Deserts & Winds: Fluvial and Eolian Processes in Arid Zones Why Do Deserts Exist?
Desert any spot on Earth that receives less than 25 cm (10 in) rain per year Semi-desert 25-50 cm (10-20 in) Global Distribution of Arid/Desert
and Semi-arid/Steppe Regions Types of Deserts Subtropical High, Rain-shadow, Coastal, and Interior deserts. Erg Sea of Sand
Largest are in Sahara and Arabian Deserts Sand covers about 20% of worlds deserts Vegetation is rare Sand probably formed during more
humid climate Namib Desert, Namibia Death Valley N.P., California
Hamada Bedrock predominates Death Valley N.P., California Desert varnish
oxidation of iron and manganese, useful for dating desert surfaces Arid Landscapes: Basic
Concepts I. Most deserts exhibit highly angular landscapes. mechanical weathering dominates angular particles of weathered rock; bedrock
outcrops with wind erosion little soil or vegetation to soften landscape II. Softer, more rounded landscapes appear where deposition is predominant. dunes alluvial fans
III. Desert rainfall is infrequent, intense, and unreliable. intense flash flooding, intense erosion ephemeral streams Joshua Tree N.P.,
California Las Vegas Flash Flood Wash Arroyo
The Nile River meanders through the Sahara A braided stream near Chinle,
Arizona FLUVIAL LANDFORMS IN ARID REGIONS Fluvial Erosion sporadic nature of desert runoff
(rare, intense flash floods), and lack of vegetation to protect surface materials causes rapid erosion. Ephemeral stream channels washes or arroyos (orwadis in the Middle East, and barrancas in Latin America) Even streams originating in humid regions cannot
maintain sufficient discharge and eventually disappear Steep slopes steep-walled canyons Mesas and Buttes
Slot Canyon The Narrows, Zion N.P.,
Utah Differential Erosion MESA AND SCARP
TOPOGRAPHY Pediments and Inselbergs eroded bedrock and resistant knobs Inselberg - island mountains
rise abruptly out of desert landscapes Uluru (Ayers Rock), Australia Bornhardt highly resistant rock
landform; a type of inselberg. Kata Tjuta (The Olgas), A Bornhardt in Arches National Park,
The Badlands of South Dakota Impermeable clays and lack of soil cover produced rapid runoff, and a dense network of
barren ridges dissected by a maze of steep, dry gullies and ravines Badlands topography. Fluvial Transportation large amounts of rock and
sand moved short distances during sporadic flows Fluvial Deposition alluvium deposits at the base of mountains or in closed basins does as much to level the land as does erosion differentiated from coarse to fine
Alluvial fans Bajada Piedmont Alluvial Plain Rock debris in piedmont region Debris flow fans Desert basins of interior drainage Bolsons Playas Also playa lakes, clay pans, salinas, salt flats (or salt-crust playas)
Piedmont gently sloping land that extends out from the base of mountains, composed of a series of alluvial fans (depositional). Pediment sloping bedrock at base that develops due to
gradual retreat of the adjoining mountain slope (erosional). Badwater, Death Valley, CA
Bajadas Death Valley N.P., California BASIN AND RANGE
TOPOGRAPHY Playas EOLIAN (WIND) PROCESSES AND LANDFORMS
Eolian Erosion - much less effective than fluvial deflation picking up and moving small particles of rocks; helps produce deflation hollows and desert pavements
abrasion like sandblasting effect; polishes and etches exposed surfaces; produces ventifacts and yardangs Eolian Transportation - moves nothing bigger than coarse sand
duststorms - can extend thousands of feet up sand storms - only very near the surface Eolian Deposition - dunes form and shift where Wind Transportation Mechanisms
Dust Storm, Senegal Sand Storm EOLIAN (WIND) EROSION FEATURES
Desert pavement of pebbles and stones result of deflation all small materials and sand removed by wind very thin coating of stones
Desert pavements: Also known as: gibber in Australia reg in North Africa
Wind Erosion Features: An Yardang in the Kharga Depression, Egypt
Ventifacts, Yardangs, Pedestal Rocks A sand-blasted
rock, or Ventifact, on Mars Pedestal Rocks (Rock Mushrooms) in Utahs Goblin Valley S.P.
EOLIAN (WIND) DEPOSTION FEATURES Active/Shifting/ Migrating Dunes
Types of Dunes BARCHAN TRANSVERSE DUNES
LONGITUDINAL DUNES (SEIFS) Types of Dunes Barchans - migrating crescent dune
with horns pointing downwind; form where strong winds blow in a consistent direction. Move fast. Common in central Asia and Sahara. Can reach hundreds of feet in height.
Transverse Dunes - parallel waves of crescent dunes perpendicular to wind direction; migrate downwind. Found in areas with large supply of Seifs multiple, very long, narrow, parallel dunes. They occur in areas with at least two
dominant wind directions. Can be tens of miles long. More common outside the U.S. Other Types of Dunes Star Dunes
tabilized Dunes when plants establish themselves in a dune area Loess Deposits: Wind-deposited silt,
transported by dust storms; vary in thickness from few inches to over 300 feet Gully eroded thick loess in Northern China
Unstable loess bluffs in Vicksburg, Mississippi Major Loess Regions of the World (Most loess deposits are peripheral to deserts and recently glaciated
areas) Finally, Combined effects of Weathering, Mass
Wasting + Fluvial and Eolian processes! Delicate Arch, Arches National Park, Utah
