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48 Cards in this Set

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What percent of Earth's land surface is covered by arid landscapes?



26%

What percent of Earth's land surface is covered by semi-arid and arid landscapes?

35%

What are Hadley cells and why do they produce dry areas?

Large atmospheric convection currents
Produce dry areas underneath the descending part of the current
Occur around 20-30 degrees north and south
Example: The Sahara Desert


  • Large atmospheric convection currents
  • Produce dry areas underneath the descending part of the current
  • Occur around 20-30 degrees north and south

Why are places in the Lee of Mountain ranges dry?


  • Air loses its moisture as it passes over the mountains.


  • Example: Easter Washington State (and to some extent the desert southwest of the US)

Why are continental interiors dry?


  • Because they are a long way from sources of moisture (the oceans).


  • Produce deserts like the Gobi and Taklimakan deserts in China and Mongolia.

Wind Action In Deserts

Why is wind important in deserts?

Simply because there is little water.

Wind Action In Deserts

Why is wind common in arid desert regions?




  1. Air near the surface is heated and rises.
  2. Cooler air comes in to replace hot air rising.


  • This movement of air results in winds.
  • Arid regions have little or no soil moisture to hold rock and mineral fragments.

Wind Action In Deserts

Water is a much more powerful agent of landscape formation, but in its absence, even weak agents such as the __________ become significant.

Water is a much more powerful agent of landscape formation, but in its absence, even weak agents such as the wind become significant.

Wind Action In Deserts

The __________ is a fluid, like __________ only much less dense.

The atmosphere is a fluid, like water only much less dense.

Wind Action In Deserts

Wind has the ability to…?

Transport

Erode

Deposit Sediment

Note: These processes are a lot like the ones you'll find in rivers.

Wind Action Deserts - Transportation Process

What are the three processes of transportation by wind?


  1. Surface Creep (larger particles)
  2. Saltation (medium particles)
  3. Suspension (smallest particles)

Wind Action Deserts - Transportation Process

Define: Surface Creep (large particles)

The rolling and sliding of larger soil particles along the ground surface.
The movement of these particles is aided by the bouncing impacts of the saltating particles.
  • The rolling and sliding of larger soil particles along the ground surface.
  • The movement of these particles is aided by the bouncing impacts of the saltating particles.

Wind Action Deserts - Transportation Process

Define: Saltation (medium particles)

Movement of particles by a series of short bounces along the surface of the ground.
Dislodges additional particles with each impact.
  • Movement of particles by a series of short bounces along the surface of the ground.
  • Dislodges additional particles with each impact.

Wind Action Deserts - Transportation Process

Define: Suspension (smallest particles)

Fine particles are moved parallel to the surface and upward into the atmosphere by strong winds.
  • Fine particles are moved parallel to the surface and upward into the atmosphere by strong winds.

Wind Action In Deserts - Erosion Process

What are the two ways wind can erode?


  1. Deflation
  2. Abrasion

Wind Action In Deserts - Erosion Process

Define: Deflation

When the wind moves loose soil and dirt particles.
Leads to desert pavement surfaces (regular deserts).
  • When the wind moves loose soil and dirt particles.


  • Leads to desert pavement surfaces (regular deserts).

Wind Action In Deserts - Erosion Process

Define: Abrasion


  • When airborne particles cause land mass to wear away or erode.


  • An area is eroded directly by airborne particles.

Wind Action In Deserts - Erosion Process

What are two formations Abrasion can make?


  1. Ventifacts
  2. Aerodynamically-Sculpted Yardangs

Wind Action Deserts - Erosion Abrasion Process

Define: Ventifacts

Any bedrock surface or stone that has been abraded or shaped by wind-blown sediment.
  • Any bedrock surface or stone that has been abraded or shaped by wind-blown sediment.

Wind Action Deserts - Erosion Abrasion Process

Define: Yardangs

Streamlined wind-eroded ridges
Commonly found in deserts.
  • Streamlined wind-eroded ridges


  • Commonly found in deserts.

Wind Action Deserts - Deposition Process

What are two formations Deposition can make?


  1. Sand Ripples
  2. Sand Dunes

Wind Action Deserts - Deposition Process

Define: Sand Ripples

Occur as a result of the largest grains accumulating as smaller grans are transported away.
Ripples form in lines perpendicular to wind direction.
  • Occur as a result of the largest grains accumulating as smaller grans are transported away.
  • Ripples form in lines perpendicular to wind direction.

Wind Action Deserts - Deposition Process

Define: Sand Dunes

Mounds with gentle slope in the upwind direction
Steep slope (slip face) on the downwind side.
  • Mounds with gentle slope in the upwind direction


  • Steep slope (slip face) on the downwind side.


Wind Action Deserts - Deposition Process

Define: Slipface

Sand brought up the windward slope.
  • Sand brought up the windward slope.


Wind Action Deserts - Deposition Process

Define: Angle of Repose

The right steepness where the dune becomes stable. 
This angle is usually 30-34 degrees.
  • The right steepness where the dune becomes stable.
  • This angle is usually 30-34 degrees.

Wind Action Deserts - Deposition Process

What are three different types of sand dunes?


  1. Crescentic
  2. Linear
  3. Star

Wind Action Deserts - Deposition Process

How do dunes migrate?


  • Downwind as sand piles up on a gentle windward slope.


  • Drops off steeper slip face on the downwind side (Angle of Repose is important here).

