Deserts and Wind Action

Prepared by Betsy Conklin forDr. Isiorho

Deserts

desert: any region with low rainfall

World distribution of nonpolar deserts. Most deserts lie in two bands near 30o N and 30oS.

Distribution of Desertsthe location of most deserts is related

to descending air - the equator receives the sun’s heat more directly than the rest of the earth, the air warms and rises then moves both northward and southward to sink near 30oN and 30oS

Distribution of Deserts (cont.)rain shadow: deserts resulting from

mountain ranges - moist air is forced up to pass over a mountain range, it expands and cools, losing moisture as it rises, dry air coming down on the other side of the mountain compresses and warms, bringing high evaporation with little or no rainfall to the downwind side of the range

Distribution of Deserts (cont.)

great distance from the ocean: since most rainfall comes from water evaporated from the sea, a great distance from the ocean is another factor that can create deserts

Characteristics of Deserts

lack through-flowing streamsinternal drainage - the streams drain

toward landlocked basins instead of toward the sea

flash floods - because of the lack of vegetation, heavy rainfall runs rapidly over the surface and can create sudden local floods of high discharge

Characteristics of Deserts (cont.)

arroys or dry washes: channels created by the rapid downcutting by sediment-laden floodwaters which tend to produce narrow canyons with vertical walls and flat, gravel-strewn floors

Desert Landforms plateaus: a broad, flat-topped area

elevated above the surrounding land and bounded, at least in part, by cliffs

mesa: a broad, flat-topped hill bounded by cliffs and capped with a resistant rock layer

butte: a narrow hill of resistant rock with a flat top and very steep sides

Desert Landforms (cont.)

monocline: bends in rock layershogback: a sharp ridge that has

steep slopescuesta: a gently tilted resistant

layer with one steep side and one gently sloping side

Desert Landforms (cont.) playa lake: a shallow temporary lake (following a

rainstorm)on a flat valley floor in a dry region playa: a very flat surface underlain by hard, mud-

cracked clay bajada: a broad gently-sloping depositional surface

formed by the coalescing of individual alluvial fans pediment: a gently sloping surface, commonly covered

with a veneer of gravel, cut into the solid rock of the mountain

Wind Action

wind can be an important agent of erosion and deposition in any climate, as long as sediment particles are loose and dry

wind differs from running water in two ways: because air is less dense than water, wind can

erode only fine sediment - sand, silt and clay wind is not confined to channels as running

water is, so water can have a widespread effect over vast areas

Wind Erosion and Transportationdust storms: when loose silt and clay are

easily picked up from barren dry soil, such as in a cultivated field - silt and clay can remain suspended in turbulent air for a long time, so a strong wind may carry a dust cloud hundreds of meters upward and hundreds of kilometers horizontally

An approaching dust storm in Prowers Country, Colorado (1930s)

Wind Erosion and Transportation (cont.)

volcanic ash: ash from a volcanic eruption that can be carried by wind for a very great distance - the ash can be caught in high altitude jet streams which have been known to carry ash as far as 3,000 miles

jet stream: a narrow belt of strong winds with velocities sometimes greater than 200 mph

saltation: a mode of transportation that carries sediment down current in a series of short leaps or bounces

Wind Erosion and Transportation (cont.)sandstorms: clouds of sand moving

rapidly near the land surfaceventifacts: rocks with flat, wind-

abraded surfaces

Ventifacts eroded by blowing sand

Wind erosion near the ground has sandblasted the lower 3 ft. of this basalt outcrop, Death Valley, CA. (Hammer for scale)

Deflationdeflation: the removal of clay, silt, and

sand particles from the land surface by wind

blowout: a depression on the land surface caused by wind erosion

pillar: erosional remnant of the former land that may be left at the center of a blowout

Large blowout near Harrison, Nebr.

Deflation by wind erosion can form a blowout in loose, dry sediment. Deflation stops at the water table. A pillar, or erosional remnant, may be found in the center of a blwout

Wind Depositionloess: a deposit of wind-blown silt and clay

composed of unweathered, angular grains of quartz, feldspar, and other minerals weakly cemented by calcite - have a high porosity, typically near 60%

Major loess-covered areas in the world

Vertical road cuts in loess - Vicksburg, MississippiDefinite

Possible or probable

Sand Dunes

sand dunes: mounds of loose sand grains heaped up by the wind, most likely to develop in areas with strong winds that generally blow in the same direction

sand grains found in dunes are commonly well-sorted and well-rounded because wind is very selective as it moves sediment

Sand Dunes (cont.)slip face: the steep downwind slope

of a dunewind ripples: small, low ridges of

sand produced by saltation of the grains

Wind ripples on sand surface - Monument Valley, Utah

Types of Dunes

barchan: a crescent-shaped dune with a steep slip face on the inward or concave side which is formed where the sand supply is limited

Barchans

These barchan dunes are advancing as much as 50 ft. A year over this barren valley floor in southern CA

Types of Dunes (cont.)

transverse dune: a relatively straight, elongate dune oriented perpendicular to the wind direction

Transverse dunes

Transverse dunes, Oregon

Types of Dunes (cont.)

parabolic dune: somewhat similar in shape to a barchan dune, except that it is deeply curved and is convex in the downwind direction

Parabolic dunesParabolic dunes near Pismo Beach, central California.Wind blows from left to right. The ocean and a sandbeach are just to the left of the photo

Types of Dunes (cont.)

longitudinal dune: one of the largest types of dunes which is a symmetrical ridge of sand that forms parallel to the prevailing wind direction

Longitudinal dunes

Longitudinal dunes in the Sahara Desert, Algeria.Photo from Gemini spacecraft at an altitude of about 65 miles

Dunes Types(sketches)

Pictures

All pictures used in this power point presentation were taken from the following:Carlson, Diane H., David McGeary and Charles C. Plummer. Physical Geology: Updated Eighth Edition. New York City, McGraw-Hill Higher Education, 2001.