Deserts and Wind Action

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

2. Deserts • desert: any region with low rainfall World distribution of nonpolar deserts. Most deserts lie in two bands near 30o N and 30oS.

3. Distribution of Deserts • the 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

4. 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

5. 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

6. Characteristics of Deserts • lack through-flowing streams • internal 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

7. 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

8. 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

9. Desert Landforms (cont.) • monocline: bends in rock layers • hogback: a sharp ridge that has steep slopes • cuesta: a gently tilted resistant layer with one steep side and one gently sloping side

10. 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

11. 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

12. Wind Erosion and Transportation • dust 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)

13. 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

14. Wind Erosion and Transportation (cont.) • sandstorms: clouds of sand moving rapidly near the land surface • ventifacts: 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)

15. Deflation • deflation: 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 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 Large blowout near Harrison, Nebr.

16. Wind Deposition • loess: 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% Definite Vertical road cuts in loess - Vicksburg, Mississippi Possible or probable Major loess-covered areas in the world

17. 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

18. Sand Dunes (cont.) • slip face: the steep downwind slope of a dune • wind ripples: small, low ridges of sand produced by saltation of the grains Wind ripples on sand surface - Monument Valley, Utah

19. 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

20. Types of Dunes (cont.) • transverse dune: a relatively straight, elongate dune oriented perpendicular to the wind direction Transverse dunes Transverse dunes, Oregon

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

22. 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

23. Dunes Types(sketches)

24. 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.