Wind Action Deserts - Deposition Process

Three General Types of Dunes 1/3

Define: Crescentic (Dune Type)

Most common type of sand dune
Shaped like a crescent moon with points at each end.
It is usually wider than it is longNote: Also called Barchan Dunes
  • Most common type of sand dune
  • Shaped like a crescent moon with points at each end.
  • It is usually wider than it is long

Note: Also called Barchan Dunes



Wind Action Deserts - Deposition Process

Three General Types of Dunes 1/3

Where do Crescentic Dunes form?


When winds blow from one direction.
Example: Salton Depression, California
  • When winds blow from one direction.


  • Example: Salton Depression, California

Wind Action Deserts - Deposition Process

Three General Types of Dunes 2/3

Define: Linear (Dune Type)

Long straight dunes (snakelike)
Ridges are its prominent feature
Longer than they are wide
  • Long straight dunes (snakelike)
  • Ridges are its prominent feature
  • Longer than they are wide

Wind Action Deserts - Deposition Process

Three General Types of Dunes 2/3

Where do Linear Dunes form?

Form in areas with a limited sand supply and converging wind directions.
Example: Namib Sand Sea
  • Form in areas with a limited sand supply and converging wind directions.


  • Example: Namib Sand Sea

Wind Action Deserts - Deposition Process

Three General Types of Dunes 3/3

Define: Star (Dune Type)

Has arms that radiate out from a center pyramid-shaped mound. 
Dunes with variable slip face directions
Grow upwards instead of outward.
  • Has arms that radiate out from a center pyramid-shaped mound.
  • Dunes with variable slip face directions
  • Grow upwards instead of outward.

Wind Action Deserts - Deposition Process

Three General Types of Dunes 3/3

Where do Star Dunes form?

Where there is abundant sand and variable wind directions.
  • Where there is abundant sand and variable wind directions.

Desert Atmosphere Processes

Define: Wind Action

  • Responsible for deposition of very fine particles.


  • These large deposits are known as loess deposits, and make for fertile farmland.

Wind Action Deserts - Deposition Process

Large accumulations of sand are not common in the deserts, but when encountered are known as __________?

Ergs (sand seas)

Wind Action Deserts - Deposition Process

Define: Ergs (sand seas)

Area of large accumulation of sand.
Generally in the bottom of a huge basin.
Area of actively shifting sand dunes. 
  • Area of large accumulation of sand.


  • Generally in the bottom of a huge basin.


  • Area of actively shifting sand dunes.

Wind Action Deserts - Deposition Process

__________ action is also responsible for deposition of very fine particles at times in the past. These large deposits are known as _________ deposits, and make fertile farmland.

Wind then Loess

Wind Action Deserts - Deposition Process

Define: Loess Deposits

  • A very fine grained silt or clay


  • Thought to be deposited by the wind

Desert Fluvial Features

What can the presence of water produce in the desert?

  • Dramatic landscape features (quickly)


Remember: Water is a far more effective agent of landscape formation than wind is.

Desert Fluvial Features

Although it does not rain often in the desert...

...When it does rain, it usually rains hard.

Deserts - Fluvial Features

What are the four desert fluvial features?


  1. Wadis/Arroyos
  2. Alluvial Fans
  3. Playas
  4. Differential Erosion

Desert Fluvial Features Produced by Water

Define: Wadis/Arroyos

River channels cut into often unstable materials.
Lack of vegetation and thin or absent soil means water travels very quickly into river channels (mainly by overland flow).
Flash floods are not uncommon


  • River channels cut into often unstable materials.
  • Lack of vegetation and thin or absent soil means water travels very quickly into river channels (mainly by overland flow).
  • Flash floods are not uncommon

Desert Fluvial Features Produced by Water

Define: Alluvial Fans

Like deltas, but no lake or ocean is needed.
Rivers come pouring out of steep, narrow canyons onto plains, spread out, lose energy and deposit material. 
Just like a delta forming where a river enters the sea.
Alluvial fans are prone to flooding, ...


  • Like deltas, but no lake or ocean is needed.
  • Rivers come pouring out of steep, narrow canyons onto plains, spread out, lose energy and deposit material.
  • Just like a delta forming where a river enters the sea.
  • Alluvial fans are prone to flooding, and are therefore not good places to build a house.

Desert Fluvial Features Produced by Water

Define: Playas

Occasional Lakes that are sometimes wet, sometimes dry - depending on rainfall.
Usually the lake bed dry up to reveal salt deposits. 
Example: Bonneville Salt Flats
 
 
 
 
  • Occasional Lakes that are sometimes wet, sometimes dry - depending on rainfall.
  • Usually the lake bed dry up to reveal salt deposits.

Example: Bonneville Salt Flats





Differential Erosion

Define: Differential Erosion

  • Erosion that occurs at irregular or varying rates.
  • Caused by differences in the resistance and hardness of surface materials.
  • Softer and weaker rocks are rapidly worn away.
  • Harder and more resistant rocks remain to form ridges, hills, and mountains.

Differential Erosion

Define: Balanced Rocks (Hoodoos)


  • Tall spire of rock that protrudes from the bottom of an arid basin.
  • Typically consists of relatively soft rock topped by harder, less easily eroded stone that protects each column from the elements.

Differential Erosion: Example 1

Picture of Balanced Rocks (Hoodoos) in Goblin Valley State Park.

(similar can be found in Arches National Park)

Differential Erosion: Example 2

Picture of Monument Valley's Buttes

Landforms of differential erosion but on a huge scale. 
 
  • Landforms of differential erosion but on a huge scale.