Understanding Earth Chapter 19: WINDS AND DESERTS Grotzinger • Jordan Seventh Edition Chapter 19: WINDS AND DESERTS © 2014 by W. H. Freeman and Company

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. Eolian processes shape the land surface in areas, such as deserts, where few plants live. 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 is unconfined, except in narrow valleys Global Wind Patterns ● Wind (the flow of air) ● wind is unconfined, except in narrow valleys ● wind obeys all the laws of flow ● wind is parallel to surface, and may go upward too

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

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

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

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

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

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

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

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

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

At the poles, the Sun’s rays are spread over greater areas, yielding colder temperatures. North Pole There is little surface wind at the equator, and the air rises, forming clouds and rain as it cools. Westerlies At the equator, the Sun’s rays are perpendicular to the surface, concentrating heat. 30°N latitude NE trade winds At 30°N and 30°S latitudes, the cooled air sinks, warms up, absorbs moisture, and yields clear skies. Equator SE trade winds 30°S latitude 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 2. Wind as a Transport Agent ● Factors in how wind carries things ● wind strength ● particle size ● surface material

Wind as a Transport Agent: Rate of Sand Movement versus Wind Speed

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

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

Wind as an Agent of Erosion: Ventifacts

Wind as an Agent of Erosion: Rounded and Frosted Grains of Sand

Wind as an Agent of Erosion: Desert Deflation Surface

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

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

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.

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

Wind as a Depositional Agent: Sand dunes

Wind as a Depositional Agent: Ripples at Stovepipe Wells, California

Wind as a Depositional Agent: Sand Dunes in Gusev Crater, Mars

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

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

4. Wind as a Depositional Agent: Barchans Wind

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

4. Wind as a Depositional Agent: Blowout Dunes

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

4. Wind as a Depositional Agent: Transverse Dunes

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

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.

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

Wind as a Depositional Agent

Wind as a Depositional Agent: Pleistocene Loess 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 barchan: cross-bedding or the orientation of the dune’s shape on a map? Why?

Thought questions for this chapter Mars has large areas of sand dunes. 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 5. The Desert Environment ● Where deserts are found ● areas of low rainfall ● rain shadows ● far from oceans ● in polar regions

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

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

The Desert Environment: Rusty Colors and Desert Varnish (carved by aboriginal people), Canyonlands, Utah

The Desert Environment: Stream Erosion (when water is present), Saguaro National Monument, Arizona

The Desert Environment: Stream Erosion (when water is absent), Saguaro National Monument, Arizona

The Desert Environment: Desert Playa Lake, Death Valley, California

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

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

The Desert Environment: Pediment, Cima Dome, Mojave Desert, California

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

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

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

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

Key terms and concepts Desert pavement Desert varnish Desertification Deflation Desert pavement Desert varnish Desertification Dry wash Dust Eolian Loess Pediment Playa Playa lake Sandblasting Slip face Ventifact Wadi