PART III : AEOLIAN Or WIND GEOMORPHOLOGY & LANDFORMS
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1 GEOMORPHOLOGY 201 READER PART III : AEOLIAN or WIND GEOMORPHOLOGY & LANDFORMS While water is the most important landscape-forming agent in temperate and humid climates, ice fulfils this role in colder climates. Coastal geomorphic processes (wave and tidal action) likewise are the major processes where fluvial systems and oceans meet. The landscaping effects of groundwater action are limited to karst areas, which is about 10% of the earth,s surface where limestone and dolomite formations occur. The remaining landscape creating work is done by wind in all areas where water is in short supply. As South Africa is considered a water scarce country, aeolian, or wind-driven processes are therefore a key player in determining the form and function of our landscapes. Unique characteristics of arid regions include that chemical weathering occurs at a much slower rate than where water is abundant. Physical weathering is therefore the primary weathering agent and results in rocks and talus being deposited on the land surface. These materials are largely chemically unchanged and are further characterised by a low content of organic material and water, which in turn is responsible for very slow rates of soil formation. Dry regions are also characterised by sparse vegetation, which would normally protect and anchor soil and talus; the conditions in arid regions are therefore especially favourable for wind action to blow away dry talus and to be a major force in landscape genesis. 2 The distribution of Aeolian geomorphic action on earth While wind occurs everywhere on the earth’s surface, it is only where surface water is scarce that wind action becomes a major landform- creating agent. The main regulators determining if a landscape will be prone to mainly water or wind erosion are therefore climate and more specifically temperature and rainfall. Hot deserts and semi-deserts (BSh & BWh regions according to the Koppen classification system) are the regions most prone to wind as landform creating agent. While wind is the more prevalent force in shaping desert landforms, water is the more powerful. Despite water being so scarce in these environments, it is the primary agent of erosion in the desert, with many features owing their formation to mass wasting and running water as sheet floods. Even the driest deserts receive occasional rainfall, which then usually occurs as downpours which cause landform creation processes over a large scale in a short time period. Mass action and fluvial action are therefore the main landscape-forming processes in deserts with wind being only responsible for secondary, localised and other minor landforms. The wind action, however, occurs over a persistent and ongoing basis and has an enduring presence and signature in arid landscapes. Due to climate changes over time, the landscape bears evidence of former landforms that were created when different climatic conditions controlled the prevailing landforms, such as the sandstone deposits in the Eastern Free State and Lesotho: these date from times when wind action and deposition by water were the chief landforming agents. In Africa there are many hot, arid regions, such as the Namib, Sahel and Sahara deserts; and also many semi-arid regions including Namaqualand, the Kalahari and parts of Ethiopia and Somalia. These regions are all have many features in their landscape that were created by the wind-cascade system. Other parts of Africa have long, dry seasons and are subject to prolonged droughts, even though they are not classified as semi-arid regions. In these regions, as well as those where dryland cultivation leaves large areas uncovered by vegetation or crops for part of the year, large volumes of surface material is lost annually and wind erosion plays a major role in forming the landscape. Furthermore, landforms created by wind can also occur in dry river valleys, along coasts and along river floodplains where large quantities of talus are present. Wind is one of the three dominant agents in hot deserts, after mass wasting and occasional water action. The desert floors get heated up rapidly as they are dry and consist of mainly mineral silicas, while being virtually devoid of organic matter and vegetation. These heated floors heat the air directly above them, resulting in upward movements in the hot lighter air with turbulence, and any obstructions in its path sets up eddies, whirlwinds, updrafts and downdrafts. Winds also move along the desert floors with great speed and the obstructions in their path create turbulence, while very destructive storm winds also occur. Some fluvial processes and landforms found in deserts. Deserts are defined by their lack of water and while coastal deserts may experience one or two rainfalls a year; those further inland may get rain once or twice a decade only. Although rain is scarce in deserts (the