Understanding Earth Sixth Edition Chapter 19: WINDS AND DESERTS © 2011 by W. H. Freeman and Company Grotzinger Jordan

Chapter 19 Winds and Deserts

About Winds and Deserts Wind is a major agent of erosion and deposition that moves vast quantities of sand, silt, and dust over large areas.Wind is a major agent of erosion and deposition that moves vast quantities of sand, silt, and dust over large areas. Eolian processes shape the land surface in areas like deserts where few plants live.Eolian processes shape the land surface in areas like deserts where few plants live. Deserts expand and contract, depending upon climatic changes.Deserts expand and contract, depending upon climatic changes.

Lecture Outline 1. Global wind patterns 2. Wind as a transport agent 3. Wind as an agent of erosion 4. Wind as a depositional agent 5. The desert environment

● Wind (the flow of air) ● wind is unconfined, except in narrow valleys narrow valleys ● wind obeys all the laws of flow ●wind is parallel to surface, and ● wind is parallel to surface, and may go upward too may go upward too 1. Global Wind Patterns

● Characteristics of wind ● turbulence ● wind belts ● trade winds ●westerlies ● westerlies 1. Global Wind Patterns

● Complications to simple circulation ● Earth’s rotation ● Coriolis effect ● equatorial upward movement ● dry, sinking air over deserts 1. Global Wind Patterns

North Pole Westerlies 30°N latitude Equator 30°S latitude NE trade winds SE trade winds Wind as a Flow of Air: Earth’sWindBelts

North Pole At the equator, the Sun’s rays are perpendicular to the surface, concentrating heat. Westerlies 30°N latitude Equator 30°S latitude NE trade winds SE trade winds Wind as a Flow of Air: Earth’sWindBelts

North Pole At the equator, the Sun’s rays are perpendicular to the surface, concentrating heat. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures. Westerlies 30°N latitude Equator 30°S latitude NE trade winds SE trade winds Wind as a Flow of Air: Earth’sWindBelts

North Pole There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools. At the equator, the Sun’s rays are perpendicular to the surface, concentrating heat. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures. Westerlies 30°N latitude Equator 30°S latitude NE trade winds SE trade winds

North Pole There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools. At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies. At the equator, the Sun’s rays are perpendicular to the surface, concentrating heat. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures. Westerlies 30°N latitude Equator 30°S latitude NE trade winds SE trade winds

North Pole There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools. At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies. At the equator, the Sun’s rays are perpendicular to the surface, concentrating heat. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures. These two motions set up the horizontal circulation between the equator and the North and South Poles. Westerlies 30°N latitude Equator 30°S latitude NE trade winds SE trade winds

North Pole There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools. At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies. At the equator, the Sun’s rays are perpendicular to the surface, concentrating heat. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures. These two motions set up the horizontal circulation between the equator and the North and South Poles. In the temperate zones, the prevailing wind belts come from the west. Westerlies 30°N latitude Equator 30°S latitude NE trade winds SE trade winds

North Pole Westerlies 30°N latitude Equator 30°S latitude NE trade winds SE trade winds There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools. At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies. At the equator, the Sun’s rays are perpendicular to the surface, concentrating heat. At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures. These two motions set up the horizontal circulation between the equator and the North and South Poles. In the temperate zones, the prevailing wind belts come from the west. In the tropics, the prevailing wind belts blow from the east. The Coriolis effect is responsible for the deflection of air eastward or westward as illustrated by the curved arrows.

● Factors in how wind carries things ● wind strength ● particle size ● surface material 2. Wind as a Transport Agent

Wind as a TransportAgent: Rate of SandMovementversus Wind Speed

Wind as a TransportAgent: Sand Being Blown from the Desert in Namibia toward the South Atlantic

● Materials carried by the wind ● windblown dust ● volcanic, organic, soil ● aerosols ●windblown sand ● windblown sand 2. Wind as a Transport Agent

Wind as an Agent of Erosion: A Ventifact

Wind as an Agent of Erosion: RoundedandFrostedGrains of Sand

Wind as an Agent of Erosion: DesertDeflationSurface

Thought questions for this chapter What evidence might you find in an ancient sandstone that would point to its eolian origin?

3. Wind as an Agent of Erosion: Formation of Desert Pavement

● What the wind does to erode surficial materials surficial materials ● sand blasting (ventifacts; frosting on sand grains) frosting on sand grains) ● deflation (desert pavements) 3. Wind as an Agent of Erosion

Thought questions for this chapter You have just driven a truck through a sandstorm and discover that the paint has been stripped from the lower parts of the truck, but the upper parts are barely scratched. What process is responsible, and why is it restricted to the lower parts of the truck? Compare the heights to which sand and dust are carried in the atmosphere and explain the differences and similarities?

