Deserts and Wind Action Prepared by Betsy Conklin for Dr. Isiorho
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.