Transcript 16. Deserts and Winds

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 world’s 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), Australia
A Bornhardt in Arches National Park, Utah
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 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
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 Utah’s 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 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