Lecture Outlines Physical Geology, 15/e Plummer, Carlson & Hammersley
Deserts & Wind Action Physical Geology 15/e, Chapter 13
Deserts Desert any arid region that receives less than 25 cm of precipitation per year running water is the predominant force shaping most desert landscapes rare and often violent flash flood events produce most desert erosion
Where and How Deserts Form Deserts can be found anywhere that the atmosphere (air) is usually dry Most deserts are associated with areas where air is descending Rain shadow deserts form downwind of where moist air rises over high mountain ranges
Some Characteristics of Deserts Intermittent stream flow Streambeds are dry most of the year Lack through-flowing streams Exceptions include the Colorado and Nile Rivers Internal drainage streams flow to land locked basins Flash Floods common in arid regions due to short-lived high volume rain storms Desert washes or arroyos are commonly steep-sided, with flat floors covered by loose sediments - a result of rare but highly erosive flash flood events
Desert Features in the Southwestern United States Two distinct landscapes: Colorado Plateau Basin and Range Province Colorado Plateau centered on the four corners region of Utah, Colorado, Arizona and New Mexico. 1500 meters above sea-level Flat-lying sedimentary rocks that are heavily eroded into plateaus, mesas and buttes
Desert Landforms of the Southwestern United States Basin and Range province has rugged, linear, fault-bounded mountain ranges separated by flat-floored valleys narrow canyons carry sediment down to valley floors during heavy rains sediment gets deposited into alluvial fans alluvial fans may overlap to form a bajada finest sediments travel to basin center where water ponds and evaporates in playas
Wind Action Wind Erosion and Transportation large daily temperature and pressure differences lead to strong wind dust storms may occur if fine-grained sediments are readily available dust can be transported 1000s of km by atmospheric winds Dust Bowl continuing dust storms in the prairie states during the droughts of the 1930 s Saharan Desert sediments have carried across the Atlantic Oceana Volcanic Ash
Wind Action Wind Erosion and Transportation can keep dust in suspension, but larger sand grains move by saltation Sand grains moving in high-speed winds can effectively sand-blast rocks into ventifacts Deflation of fine sediments blowouts desert pavement
Wind Action Wind Deposition Loess - deposit of wind-blown silt and clay composed of unweathered grains of quartz, feldspar and other minerals sediment sources include glacial outwash plains and desert playas Thick loess deposits exist in China and the United States loess typically forms soils that are very fertile, yet easily eroded
Wind Action Wind Deposition Sand dunes mounds of loose sand piled up by the wind most likely to develop in areas with large sand supply and winds that generally blow in the same direction small patches of dunes are common in southwestern U.S., but huge sand seas exist in the Sahara and Arabian deserts dunes may also form just inland of beaches along the coasts of seas and large lakes
Wind Action Wind Deposition Types of Dunes Shapes depend upon: Wind velocity and direction(s) Amount of available sand Distribution of vegetation cover
Wind Action Wind Deposition Types of Dunes Barchan dunes crescent-shaped, with horns that point downwind and a steep slip face on the concave side form in areas with one dominant wind direction and a limited sand supply exist on Mars
Wind Action Wind Deposition Types of Dunes Transverse dunes relatively straight, elongate dunes that form in areas with large sand supply and one dominant wind direction Parabolic dunes deeply curved dunes convex in the downwind direction, forming around blowouts, and have horns anchored by vegetation
Wind Action Wind Deposition Types of Dunes Longitudinal dunes form in areas with large sand supply, parallel to the prevailing wind direction extremely long, high, straight and regularly spaced crosswinds may play a part in their development area between parallel dunes is swept clean of sand by winds formation mechanism still not fully understood
End of Chapter 13