Deserts & Winds: - PowerPoint Presentation

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 yearSemi-desert – 25-50 cm (10-20 in)

Global Distribution of Arid/Desert and Semi-arid/Steppe Regions

Types of Deserts Erg – Sea of Sand Largest are in Sahara and Arabian Deserts Sand covers about 20% of world’s deserts Vegetation is rare Sand probably formed during more humid climate Subtropical High, Rain-shadow, Coastal, and Interior deserts.

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

Arroyo Wash

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 ( or wadis 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

Uluru ( Ayers Rock), AustraliaInselberg - “island mountains” rise abruptly out of desert landscapes

Bornhardt highly resistant rock landform; a type of inselberg. Kata Tjuta (The Olgas ), Australia

A Bornhardt in Arches National Park, Utah

Fluvial Transportation – large amounts of rock and sand moved short distances during sporadic flows The Badlands of South Dakota –Impermeable clays and lack of soil coverproduced rapid runoff,and a dense network of barren ridges dissected by a maze of steep, dry gullies and ravines – Badlands topography.

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 winds and terrain slow the winds or block movement of dunes; loess deposits – wind-deposited silt ← dust storms

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

Also known as: “gibber” in Australia “reg” in North AfricaDesert pavements:

Wind Erosion Features: Ventifacts ,Yardangs,Pedestal Rocks An Yardang in the Kharga Depression, Egypt Pedestal Rocks (Rock Mushrooms) in Utah’s Goblin Valley S.P. A sand-blasted rock, or Ventifact , on Mars

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 sand.

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

Stabilized 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