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Deserts and Wind

What is a desert?Where and why do deserts form?The action of wind in deserts.Structures in wind blown sand.

Desert Mars

Desert: any area where annual precipitation is below 250 mm.

What is a desert?

A better definition…..

Desert: any area where annual precipitation is less than potential evaporation.

Includes hot areas with little precipitation and cold regions with low rates of evaporation (Arctic and Antarctic).

Approximately 30% of Earth’s land surface.

The global distribution of non-polar deserts.

Most common 30 degrees north and south of the equator.

Where do deserts develop? Causes of desert formation

Hadley cells: a part of the structure of atmospheric circulation.

Warm moist air rises upwards at the equator and 60 degrees north and south.

Dry air descends at 30 degrees and 90 degrees north and south of the equator.

Deserts are common 20-30 degrees north and south of the equator due to the downward flux of dry Air.

The global distribution of non-polar deserts.

Rain Shadows: in the lee (downwind) of mountains.

Warm, moist air blowing towards a mountain belt rises.

As it rises it cools and the moisture condenses to fall as precipitation.

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Air that moves over the mountains is very dry.

The land in the path of the dry air has potential to become a desert (depending on temperature and evaporation rates).

North American deserts are largely in the lee of the Rockies.

Mojave Desert, USA

Cold Ocean Currents: can cause deserts to form in coastal area.

Upwelling of cold ocean currents cools the overlying air.

Landward winds are dry and have potential to form deserts.

The Peruvian and Namib deserts are coastal deserts where cold waters well up from the deep ocean.

Peruvian Desert

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Peruvian DesertDry continental air: winds that blow over long stretches of land with little water.

In the central regions of continents the air tends to be dry, having lost its moisture to precipitation along the way.

The Gobi desert is inland, well away from oceans, and prevailing winds have passed over the Himalayas (rain shadow).

Gobi Desert Australia: about 35% of the continent receives less than 250 mm of rain per year.

Wind

direction

High temperatures and evaporation rates yield a vast desert, inland away from the ocean.

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Uluru (formerly Ayer’s Rock) rises 318m above the surrounding land surface and is 8 km in circumference.

It is an outcrop of arkosic sandstone that has been eroded by desert processes.

Lake Eyre covers up to 9,500 km2 when it is full but its maximum depth is only 6 m; the lake is 17 m below sea level.

This large marginal desert lake is a “salt lake”.

Waters collect during rain storms and evaporates to concentrate the salts that are in solution in the lake waters.

Two or more mechanisms may act together to result in a desert.

Human activity also contributes to the growth of deserts in marginal areas flanking true deserts.

After decades of relative climatic stability the Sahel began to experience drought in the 1960s.

The Sahel is an area extending across Africa, just south of the Sahara Desert.

Over the long term, the average rainfall varies from close to 500 mm/yr along the southern Sahel to less than 250 mm/yr at the Sahara.

Concurrent with the decrease in rainfall was an increase in the area of the area of the Sahara Desert.

With decreasing rainfall the total area of the Sahara desert increased by up to almost 1,300,000 km2.

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Grazing by domestic animals removes the sparse plant life that helps to bind the soil and hold moisture in it.

Agricultural practices in the Sahel were thought to have led to the southern migration of desert conditions.

Runoff during precipitation events is very rapid and little water is retained.

The situation raises the evaporation rates and reduces the amount of water retained.

Rainfall reduction ranges from 20 to 50% in the Sahel area.

A simulation of global climate that included interactions betweensulphur dioxide emissions and cloud formation.

The actual cause of the draught is thought to be due to a reduction in ground and sea surface temperatures due to the pollution’s tendency to reflect sunlight back to space. Rainfall is now on the rise and the desert is retreating

northward….possibly due to North American clean air laws.

Lower temperatures reduce evaporation rates over the ocean (which would fall as rain on the Sahel).

Reduced temperature differences between the land and the ocean reduce winds that bring the moisture over the land.

The action of wind in deserts

While we often think of deserts as being dominated by sand dunes, they actually make up a small part of most deserts

Subenvironments of the Sahara

In each subenvironment wind is the major process of change.

Wind plays a major role in erosion and weathering in desert areas (there is little water available for either).

Erosion by wind involves the blowing of sand from the surface.

Many deserts areas are characterized by desert pavements (also called deflation surfaces)

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Time 1

Desert pavement forms when wind erodes a surface made up of a wide range of particle sizes (gravel down to silt).

Fine grains are moved from the location by the wind whereas the heavy, large particles remain in place.

Time 2Time 1

Over time, the sand is slowly removed from the surface and it becomes enriched in the coarser particles (a pavement).

As the sand is removed the surface is lowered due to the loss of solid material.

Time 2 Time 3Time 1

The lowering of the surface led to the term deflation surface.

Eventually the gravel cover on the surface excludes the sand andsilt from the wind and the process stops.

