CHAPTER 21: DESERTS AND WIND

What is the annual precipitation in your region? A desert is a region that receives less than 25 cm of precipitation per year. What is the annual precipitation in your region?

Life Support Vegetation is so sparse, and water so rare, that a desert is unable to support significant populations of animals.

Temperature Desert regions can have extreme temperatures ranges because the atmosphere contains little humidity to block the Sun's rays or retain heat at night.

Natural Climate Change Climates can change naturally. The Sahara region was humid 5,000 to 10,000 years ago

Deserts may be hot or cold, but low precipitation is a common trait

Arctic circle Some would argue that the “Arctic Desert” isn’t really a desert. Why do most cool dry areas have more vegetation than hot dry areas?

Atmospheric moisture circulation determines the location of most deserts Polar Cells Ferrel Cells Hadley Cells

Formation Air warms and contracts as it sinks closer to Earth’s surface. Evaporation exceeds condensation. Deserts form, clustered around 30°N and 30°S latitudes.

Other Factors Determining Desert Formation Not all deserts lie around 30° latitude Several other factors contribute to desert locations: • orographic effect

Other Factors Determining Desert Formation dry, cold air descending over polar regions the distance atmospheric moisture is transported

Other Factors Determining Desert Formation cold ocean current adjacent to a tropical coast

Other Factors Determining Desert Formation poor management of farmland

Other Factors Determining Desert Formation deforestation

Each desert has unique characteristics Trade Wind Deserts

Each desert has unique characteristics Mid-latitude Deserts

Each desert has unique characteristics Rain Shadow Deserts

Coastal deserts

Ripple and dune Terminology Wind is an important geological agent

Ripple and dune Terminology Suspension: Silt carried in suspension produces well-sorted deposits of progressively smaller sediments with distance.

Ripple and dune Terminology Loess: Eolian deposits of silt are left by continental glaciations Mineralogically how does loess differ from typical wind-blown sand?

Ripple and dune Terminology Desert Pavement (lag deposit of coarse sediment left after fines blow away) Yardang (rock outcrop sculpted by sand abrasion) Suggested tips to give students: Pebbles, cobbles, and boulders left behind in Desert Pavements are called Ventifacts (can be seen upper left).

Sand Dunes reflect sediment availability and dominant wind direction Abundant loose sediment (usually sand) Energy to move sediment (usually wind) An obstacle to trap sand (often a bush) A dry climate Suggested tips to give students: In this illustration, parent and daughter isotopes can migrate in and out of a sample causing "uncertainty" in the sample's specific age. The time of metamorphism, however, can be determined.

Ripple and dune Terminology Barchan (Crescentic Dune) Strong wind in one direction, small amounts of sand, limited vegetation Transverse (Crescentic Dune) Weak wind in one direction, large amounts of sand deposited perpendicular to wind, limited vegetation

Ripple and dune Terminology Parabolic Dune Arms stabilized upwind by vegetation, often start as semi-circular blowouts that elongate Longitudinal Dune Winds flowing in opposing directions, two slip faces, aligned with wind

Ripple and dune Terminology Star dune Multidirectional winds, largest dunes, grow tall instead of moving Sand seas Large regions (>125 km2) of windblown sand numerous, very large dunes, sand covers >20% of the ground surface

Sand Dunes What are the differences in shape and wind characteristics of Barchan and Parabolic dunes?

Arid Landforms Arid landforms can be shaped by water Flash floods are common Streams tend to be ephemeral, flowing only after heavy rain.

Evolution of Desert Landscapes Suggested tips to give students: The Basin and Range region of the southwestern United States (recall Chapters 11 and 15) includes many particularly well-developed examples of desert landscapes and their evolution.

What is the evidence that this area was recently wet? Playa Lakes Playa Lakes are products of rainfall and evaporation What is the evidence that this area was recently wet?

Desertification Desertification is the process by which land loses its vegetation and turns into a desert Both natural and anthropogenic factors lead to desertification. What are some of the factors?

Environmental refugees

Desertification Desertification threatens all six inhabited continents

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