Deserts & Wind Arid Landscapes

Deserts Land of extremes Little life So harsh - named: Extreme heat Extreme dryness Little life Sparse vegetation Little animal life So harsh - named: Death Valley Badwater Devil’s playground

What is a Desert? An area where evaporation exceeds precipitation < 10” of rain per year

Where Do Deserts Occur? Approximately 30° N&S Covers about 20% of Earth’s surface Saharan Desert – largest (3,500,000 mi2) Atacama – driest < 0.5”/year)

Types of Deserts Low Latitude Deserts Mid-latitude Deserts Subtropical Rainshadow deserts

Low Latitude Deserts A band circulating Earth around 30° N&S Examples: Sahara, Arabian Deserts, Africa Mojave Desert, North America Controlled by global air pressure and wind circulation

Low Latitude Deserts - Causes Depends on angle sunlight hits surface Sunlight at polar latitudes covers wider area; therefore, less heat At equator, sunlight covers less area; more heat

Low Latitude Deserts - Causes Excess heat is transferred from equator to poles Air Circulation Ocean currents

Low Latitude Deserts - Causes At equator, warm air rises Zone of low pressure Clouds and precipitation Reaches troposphere and moves poleward As it spreads, it cools 30° N&S, cool air sinks Area of high pressure Dry conditions Location of world deserts

Low Latitude Deserts - Causes At equator, warm air rises, expands, and cools Warm air holds more H20 than cool air condenses and precipitates At 30°, cool air sinks, compresses and warms Dry and hot

Mid-latitude Deserts Deep interiors of large landmasses Examples: Gobi Deserts, Central Asia Great Basin & Sonoran Deserts, North America Controlled by location Far inland away from ocean Behind large mountain belts

Mid-latitude Desert - Cause Rainshadow Effect Windward  high precipitation Air rises, expands and cools  H20-holding capacity, condenses, precipitates Leeward  much drier Air descends, compresses and warms H20 has been removed by precipitation  dry

Geologic Processes in Arid Climates The Role of Water Does most of the erosional work The Role of Wind Main role is transportation and deposition

Water - Ephemeral Streams Flow intermittently Only lasts a few hours to days Produce flash floods However infrequent, main shaper of topography

Water – Alluvial Fan Cone-shaped deposit Rivers flow out of canyon, soak into dry ground and sediments deposited at mouth Bajada – coalescing alluvial fan

Water - Playas Shallow, dry lake Closed basin - H20 cannot escape H20 evaporates leaving its salts behind

Owens Lake

Racetrack Playa & Devil’s Golf Course

Basin and Range Alluvial fans and playas are typical of Basin & Range province Fault block mountains formed by extension Form horsts (ranges) – uplifted blocks and grabens (basin) – down-dropped blocks

Basin & Range Stages Early Stage Advanced Middle Stage Late Stage Recent uplift; early erosion Relief is greatest Advanced Middle Stage Continued erosion Relief decreases Late Stage Mountains are gone Few knobs - inselbergs

The Role of Wind Erosion Transportation Deposition Deflation Abrasion Suspended load Bed load Deposition Sand Dunes

Erosion - Deflation Blowouts – shallow depressions Strong winds pick up and transport sand creating hollow depressions

Erosion - Deflation Desert Pavement – Layer of pebbles and gravel Wind carries smaller sand-sized particle away leaving larger particles behind Forms protective layer – no more erosion

Erosion - Abrasion Wind-blown sand cuts and polishes exposed rock surfaces (sandblasts) Produces ventifacts – polished stone

Transportation – Suspended Load Fine dust particles stay suspended May travel 1000’s of miles Suspended for days to weeks Marine maintenance technicians are wearing gas masks so that they can work on aircraft in conditions that can only be described as “extreme.” The dust storm that blew in early this evening has made the air outside appear to be foggy – like it does along the Atlantic or Pacific coasts when there is a large storm offshore. It has the strange effect of turning daylight into dusk, blotting out the sun and changing the hue of every man and machine. Visibility is reduced to less than 20 yards. The wind, blowing steadily at 25 to 30 knots, howls like a banshee through the antenna guy wires. But the “fog” in the air isn’t water vapor – it’s particles of sand that the Marines inhale with every breath, that they swallow with every mouthful of food. It whips in beneath the sidewalls and portals of the tents. It jams weapons. It seeps into every crevice and clogs the intakes of jet engines and the filters of the gas masks everyone carries everywhere, all the time.

Transportation – Bed load Wind-blown sand can skip, bounce, roll or scoot - saltation

Deposition – Dune Formation Sand dunes migrate Windward side  low slope face Faces wind direction Sediment eroded by saltation Leeward side  steep slip face Points downwind Wind slows and deposits load

Deposition – Sand Dunes Barchan – crescent-shaped dunes that concave downwind limited sand and constant wind

Deposition - Sand Dunes As sand supply increases, barchan dunes begin to connect – Barchanoid dunes When ridges straighten, they are called transverse – linear ridges that are perpendicular to wind direction form with abundant sand and no vegetation

Deposition - Sand Dunes Longitudinal – linear dune that is parallel to wind direction Sand supply is good and wind is not constant

Deposition - Sand Dunes Parabolic – crescent-shaped dune that is concave upwind Some vegetation and good sand supply

Deposition - Sand Dunes Star Dunes – complex, pyramidal-shaped, with 3+ slip faces Grow upward Several wind directions Namib Desert