Aeolian Landforms and Processes
Morphology, Occurrence, and Origin
In geomorphology, moisture and vegetation are primary variables, especially on sand islands. This leads to partly filled dunes, some merging into elongated features, often superimposed, where one dune climbs over another. These features are also observed on sand islands where dunes overlap.
Bachan Dunes
Bachan dunes are found on coasts. For example, on the West Coast of the North Island in New Zealand, the sand is dark due to its volcanic origin. Onshore winds cause the dune to migrate, with trailing wings anchored by vegetation. The stoss slope is where wind transports sediment, which then cascades over, causing the dune to migrate. Councils often manage beach access to protect vegetation and prevent parabolic dune formation that can encroach on residential areas.
Dune Protection and Management
Protecting dunes near the coast is crucial. Disturbance, such as racing on dunes, can expose sediment to wind, threatening nearby properties. Coastal management includes controlling beach access to preserve vegetation cover. In Oregon, disturbance from vehicles has activated large dune areas, leading to extreme measures like walls and bulldozers to protect buildings. Regulations and appreciation for robust coastal dune systems are essential.
Parabolic vs. Barkhan Dunes
Parabolic dunes have trailing horns, while barkhan dunes have horns pointing forward. Barkhan dunes form in arid environments with limited sand and a dominant wind direction, typically on the periphery of sand seas or in sand transport corridors. These dunes are symmetrical, and increased sand supply can cause them to merge. They consist of mostly sand with no vegetation.
Barkhan Dune Examples
Barkhan dunes vary in size. Some may be relatively small, around six meters high, while others can be much larger. The surface is generally hard and devoid of vegetation, with sand moving from a source to a sink area. Even very small dunes (e.g., 12 cm) can be found moving across frozen surfaces. In Antarctica, abundant sediment leads to coalesced dunes forming sinuous barkanoid ridges. These ridges move until obstructed by water sources like melt channels from glaciers.
Linear Dunes
Linear dunes are common in the Simpson Desert and can be activated by overgrazing and fire. These dunes are aligned parallel to the dominant wind direction, unlike transverse dunes that are perpendicular. Linear dunes cover about two-thirds of all sand seas and range from to kilometers wide and to kilometers long. They feature multiple asymmetric parallel ridges with wide interdune areas called swales, which in Western Queensland often contain clay pans.
Wind Regimes and Dune Formation
Transverse dunes form when wind blows perpendicular to the dune crest. Linear dunes, common in the Simpson Desert, form under wind regimes that oscillate between two directions. These dunes often bifurcate, forming tuning fork shapes, with active crests and vegetated swales. The Simpson Desert is relatively well-vegetated, except in areas affected by fires. The dune crests often exhibit a snake-like pattern due to wind action.
Star Dunes
Star dunes form in areas with abundant sediment and complex wind regimes. They feature a central peak with three or four radial arms and can be very large, such as those in Namibia that reach 300 meters high and three kilometers across. These dunes can occur singly or in clusters, often along the leeward margins of sand seas, where there are marked seasonal changes in wind direction.
Other Dune Forms and Aeolian Features
Climbing dunes accumulate against topography on the windward slope, while lee dunes form in the leeward of vegetation, which slows wind speed and filters sand. Lunettes are formed by clay balls—sand grains coated with clay—that accumulate on the shoreline of clay pans. When it rains, the clay breaks down, creating a clay deposit with sand. Wind deflation causes blowouts called Net gaps. This occurs when sand supply overwhelms vegetation, leading to its burial, or when vegetation is insufficient to trap sand.
Erosional Landscapes and Desert Pavements
Poor land management, such as overgrazing, can lead to erosional landscapes where vegetation is removed, exposing sand to wind and water erosion. Desert pavements, or gibber plains in Australia, are formed when wind selectively deflates fine particles, leaving behind a lag of coarse material. This armored surface protects against further deflation. Wetting and drying of clay can also heave larger particles to the surface, contributing to pavement formation. The surface is fragile.
Dust Storms
Dust storms result from the removal of fine particles, often due to human activities such as agricultural expansion. The Dust Bowl era in the US during the 1930s was caused by breaking up prairie grasslands for wheat production, followed by a drought. Dust storms can transport vast amounts of sediment over long distances, impacting air quality and even requiring lights to read newspapers at midday. Dust storms also affect marine ecosystems by transporting nutrients and pollutants.
Summary of Arid Landforms and Fragility
Arid landforms reflect weathering processes influenced by temperature, rainfall, wind, and fluvial action. Australian landscapes often experience long periods of dormancy punctuated by extreme events. Aeolian systems are fragile; activities like driving on dunes can reactivate them, causing damage, especially in populated coastal areas. The study of dunes helps understand processes on other planets like Mars and can provide records of past droughts and pollution.
Applications and Environmental Impact
Australian dust storms carry nutrients like iron into the Southern Ocean, fertilizing marine ecosystems. Experiments have been conducted to simulate this effect to stimulate phytoplankton blooms and sequester carbon dioxide (). However, large-scale implementation faced challenges. Aeolian systems offer insights into drought history, pollutant transport, and ecosystem fertilization, highlighting the interconnectedness of earth's systems.