Deserts

Deserts

PART 1Introduction to deserts(not desserts!!!)What is a desert really?The common definition of desert is a region that receives less than 25cm of rain a year on average

Compare this to Vancouver: approximately 110 cm a year

Total yearly evaporation > precipitation

Tropical desert characteristicsSparse vegetation

Strong daily winds

Extreme heat

Flash floods

Hard rocky surfaces

Did you know?

Only 15% of the world' s desert surface is pure sand. (Parts of the Sahara and Arabia desert) How do deserts come form?Rainfall cycleEvaporation lifts pure water up from the sea.. Onshore winds from the oceans carry air that is rich in moisture over the land. Air rises and cools.Water vapour condense and falls as rain. It provides vital sustenance before returning to the sea through ground water, streams and rivers. 1) LatitudeThe equator is an area of constant low pressure

Hot air at the equator rises and spreads (up and away) north and south before cooling, condensing and releasing its moisture over the tropical zones (high pressure belts)

Deserts occurs in 2 broad belts: at 20-30 degrees north and south of the Equator, along the Tropics of Cancer and Capricorn. Desert areas develop under the influence of the quite permanent high-pressure areas.

2) Mountain barriersWinds carry moisture-laden air into the mountains where it is forced to rise over mountain barriersRising air cools, water vapours condense, and heavy rain falls Moving past the range, the air descends, heats up and dries out

No clouds and sunny skies on the other side!

3) Cold ocean currentsWarm air over cold water air cools, condenses and falls as rain

Air drops moisture over the water

As it travels to warm land, air temperature rises

Warm air can hold a lot of moisture no cloud or rain over desert

Cold ocean currents responsible for creating:Namib Desert Atacama Desert

4) Ocean influenceLands far away from oceanic influence

The longer an air mass is travelling, the greater its chances of losing moisture

An example is the Gobi Desert

How does wind affect deserts?Wind erosion 1Wind picks up light surface materials and carry them in suspension

Saltation: a method of transport where particles roll and bounce along the ground

Wind erosion 2Sparse vegetation = strong winds

Lighter materials: silt, clay- carried higher up by wind

Heavier: sand- carried close to the ground by stronger wind- Grinds and scours

What is a toadstool?

No, silly

Even telephone poles are affected!

What is deflation?

Deflation Wind blows away sand, silt and clay so rocks underneath are exposed

Creates blowouts in Badlands

Blowouts can form oases if it is deep enough to reach an underground water source (aquifers)Badlands can have spectacular coloured rock layers

Did you know?Desert occupies about one fifth to one third of the earth' s surface. How does water affect deserts?

Rain is rare but when it happens it usually does in the form of sudden, intense thundershowers

Flash floods

Alluvial fan (did you get this right?)Alluvial fans

Alluvial fans (in colour!)

PART 2Desert sand dunes and landforms

What is a dune?A dune is any accumulation of sand-size, windblown materials.Where are dunes found?Dunes occur where there is a large supply of sand, wind to move it and a place where it can accumulate. Dunes are found on coasts, near rivers and in desert basins

Desert sand dunesIndividual sand granules (grains of sand) accumulate through both water and wind (eolian) transportation

Individual granules form perpendicularly to the wind's direction, forming small ripples.

As more granules collect, dunes form.

Dunes are the most common depositional landform of the desert

Windward slope: long, gentle(saltation)Leeward slope (SLIP FACE): steep(rolling and sliding rocks)

Did you know?Quick sand is wet sand held up by the water within it

Usually not more than thigh deep

There are four types of domesBut first please note thatDunes are influenced by the strength and direction of the wind

The vegetation

The landforms in the area1) Transverse duneAbundant sand is available

A continuous sand ridge Right angles to the wind

2) Barchan duneCrescent-shapedOver a small obstructionHorns point downwardMigrate slowly in direction of windMay be a part of transverse dunes if they align together

3) Parabolic duneCrescent-shapedForms around a blowoutHorns point up windVegetation anchors horns

4) Longitudinal duneThin layer of sand

Winds blow continuously one-way

Long, straight and parallel to the wind

Try not to meanwhile in class

Physical weatheringBecause of the limited precipitation, physical weathering is relatively more important

Heating-cooling, freeze-thaw and exfoliation are common

Produces angular rock materials

Paradoxically, Water is the chief agent of erosion in desertsHeavy rains occur during intense thunderstormsHard, bare ground is easily eroded by run-off Few plants = cannot break the fall of raindrops, slow down waters movement, and hold on to the soil

A flash flood in Southern Utahhttp://www.youtube.com/watch?v=kV4aF4AZtY0

A reminder

Unique desert landformsRunning water is responsible for creating many of these landscapesAlluvial Fans and BajadasAlluvial fan + Alluvial fan = Bajadas

Wadisis the Arabic term traditionally referring to a valley or channel.

A dry riverbed that contains water only during times of heavy rain

Playa LakesShallow lakesHeat evaporation salt depositsPresent when playas fill with water only rainstorms causing flash floods

CanyonsDeep, steep-sided valleysSedimentary rocksVertical erosionE.G. Grand Canyon

Mesas and ButtesMesa: an elevated area of land with a flat top and steep sides table-top shapeButtes: isolated hill with steep, vertical sides and a small, relatively flat topPLEATEAU > MESA > BUTTES

Deserts and Desserts

HamadasSometimes also known as stone or rock pavementWind removes all sand from the basin (deflation) leaving behind only the underlying rock layer

BolsonSemiarid, flat-floored arid valley surrounded by hills or mountains A large desert basin that is slowly filling up with debris

Ergsa large, relatively flat area of desert covered with wind-swept sand with little or no vegetative cover

LoessFormed by the accumulation of wind-blown silt

And thats all folks!