● Sand dunes ● deserts, beaches, lake shores, floodplains, etc. floodplains, etc. ● need a ready supply of loose sand sand ● vegetation stabilizes dunes 4. Wind as a Depositional Agent

Wind as a DepositionalAgent:Linear Sand Dunes of the SouthernArabianPeninsula

Wind as a DepositionalAgent:WindRipplesatStovepipeWells,California

Wind DepositionalAgent: Sand Dunes in Gusev Crater, Mars

● How sand dunes form and move ● saltation of sand ● transverse piling of sand or sand piles up behind an sand piles up behind an obstruction obstruction ● piling and avalanche of sand 4. Wind as a Depositional Agent

Wind as a DepositionalAgent: Sand Piling Downwind of an Obstruction

4. Wind as a Depositional Agent: Formation of Wind Shadow Sands

4. Wind as a Depositional Agent: Formation of Sand Dunes

4. Wind as a Depositional Agent: Formation of Sand Dunes

4. Wind as a Depositional Agent: Formation of Sand Dunes

4. Wind as a Depositional Agent: Formation of Sand Dunes

4. Wind as a Depositional Agent: Formation of Sand Dunes

● Types of sand dunes ● barchans ● blowout dunes ● transverse dunes ●linear dunes ● linear dunes 4. Wind as a Depositional Agent

Wind 4. Wind as a Depositional Agent: Barchans

Wind Barchans are crescent-shaped dunes, always the products of limited sand supply and unidirectional winds.

4. Wind as a Depositional Agent: Blowout Dunes

Blowout dunes are almost the reverse of barchans (the horns point “backwards”).

4. Wind as a Depositional Agent: Transverse Dunes

Transverse dunes form in arid regions where there is abundant sand.

4. Wind as a Depositional Agent: Linear Dunes

Linear dunes occur in areas that have a moderate sand supply, a rough pavement, and winds in the same general direction.

● Dust falls and loess ● fallen atmospheric dust creates a deposit of loess (loess blanket) deposit of loess (loess blanket) ● loess covers 10% of Earth’s surface surface 4. Wind as a Depositional Agent

Wind as a DepositionalAgent:PleistoceneLoess in the CatalinaMountains,Arizona

Wind as a DepositionalAgent:PleistoceneLoess in Central China

Thought questions for this chapter What factors determine whether sand dunes will form on a stream floodplain? Trucks continually have to haul away sand covering a coastal highway. What do you think might be the source of the sand? Could its encroachment be stopped? Which of the following would be a more reliable indication of the direction of the wind that formed a brachan: cross- bedding or the orientation of the dune’s shape on a map? Why?

Thought questions for this chapter There are large areas of sand dunes on Mars. From this fact alone, what can you infer about conditions on the Martian surface? What aspects of an ancient sandstone would you study to show that it was originally a desert sand dune? What evidence would cause you to infer that dust storms and strong winds were common in glacial times?

5. The Desert Environment: Where the Deserts Are

● Where deserts are found ● areas of low rainfall ● rain shadows ● far from oceans ●in polar regions ● in polar regions 5. The Desert Environment

● Various factors play a role in the formation of deserts in the formation of deserts ● plate tectonics ● climate change ● human actions ●in polar regions ● in polar regions 5. The Desert Environment

● Desert weathering phenomena ● desert colors (rusty, orange brown) brown) ● desert varnish ● stream erosion (when water is present) is present) 5. The Desert Environment

TheDesertEnvironment: Rusty Colors and Desert Varnish (carved by aboriginalpeople),Canyonlands,Utah

TheDesertEnvironment:DesertStreamErosion (when water is present), SaguaroNational Monument, Arizona

TheDesertEnvironment:DesertStreamErosion (when water is absent), SaguaroNational Monument, Arizona

TheDesertEnvironment: Desert Playa Lake, Death Valley,California

● Desert sediments; sedimentation ● alluvial sediments ● eolian sediments ● evaporite sediments (playa and playa lakes) and playa lakes) 5. The Desert Environment

● Desert landscapes ● playa ●desert pavements ● desert pavements ●dune fields ● dune fields ● dry washes (wadis) ● pediments 5. The Desert Environment

TheDesertEnvironment:DesertPediment, Cima Dome, MojaveDesert,California

Downfaulted lowlands Downfaulted lowlands Elevated mountains Elevated mountains Fault 5. The Desert Environment: Formation of a Pediment Time 1 The lowlands are downfaulted, and the mountains are elevated.

Stream floodplain Stream floodplain Early erosion of mountains Early erosion of mountains Alluvial fans Time 2 Erosional debris is deposited as alluvial fan and stream floodplain sediments. 5. The Desert Environment: Formation of a Pediment

Pediment Alluvial deposits 5. The Desert Environment: Formation of a Pediment Time 3 Erosion produces a pediment with thin covering of alluvial deposits.

Pediment Mountain remnants Mountain remnants Time 4 Continued erosion produces a more extensive pediment. 5. The Desert Environment: Formation of a Pediment

Thought questions for this chapter What features of a desert landscape would lead you to believe it was formed mainly by streams, with secondary contributions from eolian processes? How does desert weathering differ from or resemble weathering in more humid climates? What kinds of landscape features would you ascribe to the work of the wind, to the works of streams, or to both?

Deflation Desert pavement Desert varnish Desertification Dry wash DustEolianLoessPedimentPlaya Playa lake Sandblasting Slip face VentifactWadi Key terms and concepts