinese Loess Plateau).

Landscape Evolution and Microclimates

Over geological time, dune systems are dynamic features that grow, migrate, stabilize, and reactivate in response to climate change.

• Migrating dunes progressively bury or expose other landforms, creating complex stratigraphic records.
• Large dune systems generate their own microclimates. The lee sides of tall dunes receive less wind and can retain more moisture, supporting localized vegetation.
• Interdune areas (the low ground between dunes) often have access to shallow groundwater and develop distinct ecosystems with specialized plants and animals.
• Sabkhas (salt flats) form in low-lying interdune depressions where groundwater reaches the surface and evaporates, concentrating dissolved minerals into crusts of salt, gypsum, or carbonate.
• Where dune fields meet mountain ranges, sand ramps build against the slopes, and topographic funneling of wind can intensify local erosion and deposition patterns.