Deserts and Eolian ProcessesQuestionsWhat are desertsWhy are deserts “dry”PowerPoint Presentation2. Rainshadow effects – mountain barriers affect weather patternsEastern WA – rain shadow desert3. Coastal deserts – landmasses near cold ocean currents receive little precipitation4. Polar deserts – little to no liquid water is available. Evaporation > precipitation5. Continental Interior deserts Little precipitation is brought to deep continental interiorsDry regions -Desert drainage systemsDesert soilsSlide 14Desert vegetationSlide 16Desert ErosionSlide 18Slide 19Deflation and Desert PavementSlide 21Slide 22Slide 23Slide 24Eolian Erosion: AbrasionSlide 26Slide 27Eolian transportSlide 29Slide 30Slide 31Eolian Deposition: Sand and dustSlide 33Slide 34Sand dunesSlide 36Slide 37Slide 38Slide 39Cross-bedded SandstoneEolian Deposition: DustSlide 42Slide 43Formation of Desert SouthwestSlide 45Formation of Desert Southwest: The Basin and RangeSlide 47Slide 48Slide 49Landforms Of Deserts (Emphasis on US Southwest)Arroyo: dry stream bed, usually short, narrow, and steep-sidedSlide 52Slide 53Alluvial Fans – Intermittent streams bring sediment from highlands to desert floorBajada – Coalescing alluvial fans form along desert margins at foot of mountainsPlaya lakes – dry lake beds (sometimes temporarily hold waterPlaya depositsBonneville Salt Flats, UT Playa depositsPediments: Gently sloping bedrock along flanks of mountains that become buried by sedimentInselbergs: Erosional remnants of mountainsMesas and ButtesSlide 62Deserts and Eolian ProcessesLecture 33Questions•What are deserts and where do they form?•How are eolian processes similar to fluvial processes? How are they different?•What are typical features of deserts?What are deserts•5 Major types of Deserts•Dry regions–Evaporation > precipitation•Large temperature fluctuations–Not just “Hot” regions•Sparse vegetation–Special adaptations•Patchy, thin soils–Susceptible to erosionWhy are deserts “dry”1. Deserts generally distributed 30 degrees N and S of equatorGlobal wind/weather patterns control moisture distribution2. Rainshadow effects – mountain barriers affect weather patternsEastern WA – rain shadow desert3. Coastal deserts – landmasses near cold ocean currents receive little precipitation4. Polar deserts – little to no liquid water is available. Evaporation > precipitation5. Continental Interior desertsLittle precipitation is brought to deep continental interiorsDry regions -•Low rainfall = discontinuous streams•Low humidity leads to large temperature fluctuations–Can range from freezing Temps to > 100 ˚F in a single day•Affects soil formation•Controls vegetationStreams are:Streams are: ShortShort DiscontinuousDiscontinuous Intermittent (ephemeral)Intermittent (ephemeral)Desert drainage systems““ExoticExotic”” streams: streams: those that flow completely through a desert basin those that flow completely through a desert basin (e.g., Nile River, Colorado River)(e.g., Nile River, Colorado River)Desert soils•Mechanical weathering dominant force–Freeze-thaw; thermal expansion/contraction•Little rainfall = little chemical weathering•Desert Soils have a variety of minerals–Low chemical weathering = less breakdown•Thin organic layer–Cannot support intensive vegetation•Highly susceptible to erosionDesert soilsDesert vegetation•Discontinuous in arid regions–May be absent in areas of moving sand•Adapted to dry conditions–Thick, waxy cuticles–Spongy stems–Leaves modified as protective needles/spinesDesert Erosion•Wind and Water•Running water highly important in deserts, but, running water is rare and infrequent•Wind is most commonly observed agent–Erodes material–Transports material–Deposits materialEolian Erosion•Two forms of wind erosion–Deflation•Removal of material by wind transport–Abrasion•Mechanical weathering of rock by wind blown sandEolian Erosion: Deflation•The blowing away of loose sand, silt, and clay sized particles–Larger particles remain•Only takes place in desert areas, or in the absence of vegetation–Roots tend to hold soil in placeDeflation and Desert PavementDeflation and Desert PavementDeflation and Desert PavementDesert PavementBlowoutsEolian Erosion: Abrasion•Particles transported by wind can collide and impact other rocks•Small pieces are broken off•Remaining rocks are modified by “sandblasting”•Ventifacts – polished and abraded surfaces•Yardangs – elongate ridges parallel to wind directionYardangVentifactEolian transport•Bed Load – Rolling, dragging and sliding along ground surface–Usually particles too large to be carried by wind•Saltation – semi-airborn; semi bed load•Suspension – finer grained particles able to be carried by windBed LoadSuspension•Silt and clay particles (<0.06 mm) may be carried in suspension–Carried high in the atmosphere•Up to 2500 m–Transported long distances•Dust from the Sahara may reach the CaribbeanEolian Deposition:Sand and dust1. Sand–Wind blown sand accumulates in•Sand sheets•Ripples•Dunes–Variation in forms caused by•Sand supply•Wind speed and directionSand Sheets & Ripples•Sand sheets occur in areas of lower wind velocity and larger grain size–Grains are too large to move by saltation–Moving portion of sheet is a few cm thick•Ripples form perpendicular to wind from saltating grains–Ripples occur at regularly spaced intervals–Ripples may form on dune surfacesSand dunes•A hill of ridge of sand deposited by wind•Sand dunes can migrate•Many forms – depend on –Sand supply–Wind direction & constancy–VegetationHow do dunesForm and moveCross-bedded SandstoneEolian Deposition: Dust•Dust particles may remain suspended until:–Wind velocities drop, and particles can’t remain suspended–Dust particles collide in the air, and stick together forming larger particles–Rain washes dust from atmosphereEolian Deposits: Dust•Loess – thick deposits of wind blown dust and clay–Dominated by silt sized and smaller grains–Transported long distances–Not always associated with desert areas•Common in glaciated areas (rock flour)Loess deposits in USFormation of Desert SouthwestFormation of Desert Southwest:The Basin and RangeFormation of Desert SouthwestLandforms Of Deserts(Emphasis on US Southwest)•Arroyo (wadi) dry stream bed, usually short, narrow, and