Desert Formation and Characteristics

Desert Formation and Characteristics

• Approximately 30% of Earth's land surface is desert, characterized by low precipitation.
• Deserts' extreme temperatures result from low atmospheric moisture and lack of cloud cover, leading to greater solar absorption and heat emission.
• Desert location is not random; many are between 15° and 30° latitude due to atmospheric circulation patterns.
• Trade winds, created by sinking air at 30° N and S of the equator, contribute to deserts like the Sahara and Outback.
• Rain shadow deserts form when prevailing winds are forced over mountain ranges. Moisture condenses and precipitates on the windward side, leaving the leeward side dry.
• Polar deserts, in the Arctic and Antarctic, arise from sinking cold, dry air.

Atmospheric Circulation and Deserts

• Solar energy drives atmospheric and oceanic circulation, unevenly distributed across latitudes.
• High solar energy receipt occurs close to the equator, low energy at the poles.
• Latitude is the measure of an Earth circle parallel to the equator. Angle of incidence is the angle of sunlight on Earth's surface.
• Three atmospheric cells (Hadley, Ferrel, Polar) drive large-scale air circulation. The Hadley cell rises near the equator, cools, and releases moisture, then descends at around 30˚ N and S of the equator.
• The descending air creates the horse latitudes, regions with high pressure and low precipitation.
• Other desert formations can result from rain shadow effects.
• The Atacama Desert, the driest non-polar desert, is located in a rain shadow on the western side of the Andes Mountains.
• Polar cells form, producing cold, dry conditions at the poles.

Coriolis Effect and Wind Patterns

• Earth's rotation causes the Coriolis effect, deflecting air and water movements.
• Deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• This effect influences prevailing winds (Trade Winds and Westerlies) and larger scale air circulations (e.g., Hadley Cell).
• The effect causes ocean currents to spiral and create the patterns of the zones of dry air in connection with the atmospheric cells of both hemispheres.

Desert Weathering and Erosion

• Weathering in deserts is slower than in other climates due to low water availability.
• Chemical weathering is also slower due to lack of liquid water. Mechanical weathering, like ice wedging, is also lessened.
• Desert varnish is a unique weathering product, a dark brown layer of minerals on desert rock surfaces.
• Wind is a key erosion agent in deserts, carrying sediment in dust storms (haboobs) and creating loess deposits.
• Saltation, the bouncing movement of sand grains, results in well-rounded and frosted surfaces and wind abrasion of bedrock (yardangs and ventifacts).
• Sliding stones are a remarkable example of wind's erosive power; supported on thin layers of ice or water, these stones are moved across surfaces by the force of the wind.

Desert Landforms and Processes

• Alluvial fans are fan-shaped landforms created by streams emerging from mountains, spreading out and depositing sediment.
• Bajadas are aprons of alluvium along mountain fronts, formed by merging alluvial fans.
• Inselbergs are residual mountains buried by their own erosion debris.
• Playas are dry lake beds in enclosed basins, often very flat.
• Ephemeral, or intermittent streams, contain water only after rainfall, capable of creating flash floods.
• Flash floods are a significant risk in deserts due to the high volumes of water concentrated in a short period.

Sand and Dunes

• Ergs (sand seas) are regions of loose sand accumulated and sculpted by wind.
• Dunes are sculpted by wind and sand supply; their forms vary with wind direction and sand availability, including barchans (crescent-shaped), longitudinal (linear), parabolic, and star dunes.
• Cross-bedding is a characteristic feature of dunes, formed by layers of sand parallel to the wind’s direction.
• Ancient sand dunes in various locations worldwide, such as Utah, lead to deposits like the Navajo Sandstone, an example of cross bedding.

Basin and Range Region (Great Basin)

• The Great Basin is an interior drainage basin, meaning precipitation doesn't flow to the ocean.
• The Basin and Range region is a subregion, characterized by normal faults, creating horsts and grabens (block mountains).
• Landscapes transition from mountains, alluvial fans, bajadas, to subdued inselbergs across regions like the Mojave Desert.
• Desertification is the transformation of arable land into desert, often caused by unsustainable agricultural practices, overgrazing, unsustainable water use and global climate change.
• Examples include severe droughts that transform once productive agricultural areas into barren land.