GSC 1530 Chapter 19

Deserts and Winds

Deserts

Deserts (arid) and steppes (semiarid) lands occupy about 30-35 percent of Earth’s land surface – more than any other ecosystem type

Therefore its important to understand these environments and whether deserts are expanding or contracting (see slide)

Don’t forget – there are polar deserts (Antarctica and most of the Arctic)!

Deserts

Desert – a sparsely vegetated, arid region that typically receives less than ten inches of rainfall per year and where the rate of evaporation greatly exceeds the rate of precipitation

Note no mention of temperature in my definition (see slides)

Don’t forget – there are polar deserts (Antarctica and most of the Arctic)!

Desert Origin

Scientists recognize five major desert categories

Multiple factors contribute to desert formation – two of the most important are the Earth’s general air circulation patterns and topography (see slides)

Rising air expands and cools; encourages rainfall

Sinking air is compressed andwarmed, inhibits rainfall

Rain Shadow Desert Formation

Desert Processes/Landforms

Although most think of deserts as water deficient regions, running water (when present) performs most of the erosional activity in deserts

Much of this activity is aided by ephemeral (intermittently flowing) streams that host periodic flash floods during infrequent thunderstorms (see slides)

An ephemeral stream is called a dry wash or arroyo in the American west or a wadi In the Middle East

Note boulders moved by previousflash floods!

Desert thunderstorm over Tucson, Arizona

alluvial fans

Bajada – coalesced alluvial fans

playa lake/deposits

Desert Processes/Lanfdorms

Relevance of alluvial fans and bajadas?

See figure

Source: Earth, July 2009

Inselberg

Desert Processes/Landforms

Wind erosion and sediment deposition also significantly influence the desert landscape

Deflation – the lifting and removal of loose material by wind

Remember that fine-grained sediment transported by forceful winds is quite abrasive and can sculpt rocks uniquely (see slides)

Desert Processes/Landforms

Persistent deflation may also produce a fairly common surface known as desert pavement (see slides); desert pavement is the predominant surface feature in nearly all deserts – not dunes!

Desert Processes/Landforms

Probably the most characteristic desert landforms are the wind-deposited mounds and ridges of sands called dunes

Sherman County, Oregon

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Factors Affecting Dune Formation

Major Dune Types

Consequences of migrating dunes?

migrating dunes, Egypt

Desert Processes/Landforms

Loess – deposit of windblown silt and clay size rock particles that mantles the topography like fallen dust

Common source of loess is deserts, but previously glaciated terrains may also host a fine-grained material called rock flour that can be eroded readily by wind

Very thick (up to 300 ft) accumulations exist in China and there are also significant deposits in the United States

Why care? Airborne loess can impact air quality and water quality; loess deposits aid soil development and agricultural productivity (see slides)

Loess

Importance of Loess

In some regions of the world, loess cave homes are not uncommon

October 2001, Geotimes

Connection between loess and airand water quality?

Possible Influence of Loess?

Loess and Plant Productivity

Connection between loess and plant productivity?

Desertification

Desertification – expansion, or spread, of deserts largely as a consequence of human activities

A worldwide problem, including U.S. In the U.S. moderate to severe desertification

has occurred in parts of the southwest, plains and western states in the last 150 years (see slide)

black = desert

Desertification

What are the major human activities that could encourage desertification? Hint: any process that would foster removal of the thin topsoil layer in an arid or semiarid region adjacent to a desert would encourage desertification

Causes?

Desertification Causes

Clearcutting of native vegetation (drought-resistant trees and grasses like the Tallgrass Prairie) (see figure)

Desertification

Historic extent of N. American Prairie(Illinois State University)

Big Bluestem Grass

Desertification

Note the huge difference between the root system of a wheat plant and that of a tallgrass prairie plant!

Desertification Causes

Overgrazing by livestock Poor farming choices (growth of water-hungry

crops prone to failure during a drought) Poor water management (excessive diversions of

surface waters/depletion of aquifers) Off-road vehicle use

Desertification

Short- and long-term consequences of U.S. desertification, regionally and nationally?

How could desertification be reduced by actions at the local, state or federal level? (see slides)

Dust Bowl storm near Elkhart, Kansas in 1937

Video Trailer for Dust Bowl Documentary