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WIND & DESERT LANDSCAPES WHAT IS A DESERT? Deserts form when yearly precipitation is less than evaporation Tropical deserts have: sparse vegetation (lack of rainfall) Strong winds (convection currents from sun heating) Flash floods FACTORS OF DESERT FORMATION Latitude: Key factor Equator is an area of low pressure, heat rises from ground level and moves to the upper atmosphere, then away from the equator This occurs until about 30 degrees North and South of the equator Deserts are common in this region FACTORS OF DESERT FORMATION Mountain barriers: As air is forced to rise over a mountain barrier, the rising air cools and condenses, causing heavy rainfall on the windward side of the mountain On the leeward side, the drier air warms as it sinks The higher the mountain range, the drier the descending air – it can be so dry that it pulls moisture from the ground FACTORS OF DESERT FORMATION Cold ocean currents: As warm air crosses cold water, the air cools rapidly causing water vapour to condense and rain clouds/fog form offshore Air drops moisture over the water As the air crosses onto warm land, air temperatures rise – warm air can hold more moisture Because of this, there is no rain FACTORS OF DESERT FORMATION Ocean influence: Oceans are the main source of atmospheric moisture, so areas far way from ocean influence are very dry These areas can be classified as deserts Usually, the longer an air mass is over land, the more moisture it will lose WIND EROSION IN DESERTS Trees and vegetation protect the soil, acting as a windbreak, holding the soil in place with their roots, and slowing heavy rain with leaves Because of this, areas with sparse vegetation experience strong winds that pick up surface materials (silt, clay, sand) and carry them in suspension WIND EROSION IN DESERTS Sand can only be carried in suspension in stronger winds, and remains closer to the ground than silt and clay The movement of sand is largely by saltation Strong winds in deserts can generate dust clouds that are carried long distances by air currents WIND EROSION IN DESERTS Silt and clay are too soft to have significant abrasive power Sand easily grinds/scours rock surfaces Sand abrasion can cause mushroomshaped rocks called toadstools WIND EROSION IN DESERTS Deflation is when wind blows away sand, silt and clay in deserts so that underlying rock is exposed These stony landscapes are called Badlands Deflation can lead to deep hollows called blowouts WIND EROSION IN DESERTS Blowouts may be deep enough to reach the water table and contain water for at least part of the year Many oases form this way A desert oasis fed by a spring line from an underground aquifer will never run dry unless pressure in the aquifer drops enough to stop the water flow WATER EROSION IN DESERTS Rain is rare but often comes as a sudden downpour in mountains surrounding deserts With no vegetation to anchor soil, rainwater rushes down valleys carrying sediment toward the valley floor When water reaches the flat land, the flow slows and debris are deposited Water is no longer confined to a valley and spreads to create an alluvial fan of deposits DESERT SAND DUNES As wind blows across the desert, deposits of sand can collect on an obstruction (rock, plant, etc.) and over a time a sand dune forms As the dune grows, it acts as a windbreak, slowing the velocity of wind and allowing for more deposition DESERT SAND DUNES Sand dunes have a long, gentle slope on the windward side and a steep slope on the leeward side Sand particles move by saltation up the windward slope, and roll/slide down the leeward slope (slip face) DESERT SAND DUNES In areas where wind predominantly blows in the same direction, dune migration can take place This can be a problem in areas were roads, buildings, and parking lots are located Vegetation is often planted to slow wind FOUR TYPES OF DUNES The type of dune is determined by the strength and direction of the wind, vegetation, and local landforms If wind blows consistently from one direction, dunes will have a long, gentle windward slope and a steep leeward slop FOUR TYPES OF DUNES 1. transverse dune: These dunes form in regions where abundant sand is available for dune formation Continuous sand ridge formed at right angles to the wind FOUR TYPES OF DUNES 2. Barchan dune: Crescent-shaped dune Forms over obstruction Horns point downwind and migrate slowly in the direction of the wind Can be incorporated into transverse dunes FOUR TYPES OF DUNES 3. Parabolic dune: Crescent shaped, but forms around a blowout Shaped like a barchan dune, but horns point upwind FOUR TYPES OF DUNES 4. Longitudinal dune: Found in deserts where only a thin layer of sand is present and winds blow continuously in the same direction Long and straight, form parallel to the wind DESERT LANDFORMS Alluvial fans may become so large they join together to form bajadas Wide, dry ditches that channel water out to the desert floor and contain water only during flash floods are called wadis DESERT LANDFORMS Wadis extend to a depression in the desert basin, where water accumulates after floods The result is the formation of playa lakes The extreme heat of the desert evaporates the water quickly and only salt deposits remain DESERT LANDFORMS Streams and rivers erode through plateaus of sedimentary rock to form deep, steep sided canyons Because of the lack of rainfall, almost no lateral erosion occurs DESERT LANDFORMS When a large area of plateau is separated as a result of erosion from the main plateau, a flat topped feature called a mesa is formed Erosion can break apart mesas into smaller flat topped features called buttes DESERT LANDFORMS Wind can remove all sand from a basin leaving behind only the underlying rock layer over time These rock surfaces are called hamadas Sometimes eroded sediment of upland areas is carried off by wind and deposited in lower areas. A desert basin filling with debris in this way is called a bolson DESERT LANDFORMS When sand is abundant, sand seas are common These sand seas are called ergs They migrate in the direction of the wind Difficult to stabilize DESERT LANDFORMS All deserts, regardless of amount of sand, have some amount of windblown dust Windblown dust is called loess, and can be found in thick deposits on the leeward side of deserts where prevailing winds subside and the dust settles Loess deposits are very fertile