11

Deserts and Wind Erosion

GLY 2010 – Summer 2012

Lecture 22

Photo by Bruce R. Thomas. copyright © 2005 - 7

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What Is a Desert?

• May be hot, warm, or cold

• Vegetation is generally sparse

• Must be arid or semi-arid

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Desert and Steppes

• Deserts are arid, steppes are semi-arid

Wind Circulation

Coriolis Effect

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Wind Circulation

5Map of desert regions Rain Shadow effect Aridity Index

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Rain Shadow Deserts

• On the leeward side of mountains, particularly those near the coast, there is often an arid region

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Pacific Northwest

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Rain Shadow on the Olympic Peninsula

8Return to Wind Circulation

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Aridity Index

• Aridity Index = area’s potential annual evaporation divided by recorded average annual precipitation Humid - up to 1.5 Semi-arid 1.5 to 4.0 Arid greater than 4.0 Hyper-arid - no exact definition - numbers over

100 - Eastern Sahara and Atacama Deserts can reach 200

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Physical Weathering Dominates

• With little available moisture, chemical weathering rates quickly slow to near zero

• Physical weathering also slows, but not to zero

• Therefore, physical weathering becomes the dominant process

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Evaporite Deposits in Basins

• Interior drainage possible Streams run into a basin, which may

accumulate some water Evaporation rates are so high, the lake never

gets deep enough to spill over the lowest point in its rim

Rather, the water simply becomes more saline, or dries up entirely

Salt deposits (evaporites) form

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Playa Lake

• A playa lake is a naturally occurring wetland that contains water only seasonally

• Playas often form when water collects in a natural depression

• Over time, a chemical reaction dissolves the underlying caliche layer, the land surface subsides (sinks), and a playa is created

• Ten’s of thousands of these lake exist on the western edge of the Great Plains

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What Are Playas?

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Playas and the Ogallala Aquifer

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Salina

• A salina is an ephemeral lake, created by interior drainage and evaporation

• Near Shiprock, New Mexico

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Bonneville Salt Flats

• The area west of Salt Lake Utah to near the Nevada border is a large, flat expanse of evaporite salt

• It supports almost no vegetation• The salt flats are a remnant of Pleistocene glacial Lake

Bonneville, which covered a much larger area• As the glacial lake evaporated, salts began to accumulate,

and are still accumulating today

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Glacial Lake Bonneville

• Glacial Lake Bonneville was one of several large Pleistocene lakes

• Great Salt Lake and the Bonneville Salt Flats are remnants

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Erosional Landforms

• Physical weathering dominates, so landforms in arid environments are much different than those in humid areas

• Arid regions, such as Monument Valley in Utah/Arizona, have often been used as sites for commercials

• Utah’s Red rock country has been used in many western movies

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Pediment

• Gradual headward erosion of streams in arid regions producing a gently sloping plane called a pediment at the foot of the mountain

• Pediments enlarge as mountains retreat, due to a thin covering of loose material which moves downslope with each flood

• Eventually whole mountain may erode

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Pediment

• Tucson, is situated in a valley, bordered by bajadas and pediments which form where eroded bedrock extends out from the mountains

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Mesa

• Isolated, nearly level landmass standing distinctly above the surrounding country, bounded by abrupt or steeply sloping erosion scarps on all sides, and capped by layers of resistant, nearly horizontal rock (often lava)

• A mesa has a more extensive summit area than a butte, and is a common topographic feature in the arid and semiarid regions of the U.S

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Checkerboard Mesa, Zion National Park

• Cross bedding in the Navajo Sandstone

• Massive sandstone layers

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Mesa in Utah

• Mesa in the Red Rock country of eastern Utah

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Inselberg

• Knobby remnant of mountain left by pediment formation, consisting of harder, more resistant rock

• Next three slides show the development of an inselberg

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Inselberg Formation

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Initial Exposure of Inselberg

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Fully Exposed Inselberg

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Spitzkoppe, Namibia

• The Spitzkoppe, an inselberg in the eastern part of the Namib, is probably the best known and most famous landmark in Namibia

• Known as the Matterhorn of Africa, the majestic form of the Spitzkoppe towers 1,784 metres above the flat surrounding plains 

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Desert Landforms, Early Stage

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Desert Landforms, Middle Stage

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Desert Landforms, Late Stage

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Desert Landforms

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Aeolian Erosion

• The lack of moisture means that water is not available to act as a glue, holding sediment together

• Lack of vegetation means there are few roots to hold sediment

• Therefore, the wind becomes a very important agent of erosion

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Deflation and Desert Pavement

• Winds blowing across dry, treeless expanses lift and remove clay, silt and fine sand (dust storms)

• Landscape level is lowered, at rates of centimeters per century

Dust Storm in Senegal

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Initial Aeolian Erosion

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Desert Pavement Development

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Final Stage of Desert Pavement

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Sediment Transportation Animation

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Aeolian Load

Diagram

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Disturbance of Dead-Air Layer

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Dust Bowl Movie

• Dust Bowl Era (1930’s) movie of aeolian erosion in the Great Plains

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Desert Pavement

• Desert Pavement is formed when winds remove the sand and smaller particles, leaving gravel behind, via deflation

