Arid Landforms - Los Angeles Mission College

Arid Landforms - Los Angeles Mission College

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


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


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

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