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Deserts. PART 1 Introduction 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.

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slide2

PART 1

Introduction to deserts

(not desserts!!!)

slide4
The common definition of “desert” is a region that receives less than 25cm of rain a year on average

Compare thisto Vancouver: approximately 110 cm a year

Total yearly

evaporation > precipitation

tropical desert characteristics
Tropical desert characteristics

Sparse vegetation

Strong daily winds

Extreme heat

Flash floods

Hard rocky surfaces

did you know
Did you know?

Only 15% of the world' s desert surface is pure sand.

(Parts of the Sahara and Arabia desert)

rainfall cycle
Rainfall cycle

Evaporation 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 latitude
1) Latitude

The 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)

slide11
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 barriers
2) Mountain barriers

Winds carry moisture-laden air into the mountains where it is forced to rise over mountain barriers

Rising air cools, water vapours condense, and heavy rain falls

Moving past the range, the air descends, heats up and driesout

 No clouds and sunny skies on the other side!

3 cold ocean currents
3) Cold ocean currents

Warm 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

4 ocean influence
4) Ocean influence

Lands 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

wind erosion 1
Wind erosion 1

Wind 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 2
Wind erosion 2

Sparse 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

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)

did you know1
Did you know?

Desert occupies about one fifth to one third of the earth' s surface.

slide29
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?)

slide32

PART 2

Desert sand dunes and landforms

what is a dune
What is a dune?

A dune is any accumulation of

sand-size, windblown materials.

where are dunes found
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 dunes
Desert sand dunes

Individual 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.

slide36

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 know2
Did you know?

Quick sand is wet sand held up by the water within it

Usually not more than thigh deep

but first please note that
But first… please note that…

Dunes are influenced by the strength and direction of the wind

The vegetation

The landforms in the area

1 transverse dune
1) Transverse dune

Abundant sand is available

A continuous sand ridge

Right angles to the wind

2 barchan dune
2) Barchan dune

Crescent-shaped

Over a small obstruction

Horns point downward

Migrate slowly in direction of wind

May be a part of transverse dunes if they align together

3 parabolic dune
3) Parabolic dune

Crescent-shaped

Forms around a blowout

Horns point up wind

Vegetation anchors horns

4 longitudinal dune
4) Longitudinal dune

Thin layer of sand

Winds blow continuously one-way

Long, straight and parallel to the wind

physical weathering
Physical weathering

Because of the limited precipitation, physical weathering is relatively more important

Heating-cooling, freeze-thaw and exfoliation are common

Produces angular rock materials

paradoxically
Paradoxically,…

Water is the chief agent of erosion in deserts

Heavy rains occur during intense thunderstorms

Hard, bare ground is easily eroded by run-off

Few plants= cannot break the fall of raindrops, slow down water’s movement, and hold on to the soil

a flash flood in southern utah
A flash flood in Southern Utah

http://www.youtube.com/watch?v=kV4aF4AZtY0

unique desert landforms
Unique desert landforms

Running water is responsible for creating many of these landscapes

alluvial fans and bajadas
Alluvial Fans and Bajadas

Alluvial fan + Alluvial fan = Bajadas

wadis
Wadis

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

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

playa lakes
Playa Lakes

Shallow lakes

Heat  evaporation  salt deposits

Present when playas fill with water only rainstorms causing flash floods

canyons
Canyons

Deep, steep-sided valleys

Sedimentary rocks

Vertical erosion

E.G. Grand Canyon

mesas and buttes
Mesas and Buttes

Mesa:an elevated area of land with a flat top and steep sides

table-top shape

Buttes: isolated hill with steep, vertical sides and a small, relatively flat top

PLEATEAU > MESA > BUTTES

hamadas
Hamadas

Sometimes also known as stone or rock pavement

Wind removes all sand from the basin (deflation) leaving behind only the underlying rock layer

bolson
Bolson

Semiarid, flat-floored arid valleysurrounded by hills or mountains

A large desert basin that is slowly filling up with debris

slide60
Ergs

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

loess
Loess

Formed by the accumulation of wind-blown silt