Closeup of desert pavement, Russell Spit, Nevada

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Desert Varnish

• Bandelier National Monument, New Mexico

• Over time, the very slow chemical weathering may also lead to a deposition of a thin coating of manganese compoundsPhotographer: Justin Gould

(used by permission)

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Abrasion

• Wearing away of hard materials but the impact of hard mineral grains

• Ventifacts are wind erosional features produced in areas where the wind is predominantly from one direction - they have flat sides, and sharp edges, with surfaces toward wind pitted

Ventifact photo - "Photograph ©2000Tom Schweich. Used by permission, http://www.schweich.com"

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Ventifact Formation

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Ventifact Facet Development

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Continued Development of Facets

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Loess• Winds may deposit the fine “dust” as they

die• Aeolian dust accumulations are known as

loess, which is often a post-glacial feature, but thick accumulations of loess can develop in deserts

• Glaciers grind the rock down into very fine particles (“rock flour”) and this becomes the aeolian dust

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Vertical Loess Bluff

• Near Mississippi River, in Southern Illinois• Wind blown material has a high

cohesiveness, and retains vertical slope

• About 3 meters high

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Depositional Landforms

• Erosion isn’t the only agent creating desert landforms

• Deposition also plays a role

• Alluvial fans and sand dunes are examples

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Alluvial Fans

• Semicircular deposits at the base of mountain streams, where the gradient decreases as they flow onto the valley floor

• They violate the Law of Original Horizontality, because deposits at the edge of the fan aren’t horizontal

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Alluvial Fans, Death Valley, Califormia

• Multiple alluvial fans are visible

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Sand Dunes

• There are several kinds of sand dunes• Important factors in their creation are sand supply,

wind direction, and obstructions• Sand dunes are another major exception to the Law

of Original Horizontality• Sand is blown up and over the dune, and deposited

on the lee side, on a non-horizontal surface

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Sand Dune Formation

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Dune Migration

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Saltating Sand Grains

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Slipface Deposition

• White Sand Dunes NM, New Mexico• Sand is sliding down the steep face of the slipface

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Dunes and Mountains

• The Sangre de Cristo mountains, in the background, reach 14,000’ a.s.l., so they tower over the dunes, and act as an obstruction, which keeps the sand from moving further east

•Dunes reach nearly 700 feet in height•Valley floor is about 7500’ a.s.l.

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Barchan Dunes

• Crescent shaped ridges, perpendicular to wind

• Form where sand supply is limited and terrain is flat

• Horns of crescent point downwind

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Barchan Dune Formation

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Transverse Dunes

• Ample wind produces dunes perpendicular to wind direction, with a constant wind direction

• May reach 100 km in length, 100-200 meters high, and 1-3 km in width

• Vegetation scarce, sand supply plentiful

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Transverse Dunes

• Sand Hills, Nebraska

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Florida Dunes

• Transverse sand dunes, anchored in place along the Florida coastline by grasses

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• Photo courtesy Philip Greenspun

• Great Sand Dunes National Park, Colorado

Dune Ridge of Transverse Dune

Barchanoid Dunes

• Intermediate between isolated barchan dunes and transverse dunes

• Form where sand supply is more plentiful then is the case of barchan dunes

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Barchanoid Dune Photo

• White Sands National Monument, New Mexico

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Seif (Longitudinal)

• Parallel to prevailing wind direction

• Moderate sand supply

• Wind direction varies within a narrow range

• May reach 100 km long, 100 meters high, but often much smaller 60 m long by 5 meters high

• Etymology: Arabic sayf, = sword

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Longitudinal Dune Formation

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Seif Dunes

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Blowout, or Parabolic

• Horseshoe shaped, horns point upwind

• Develop from transverse dunes where erosion increases, especially after vegetation is removed

• Horns are vegetated, rest is not, and center moves downwind

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Parabolic Dune Formation

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Parabolic Dune

• Photo courtesy Philip Greenspun

• Great Sand Dunes National Park, Colorado

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Star Dune Formation

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Star Dunes

• Star dunes form when the wind direction is variable

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Paleowind Direction Determination

• Cross-bedding is a remnant of non-horizontal deposition in sand sediment or sandstone

• When wind directions change, the direction of deposition changes

• Together with changes in thickness of loess deposits (thicker near the source), the cross-bedding can be used to determine the direction of dominant paleowinds

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Cross-Bedding Formation

Desertification Definition

• “Desertification means degradation of land and vegetation, soil erosion and the loss of top soil and fertile land in arid, semi-arid, and dry sub-humid areas, caused primarily by human activities and climate variation” - United Nations Convention to Combat Desertification (UNCCD)

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Desertification Effects

• Invasion of non-desert regions by desert o Example: Sahal, immediately south of the Sahara

• Drought from 1970 to mid-90's - semi-arid lands are about 35% of total land area, and are subject to desertification

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Sahara and Sahal

Desertification Video

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Reversing Desertification

• Reversing the desertification process is difficult

• Involves Wind breaks (planting forests in China) Leveling dunes, covering with topsoil Replanting, and providing water to

establish cover crop of vegetation