Deserts

Geology 1001-section 4 Dr. Dutrow (c) Abbreviated versions of overheads

Back to syllabus last updated: 04/02/98

DESERTS and WIND ACTION DESERTS: regions that receive little rainfall and are generally sparsely vegetated.

WIND:

a geologic agent of erosion, transportation and sedimentation

horizontal flow of air relative to surface of rotating planet

Eolian - geologic processes powered by wind (from the Greek god of winds Aeolus)

Circulation of Earth's Atmosphere

(a) Solar driven -

i. more heat E at

- hot

- develop density driven

ii. creates prevailing winds that

0 - 30 = tradewinds (tropics)
30 - 60 = westerlies (temperate)
60 - 90 = easterlies (polar)

e.g. 30° N, N flowing cold-dry equatorial air descends, becomes warmer, capable of

= warm climate, scarce rain, skies clear

produces arid land between

(b) Earth's rotation modifies i. Coriolis effect - an effect that

Earth to veer to the

e.g. N of 30, winds respond to Coriolis effect ii. Determines Location of Deserts

DESERTS A desert receives less than mm of ppt annually.

Classified based on

        semi-arid =
        desert =
        hyper-arid =

1. Kinds of deserts

(a) Determined by

(b) Subtropical deserts

between 15-30°

zones of continual

most

(c) Continental interior deserts

mid-latitude between

dry due to

e.g.

(d) Rainshadow deserts

in rainshadow of

mtns are

- air rises along

- air deficient in

e.g. Sierra Nevadas - Great Basin; Cascades - Idaho (e) Coastal Deserts

occur along

upwelling of cold seawater

when moves onto land,

Peru, Chile, western S.Am. Andes, altiplano

(f) Polar Deserts

little precipitation because

e.g. valleys of Antarctica

2. Geologic Processs in Desert

(a) Weathering i. physical

little chemical decay prevents

wind prevents soil

- soil

- sand, gravel bedrock

orange/brown due to

particles are

ii. Desert varnish - - distinctive dark brown, often shiny coating (patina) formed

- clays + Fe/Mn oxides

- due to

(b) Erosion agents i. wind is

ii. Streams are extremely

-runoff is unchecked

-although only intermittent

-major mechanism

Produces arroyos -

Pediments - gently sloping platform-like structure at

Inselberg - steep-sided knob of

Bornhardt - large rounded structure that forms

(c) Stream Deposition: Sediment and Sedimentation -
i. Alluvial Sediments -

floods lay down sediments

alluvial fans are produced at

-sediment load

-very porous and permeable,

ii. Eolian sediments - sand dunes, dune field, ergs
iii. Evaporite sediments - playa lakes - occasionally filled with water

(during runoff).

- water highly

-deposits of

3. Desert Landforms (varied)

Fans - alluvial fans characterize

- developed at

Bajada - coalesced

- undulating surface, convex up

Pediments - broad, gently sloping platforms of

Dry wash - dry river valleys (wadis in near East)

Mesa - flat-topped

Box Canyon - steep-sided, flat bottomed canyons

e.g. Canyon de Chelly, NM; Anasazi ruins

WIND ACTION

1. Characteristics of wind

(a) Obeys same laws of fluid motion - wind has (b) unconfined flow; unconfined by solid boundaries - spread in (c) intermittent

(d) nearly always turbulent

(e) extremely

2. Sediment Transport by wind - Erosion (a) Sediment moves by

saltation - mode of sediment

- serves to

- produces

(b) Suspended load - particles that remain

usually

can travel

Capacity - in storm,

Competence - amount of sand carried increases

e.g. 30mi/hr wind can move .5 ton/day

EROSION by WIND

1. Deflation - to blow away; removal of

(a) develops in

(b) scoops out sediment to make

e.g. Dust Bowl of 30's
(c) Deflation basin -

due to

(d) vegetation cover i.e. don't tear up desert vegetation 2. Desert Pavement

coarse, gravelly ground surface produced

- removal of

creates

3. Wind Abrasion

important

rounds and frosts

(can tell desert provenance) Sandblasting - abrasion of grains by particles by
Ventifacts - rocks which have been

Yardangs - elongate streamlined, wind-eroded ridges

DEPOSITION by WIND

Formation of Sand DUNES

(a) An obstacles creates a

(b) Wind velocity here is

(c) sand no longer can be moved

(d) creates

(e) dunes migrate

(f) slip face - downwind side of dune with steep face

(g) bed load deposition: sand dunes

TYPES of DUNES

Barchan - cresent-shaped dune,

-produced by

Transverse - long, wavy ridges that lie

- produced by

e.g. beaches

Longitudinal (Linear) dunes - long, straight dunes

Parabolic - cresent-shaped dunes, with tips

Star dunes -

Blowout - parabola-shaped dune formed when

Draas - extremely large, high, hilly forms that

ergs - "sea of sand" extensive areas of Draas e.g. Saudi Arabia 3. Dust Falls - suspended load Loess - wind deposited sediment - lacks

- deposited chiefly from

- e.g. N. Am. upper MS Valley; Sandhills, NE - St. Francisville, LA

COMPOSITION of DUNES

1. usually made of

2. other materials

(a) Calcite - Bermuda dunes

(b) Gypsum - . White Sands, NM

(c) Fe-oxides, heavy minerals - Hawaii Black Sand

Arid regions - DESERTIFICATION

account for (27.5 mill mi²)

Desertification - processes by which

- often result of human actions

Symptoms -

- declining

- increasing

- reduction in

- high

- loss of

To reverse trend must understand geologic principles involved,

- use intelligent application of measures designed to reestablish a natural balance in affected areas