Sediment transport by wind is largely by creep (gravel), saltation(sand) or suspension (silt and dust).

Creep involves the sliding of coarse particles over the ground surface.

Creep is driven by the wind, often in conjunction with impacts against the coarse particle by many smaller particles.

Saltation is a form particle motion whereby:

Particle rises steeply off the surface.

It is carried downwind as it descends.

(The path is said to follow a ballistic trajectory)

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A particle sitting on a boundary under a flowing current of air resists motion by the air due to its own weight (W) holding it in place.

The particle will move only if the force of the wind is strong enough to exceed the particles weight.

The air velocity that is “felt” by the particle varies from approximately zero at its base to some higher velocity at its highest point.

Pressure is also imposed on the particle and the magnitude of the pressure varies inversely with the velocity:

Higher velocity, lower pressure.

Maximum pressure is exerted at the base of the particle and minimum pressure at its highest point.

The pressure on the particle varies symmetrically from a minimumat the top to a maximum at the base of the particle.

This pressure distribution results in a net pressure force that acts upwards.

Thus, the pressure force (known as the Lift Force) acts oppose the weight of the particle (reducing its effective weight).

This makes it easier for the wind to roll the particle along the bed…or……..

If the particle remains immobile to the air’s flow and the velocity reaches a high enough level so that the Lift force exceeds the particle’s weight….it will jump straight upwards away from the bed.

Once off the bed, the pressure difference from top to bottom of the particle is lost and it is carried downwind as it falls back to the bed….

Following a ballistic trajectory.

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The first grain collides with the surface, knocking other particles up into saltating trajectories.

When just a few grains begin to move like this they start millions of grains moving in a very short time.

The amount of sand in transport varies with the wind speed.

Winds as low as 60 km/hr can move up to one ton of sand per meter over sand dunes.

Fine particles (silt and clay) can be taken into suspension by wind and carried into the upper atmosphere where they can be circulated world-wide.

Dust storms are due to strong winds that take up a large concentration of fine particles into suspension.

Dust storms can move sediment for hundreds to thousands of kilometers over a couple of days.

A correlation between asthma attacks and dust storms in the Sahara has been explained by global circulation of desert dust.

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The worst dust storms in North American history took place during the 1930s.

The deposits of dust storms are very fine grained and called Loess. Blowing sand also acts to cause weathering: the breakdown of a parent rock.

The impact of the moving solid particles on objects, such as rocks, slowly chips away material at the surface.

Linear outcrops of relatively hard rocks are molded into blunt forms by the constant bombardment by moving sand grains.

The blunt end of the feature faces into the wind.

Such outcrops are termed Yardangs.

It has been suggested that the Great Sphinx of Giza is, in large part, made up of a Yardang, that was modified by the Egyptians.

Other features caused by wind weathering include tapered rock columns.

These are narrower at the base than the top.

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The concentration of sand in transport is greatest near the ground, decreasing upwards.

Therefore, abrasion is greatest near the ground, decreasing upwards.

Structures in wind-blown sand

Structures like sand dunes are collectively termed bed forms.

Sediment moves over bed forms and becomes incorporated with in them.

Bed forms are structures in sand and silt that are molded by the interaction of a fluid (air or water) with the mobile bed.

Wind ripples are small, asymmetric bedforms that migrate in the direction of the wind when the wind is carrying sand.

Stoss slope: the long, low slope facing the wind.

Lee slope: the steep (about 30 degrees) slope on the downwind side of the ripple.

Ripple crest: the highest point on a ripple.

Wind ripple crest lines are very straight and parallel to each other.

Occasionally a crest will split into two separate crests: this splitting is referred to as bifurcation.

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Ripple crests are spaced centimetres to decimetres apart.

The height of ripples (vertical distance from base to top of lee slope) ranges from less than 1 cm to a few centimetres.

Ripples migrate as sand saltates or rolls up the stoss slope. At the crest it avalanches down the lee slope.

Avalanching results in deposition and the position of the lee slope surface moves slightly in the downwind direction.

Wind ripples on sand dunes in Spirit Sands Provincial Park, Manitoba.

Ripples are common on flat, sandy surfaces and superimposed on larger bed forms.

Dunes: bed forms that are distinctly larger than ripples.

Dune exert a strong influence on the structure of wind flow overthem.

Forms a strong “eddy” in the lee of the dune.

Scour of sand in the trough at the base of the lee slope.

Avalanching of sand down the lee slope.

With every avalanche the dune migrates in the direction of the wind.

The layers of sand that are deposited within the dune are distinctly different due to differences of grain size.

In cross-section a dune displays this internal layering as a form of cross-bedding.

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Windblown dune crossbedding in 260 million year old sandstone

Dune types differ in terms of their form and behavior.