Sahara’s average annual rainfall is only 127mm), it usually comes down torrentially and lasts mere minutes or a few hours. This enhances rainsplash erosion on the bare, unvegetated desert soil, loosening and spattering soil particles in all directions. The desert rock material also undergoes accelerated mechanical and chemical weathering processes due to drastic diurnal temperature changes, decaying fast, with the torrential rains helping to remove the weathered materials speedily. The dislodged soil and talus is carried off with the flowing water, which picks up and transports more sediment as it rushes along. Owing to a lack of organic-rich soil that can absorb the water, and also the absence of vegetation, the desert landscape offers little resistance to water action and erodes rapidly. Water therefore accomplishes within a few days what would take desert winds a year to accomplish, which 3 means that the weathered debris in deserts is moved by not only wind, but also significantly by rain/sheet wash. Finding natural depressions, such as gullies and canyons, the sediment-laden water gains speed and power as it is confined and flows downward. Increased velocity allows the water to pick up more and larger sediments and rock debris, eroding them and the surface below as it rushes along. Often clogged with so much debris, the water can resemble a mud-flow (a thick mixture of water, mud, and other surface fragments). Arroyos, the dry streambeds created by previous rains, again fill with water. When an arroyo finally opens onto a flat, broad plain, the rushing water flows out and drops its load of sediments, forming a new alluvial fan. In other areas, basins or depressions in the desert floor fill with water, forming playa lakes that soon evaporate, leaving a dry, cracked, salty lake bed that will remain until the next rain. Alluvial fan Precipitation that falls in higher elevations in deserts flows rapidly down to flat areas through canyons, valleys, and other narrow, confined channels. Because most desert soil lacks plants and their root systems to help hold the soil together, the flowing water easily picks up any loose material in its path. The faster the water flows, the larger the pieces of material it is able to pick up and carry along. When the rushing water finally reaches a plain or flat area, it loses power since gravity is no longer helping it flow down a steep slope. As it slows, the water is unable to carry the sediment—gravel, clay, sand, and silt—it picked up on its way downhill. Large rocks and other heavy material are deposited first at the base of the canyon, followed by other material in decreasing size. No longer confined to a narrow channel, the water spreads out the farther it moves away from the base of the canyon. The finest material it carried is deposited at the outer edge. When the water evaporates, the sediments remain behind. Over time, as more water flows onto the plain, more sediment is deposited, and a wide, fan-shaped pile known as an alluvial fan forms. When two or more alluvial fans merge on a plain to create a broad, sloping surface, they form a bajada. Arroyo An arroyo is a desert landform sculpted by the action of water. Sudden heavy downpours cut channels in the desert floor, often in canyons or other low-lying areas. These fast-moving but short-lived streams create deeper channels or gullies with steep sides and an almost flat bed or bottom. Just as quickly as the water appears, it disappears in the normally dry desert environment. What remains is an arroyo, a dry watercourse with a floor that is often gravel-strewn. Stream channels in desert areas are broad, smooth and indefinite and flow for a brief time after rains. Bajada. When two or more alluvial fans merge on a plain to create a broad, sloping surface, they form a bajada. THE WIND CASCADE SYSTEM The work performed by aeolian systems is concentrated in desert regions and therefore desert is will be used as model to describe this wind cascade system. On the basis of its morphological characteristics, the desert landscape can be subdivided into the deflation-corrasion and the aeolian-accumulation subsystems. The processes of deflation and corrasion dominate in the deflation-corrasion subsystem, while the deposition of wind-transported talus is the major process in the aeolian-accumulation subsystem. Deflation is the process of removing talus on the earth surface by wind, while corrasion is the physical weathering of rock by talus-carrying wind. Corrasion processes include abrasion, chipping and polishing. The deflation-corrasion subsystem consists of the Hamada and the serir. The hamada has 4 exposed rock surfaces and products of mechanical weathering , namely angular rock fragments scattered about. The serir is a stony desert with smaller rock fragments impacted mainly by corrosion processes. The size of the rock fragments decreases in the direction of the aeolian-accumulation subsystem, from boulders down to sand grains.