Erosion and Desert Environments

Wind as an Agent of Erosion

  • Rivers are the main force of erosion and transportation due to water while groundwater serves as storage.
  • Wind becomes the primary driver of erosion and transportation in the absence of water.

Planetary Geology and Desert Landscapes

  • Desert landscapes on Mars are studied using Earth as a reference due to known factors that create Earth's environments.
  • Aerial images from drones over Earth deserts are compared to Mars to determine if features are aeolian (wind-driven) or fluvial (water-driven).

Agents of Erosion, Transport, and Deposition

  • Wind, like water, acts as an agent of erosion, transport, and deposition.
  • The focus is on desert environments and related terminology.
  • Geomorphology courses offer further exploration of desert environments.

Causes of Wind

  • Wind is driven by the difference in pressure within the atmosphere, essentially convection.
  • Warm, moist air rises in low-pressure systems, which has a higher capacity to carry water, leading to humidity.
  • As the air rises, it cools and condenses, causing precipitation.
  • The cool, dry air then falls, creating a cycle.
  • H<em>2OH<em>2O molecules have a mass of approximately 1818, while O</em>2O</em>2 and N2N_2 are around 3030, influencing their movement in the atmosphere.

Desert Formation at 30-Degree Latitudes

  • Deserts are commonly located at 30-degree latitudes north and south due to the air cycle.
  • Air loses moisture as it moves from the equator towards these latitudes, resulting in dry conditions.
  • Temperature and humidity are the main factors influencing this pattern.

Wind Speed and Direction

  • Wind speed is driven by the pressure differential between high and low-pressure systems.
  • Wind direction is determined by where the wind originates (e.g., westerly winds come from the west).
  • The Coriolis effect, due to Earth's rotation, influences wind and water direction (to the west counterclockwise in the Northern Hemisphere).
  • Horse latitudes, around 30 degrees, were historically problematic for sailing ships due to lack of wind; horses were sometimes thrown overboard to conserve resources.

Wind Impacted Landscapes

  • Strong winds are necessary to move sediments and create wind-impacted landscapes.
  • Little vegetation and wide-open areas facilitate wind movement.
  • Loose, dry, fine-grained soils are easily transported by wind.
  • Deforestation can lead to desertification by removing topsoil.
  • Wind can move sediments in stream floodplains, beaches, deserts, and dry lake beds (playas).
  • The Salton Sea in Southern California is an example of a problematic dry lake bed with lithium deposits, creating a dilemma between dust control and mining.

Location Controls of Deserts

  • Deserts make up approximately 25% of the land surface.
  • Antarctica is the largest desert.
  • Deserts are found at subtropical latitudes, in cold water regions, and in the interiors of continents.
  • A desert is defined by aridity (low precipitation) and less than 15% vegetation, irrespective of temperature.

Hot vs. Cold Deserts

  • Hot deserts are at low latitudes and elevations, far from oceans, with extreme temperature changes.
  • Cold deserts are at high latitudes and elevations, near cold ocean currents, with temperatures below freezing.

Types of Deserts

  • Subtropical deserts: Located at 30 degrees north and south latitudes due to air circulation patterns.
  • Rain shadow deserts: Form on the leeward side of mountain ranges where air descends and warms, absorbing moisture.
  • Coastal deserts: Near cold ocean currents, which limit evaporation and precipitation (e.g., Atacama Desert).
  • Continental interior deserts: Far from moisture sources, resulting in dry conditions (e.g., Gobi Desert).
  • Polar deserts: Occur above 66 degrees latitude due to low precipitation despite the presence of ice.

Rain Shadow Deserts

  • Warm currents lead to evaporation, and coastal ranges cause uplift and cloud formation.
  • Mountains force precipitation on the windward side, creating a rain shadow on the leeward side.
  • Air warms as it descends, absorbing any remaining moisture.

Coastal Deserts

  • The Atacama Desert is the driest place on Earth due to a combination of factors: cold current, rain shadow, and subtropical latitude.
  • It is used to mimic Mars for research and is a good place to find meteorites.

Continental Interior and Polar Deserts

  • Continental interior deserts lose air moisture as it crosses landmasses (e.g., Gobi Desert).
  • Polar deserts are above 66 degrees latitude with very low precipitation.

