Desert Formation and Characteristics

Study Notes

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.

Description

Explore the fascinating world of deserts, focusing on their formation, location, and unique characteristics. Learn how atmospheric circulation patterns influence desert climates and the role of trade winds and mountain ranges in creating these arid landscapes.