Harry Williams, Earth Science1 DESERT LANDSCAPES Factors influencing desert landscapes: 1. lack of...

Harry Williams, Earth Science 1

DESERT LANDSCAPESFactors influencing desert landscapes:1. lack of soil development - why? little chemical weathering (little clay). Chemical weathering that does occur = slow oxidation; hydrolysis is very limited due to lack of water. In humid areas much chemical weathering occurs beneath the soil due to soil moisture/ground water in contact with rock; obviously this is not the case in deserts, where the water table is usually deep below the surface.


2. sparse vegetation - little organic matter for soil development; little organic acids for chemical weathering; little cohesion due to root mats.


3. mechanical weathering produces uncohesive sand and silt covered surface which can be moved by wind and running water, and is of low permeability promoting surface run-off and erosion.


Characteristic Desert Landforms. 1. Eolian Processes and Landforms:Contrary to common belief, most deserts are not covered in sand dunes; for example, only 1/9 of the Sahara is covered in sand. In fact, fluvial processes do most geomorphic work in most deserts. However, the wind is more effective in deserts than elsewhere, due to the loose surface and production of sand and dust by mechanical weathering.


a. Deflation - the erosion of a surface by wind is termed deflation. Coarse particles are left behind to form a characteristic desert pavement.


Typical desert pavement.


b. Sand Dunes - wind moves sand by creep and saltation; due to its weight sand usually is not carried more than 2 m off the ground; fine silt can be carried further aloft by wind turbulence. The sand is deposited in various types of dunes.


A dune is simply a deposit of wind-blown sand,with a characteristic gentle windward and steep leeward slope. The type of dune depends on sand supply and wind conditions.


i. Barchan Dunes - limited sand supply, wind mainly from one direction. Isolated dunes perpendicular to wind, with bare ground between them.


ii. Transverse Dunes - abundant sand supply, weak wind mainly from one direction, dunes perpendicular to wind (a “sea of sand”).


iii. Longitudinal (Seif) Dunes - moderate sand supply, strong winds from one quadrant. Linear dunes parallel with prevailing wind.


c. Loess - the fine silt carried in suspension for long distances forms loess, a blanket of silt adjacent to the source region (Note: most loess is relict, originating in the last Ice Age, when strong winds blew across silty deposits left by glacial meltwater).


A loess bluff, Kansas.

Dwelling carved into loess, northern China. Loess here is up to 250 m thick. Eroded loess ends up in the Yellow River (the name comes from the yellow color of the loess-rich water).


2. Fluvial Processes and LandformsDue to the impermeable, uncohesive surface and infrequent, but intense rains, most desert areas are dissected by numerous rills, gullies and channels, and are also subject to sheetwash. Many larger channels are dry most of the year and are called washes in the U.S. southwest. Flash floods are common during thunder storms, due to large amounts of run-off; the overland flow also transports large amounts of sediment, so that many washes carry mud flows.Where channels emerge, alluvial fans are built from mud flows;these may coalesce to form BAJADAS.

Harry Williams, Earth Science 15A dry wash


Alluvial fans, Owens Valley, California.


Bajada, Death Valley, California.


Basin and Range Province.


Death Valley, playa lake.


Erosional Landforms In Deserts.Humid areas = almost continuous cover of regolith, soil and vegetation; regolith usually fine-textured due to clay content; creep and solution produces smooth, curvilinear slopes which become lower over time.Dry areas = regolith/soil is thin and discontinuous, sparsely vegetated; regolith is coarse due to lack of clay; rockfall, sheetwash and mass wasting produce angular slopes which retreat back over time.

Humid areas

Dry areas


Landscapes are angular due to differential erosion - resistant rock forms steep cliffs; softer rock forms gentler slopes. There is little soil creep or solution to "smooth out" the landscape.


The effect of differential erosion is visible in the “stair-step” topography of the Grand Canyon.


As slopes are eroded back and maintain their angular form, Plateaus, Mesas, Buttes and Pinnacles are formed where resistant flat layers (cap rocks) are present e.g. lava flows in west Texas.

Monument Valley, Utah.

Time


Plateaus, mesas, buttes and pinnacles formed by stream erosion of uplifted strata.


Lava flow cap rock

Near Big Bend

Harry Williams, Earth Science1 DESERT LANDSCAPES Factors influencing desert landscapes: 1. lack of...

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