Great Basin Example

  • The Great Basin in the Western US is a desert due to the rain shadow effect from the Sierra Nevadas and its location in the continental interior.
  • The desiccation of ancient lakebeds results in economic opportunities and health hazards downwind.
  • Basin and range topography leads to erosion and deposition.

Death Valley Example

  • Death Valley is the lowest point in North America (-282 feet) near the highest point in the 48 states.
  • High elevation leads to high erosion rates, with the low-lying area serving as a depositional environment.
  • The area is characterized by vast temperature extremes, with the highest temperature recorded in the Western Hemisphere.

Aeolian Processes

  • Aeolian refers to wind-driven processes.
  • Wind, like water is influenced by velocity that determines the size and class of particles it can carry and transport.
  • Electrostatic forces between clay and silt particles make them harder to pick up.
  • Saltation involves particles bouncing along the surface.
  • Suspended particles can travel long distances in the air.
  • Wind can erode and deposit sediments in the same environment, shaping the desert landscape.

Weathering in Deserts

  • Physical weathering is more dominant than chemical weathering due to the lack of water.
  • Salt wedging is a form of physical weathering.
  • Desert environments lack rainfall and vegetation, exposing bedrock and geologic formations.
  • Trace elements can create dynamic colors.

Wind and Water Interaction

  • Wind is the main driver, but water (though infrequent) is a dominant force in shaping desert landscapes.
  • Violent windstorms can pick up larger particles and cause significant problems.

Terminology for Desert Deposits

  • Lag deposits: Coarse sediments left behind after wind removes finer particles.
  • Deflation: Lowering of the land surface due to wind erosion.
  • Pans: Small basins formed by deflation.
  • Ventifacts: Small rocks sandblasted by wind.
  • Yardangs: Linear ridges of bedrock carved by wind.

Sand Dunes

  • Particles roll up the windward side of dunes and fall down the leeward side.
  • Dunes migrate over time due to wind direction.
  • Cross-bedding indicates wind direction.

Dune Types

  • Barkan dunes: Limited sediment supply, horns pointing downwind, unidirectional wind.
  • Longitudinal dunes: Bidirectional winds, minimal vegetation, parallel to mean wind vector.
  • Transverse dunes: Common, parallel dunes perpendicular to prevailing wind.
  • Parabolic dunes: Horns point upwind due to vegetation anchoring.
  • Star dunes: Formed by winds from multiple directions.

Desert Varnish and Other Features

  • Desert varnish is a coating of iron and magnesium oxides on rocks, often with pictographs (paintings) or petroglyphs (carvings).
  • Talus aprons are piles of unconsolidated, angular rocks near their source.
  • Alluvial fans are fan-shaped deposits at the base of mountains.
  • Playas are desert lakes with no outlet, concentrating minerals like gypsum and salt.

Cliff and Slope Formation

  • Resistant layers (sandstone, limestone) form cliffs, while weaker units form slopes.
  • Headward erosion applies to desert environments.
  • Mesas erode into buttes and then chimneys.
  • Hoodoos are tall, long features.

Dip Slopes, Cuestas, and Hogbacks

  • Dip slopes are tilted rock units.
  • Cuestas have a low dip angle (under 30 degrees).
  • Hogbacks have a steeper dip angle (greater than 30 degrees).
  • Water gaps are areas where water flows through ridges.

Inselbergs

  • Inselbergs are isolated rock formations in a range.
  • Faulting in basin and range topography creates room for sediment deposition.

Desert Pavement Formation

  • Desert pavement is formed by lag deposits (coarser stuff left behind).
  • Fine-grained sediments are blown away.
  • Soil accumulates under the pavement.
  • The Brazilian nut theory suggests larger rocks rise to the surface through movement.

Desertification

  • Aridification (creating more deserts) can occur naturally or due to human activity.
  • Natural causes include changes in temperature, humidity, and tectonics.
  • Human causes include overpopulation, deforestation, and careless agriculture.
  • Rainforest soils are poor due to high rates of water flow, making deforestation for farming unsustainable.

Wind Transport of Nutrients

  • Sediments from the Sahara are blown over the Atlantic Ocean to replenish the rainforest with phosphorus.
  • A satellite was launched to observe dust clouds and their thickness.

The Salton Sea Example

  • The desiccation of the Salton Sea leads to toxic dust storms.