EARSCI DESERT

8/10/2019 EARSCI DESERT

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DESERTSDean Victor Montalvo


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What is a desert?

Any region with an arid climateand a rainfall less than 25cm/year.


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How deserts form?

Descending warm and moist air.

Rain shadow Distance from ocean Coasts with cold ocean currents Polar deserts


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Distribution of deserts

Most of the deserts lie along a narrowbelt 30 N and S of the Equator.

These two belts have warmer air andcan hold a lot of moisture, leading to

very low rainfalls.


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Distribution of deserts

The two belts are characterized by

clear skies, lots of sunshine, little rainand high evaporation.


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Deserts due to Rain Shadow

Rain shadow is a dry regiondownwind from a mountain range.

Example: Sierra Nevada range inCalifornia causing desert formation inNevada and N. Arizona.


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Formation of Rain Shadow Deserts

Moisture ladenwinds come

onshore

Moist airmassrise and cool,

causing

precipitation

resultingin a rainywindward

slope

Moisture-depleted airmasssink and warm,

reducing relative

humidity

forming a rain shadow on the

leeward side of the mountains.


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Distance from ocean: oceans are a great source ofevaporated water. At greater distance from theoceans, deserts occur in the continental interior.

Example: Gobi desert in China.


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Coasts with cold ocean currents: warm air which canhold more moisture and no rains on the coasts.

Example: Pacific coast of S. America.


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Polar deserts: cold air with little moisture. No rains.

Examples: Antarctica, Greenland, Alaska.


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Wind erosion and transportation

Wind can erode only finer particlessuch as clay silt and sand.

Deserts typically have strongwinds which can erode loose drysediments causing sandstorms/dust storms.


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Effects of wind action are strongonly close to the ground (uptoone meter from the groundsurface).


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Deathvalley, Ca


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Deathvalley, Ca


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Erosional features

Desert pavement: thin surfacelayer of closely-packed pebbles.


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Desert Pavementin the SonoranDesert, Arizona


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Ventifacts: rock fragments with flat, wind-abradedsurfaces.


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Blowout: depression on the land surface caused bywind erosion.


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Wind deposition

Loess Sand dunes


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Loess: deposit of wind-blown silt

and clay consisting of quartz,feldspar and clay minerals.

Weakly cemented by calcite.

A desert or glacial outwash isneeded as source material.


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Sand dunes

Mounds of loose sand grains heaped up by the wind. Composition of sand depends on;

Sand source Chemical weathering

Quartz, feldspar and calcite are generally moreabundant.

Well sorted and rounded.


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How a dune forms?

A sand dune forms with a gentle upwind slope and asteeper slip face on the downwind side.

Sand eroded from the upwind side is deposited on theslip face, forming cross-beds.


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Embryonic dunes forming on the lee-side of vegetation


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Types of dunes

Factors controlling dune type: Wind velocity and direction

Sand supply Vegetation cover


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4 types of dunes: Barchan Transverse Parabolic Longitudinal


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Barchan: crescent shaped dune convex in theupwind direction.


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Transverse: relatively straight, elongate duneoriented perpendicular to the wind direction.


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Parabolic: similar to a barchan dune except thatit is convex in the downwind direction.


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Deserts in US

Deserts in SW US are caused by; Proximity to the 30 N latitude. Rain shadow by Sierra Nevada.

Colorado plateau Basin and Range Province, Nevada.


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Associated features

Plateaus: broad, flat-topped areas elevated above thesurrounding land and partly bounded by cliffs.

Mesa: broad flat-topped hill bounded by cliffs from all sides.Butte: narrow hill of resistant rock with a flat top and very

steep sides.


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Runoff water in the valley may collect at the floorforming a Playa lake.

Mud cracks develop when a playa lake dries.


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Desert Weathering is Characterized By:

a dominance of physical weathering

minimal chemical weathering, consisting mainly ofoxidation of mafic minerals


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Desert Weathering is Characterized By:

formation of d e s er t v a rn i s h : a weathering veneer

formed of a mixture of clays and smaller amounts of ironand manganese oxides.


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Oxidation of mafic

minerals results inthe formation of ironoxides and the typicalreds and browns ofa desert landscape


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Petroglyphs scratched in desert varnish

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