Classification of Eolian Dunes

The type of dune that develops depends on:

amount of available sand

wind speed and variability

duration of sand-transporting winds

directional variation in wind

Transverse dunes: crests perpendicular to wind direction (like giant ripples).

Spacing of crests up to many kilometres.

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Barchan dunes: solitary dunes that migrate over a surface with little sand.

Strongly curved crest with “arms” extending in the direction of migration.

Barchanoid dunes: resemble barchan dunes but occur where there is abundant sand. Individual dunes are linked laterally.

Parabolic dunes: similar to barchan dunes but arms extend in the direction of the wind.

Often called “blow out dunes” because they can develop when wind blows out a hollow into the sand surface.

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http://www.nps.gov/grsa/resources/parabolic02.htm#

Great Sand Dunes National Park and Preserve

Linear dunes: dunes with long axes extending parallel to the average wind direction.

Linear dunes have a triangular profile across the average wind direction.

Form when the prevailing winds vary in directions over about 90 degrees.

Linear dunes in Australia Star dunes: complex forms with multiple crests that radiate outward from one or more points.

Draa: a bed form that is bigger than a dune and commonly has dunes superimposed in it.

Requires a large volume of sand.

Draas vary in form, many resembling gigantic dunes.

Barchanoid Draa

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Erg: a “sea of sand” with Draas and dunes superimposed on it.

The largest Erg covers almost 500,000 km2

Desert Mars

NASA

Mercury 4,880 (38%) 0.367

Venus 12,103 (95%) 0.723

Earth 12,756 1.0

Mars 6,787 (53%) 1.52

Name Diameter (km)% of Earth in brackets

Distance from Sun(AU)1

1Astronomical Units: 1 unit is the distance from Earth to the Sun, 149,597,900 km.

Atmospheric compositionMars

95% CO23% N22% O2, H2O

Earth

78% N221% O21% CO2,Ozone, H20

Density of Martian atmosphere is 1% of Earth’s atmosphere (therefore, very low atmospheric pressure).

Surface temperatures: Average: -55°CMin.: -107°CMax.: +27°C

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NASA Mars Spacecraft and date of launch

Mariner 4, November 1964Mariner 6, February 1969Mariner 7, March 1969Mariner 9, May, 1971Viking 1, August 1975Viking 2, September 1975Mars Observer, September 1992

Pathfinder, December 1996 Mars Global Surveyor, November 1996Climate Orbiter, December 1998Mars Polar Lander, January 1999 (Lost on arrival)Mars Odyssey, April 2001Mars Express, June 2003 (Lost on arrival)Exploration Rover Spacescraft, June-July 2003.

Plate Tectonics on MarsThe planet has a metallic core.

There appears to be convection of the mantle.

Convection about the core generates a weak magnetic field.

The crust is twice as thick as Earth’s crust and that inhibits plate tectonics.

The abundance of meteorite craters indicates that plate tectonics is not operating.

Plate tectonic processes have operated in the past.

Valles Marineris: a failed rift that has been modified by erosion.

4,000 km long, 100 km wide and up to 10 km deep.

Modified by running water and landslides down its steep slopes.

A Martian analog for the East African Rift.

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Volcanoes on mars: giant volcanoes are present, far larger than anything on Earth.

Water on Mars?Very little in the atmosphere.

Polar ice caps that shrink and expand annually.

South Pole: covered by an ice cap of frozen CO2 (dry ice).

North Pole: ice cap of CO2 over water ice.

Martian North Pole partially covered by sand and dust.

Unusual landforms not seen on Earth form at the south pole as the dry ice (frozen CO2 ) sublimates.

Decay of southern CO2 ice cap.

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It has been suggested that Mars recently experienced an ice age that left much of the planet covered with ice.

It is estimated that there is enough water to cover the planet with a depth of 50-100m.

Ice crystals in the atmosphere form clouds around high volcanoes.

At present the atmospheric pressure is to low to maintain liquidwater: it would boil at temperatures that would be cold enough for it to freeze on Earth.

There was running water in the past: many dry river beds resemble active rivers on Earth.

Running water was gone by about 4 Billion years ago (based on the large number of craters not eroded by water).

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Deserts and wind on MarsMars is a desert (absolutely no precipitation of water, just some CO2

snow)

Winds: 250 km/hr winds are common with gusts up to 700 km/hr.

Due to the low density of the Martian atmosphere it requires high wind to begin to move sand and dust.

Once moving at such high speeds moving sand particles have very high kinetic energy (energy to do the work or erosion).

Desert pavements are extensive on Mars.

Earth or Mars?

Earth

Mars

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Dust storms are common and very extensive on Mars.

Mars

Earth

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Ergs include dunes of all varieties found on Earth.

Transverse Dunes

Transverse dunes migrate along the floor of a valley system.

Barchan dunes

Mars Global Surveyor,Released January 26, 2005

Name: “Alien Head”

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