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.
Introduction to deserts
Compare thisto Vancouver: approximately 110 cm a year
evaporation > precipitation
Strong daily winds
Hard rocky surfaces
Only 15% of the world' s desert surface is pure sand.
(Parts of the Sahara and Arabia desert)
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.
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)
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.
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!
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
Namib Desert Atacama Desert
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 picks up light surface materials and carry them in suspension
Saltation: a method of transport where particles roll and bounce along the ground
Sparse vegetation = strong winds
Lighter materials: silt, clay
- carried higher up by wind
- carried close to the ground by stronger wind
- Grinds and scours
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)
Desert occupies about one fifth to one third of the earth' s surface.
Alluvial fan (did you get this right?)
Desert sand dunes and landforms
A dune is any accumulation of
sand-size, windblown materials.
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
granules (grains of
through both water
and wind (eolian)
Individual granules form perpendicularly to the wind's direction, forming small ripples.
As more granules collect, dunes form.
Windward slope: long, gentle
Leeward slope (SLIP FACE): steep
(rolling and sliding rocks)
Quick sand is wet sand held up by the water within it
Usually not more than thigh deep
Dunes are influenced by the strength and direction of the wind
The landforms in the area
Abundant sand is available
A continuous sand ridge
Right angles to the wind
Over a small obstruction
Horns point downward
Migrate slowly in direction of wind
May be a part of transverse dunes if they align together
Forms around a blowout
Horns point up wind
Vegetation anchors horns
Thin layer of sand
Winds blow continuously one-way
Long, straight and parallel to the wind
Because of the limited precipitation, physical weathering is relatively more important
Heating-cooling, freeze-thaw and exfoliation are common
Produces angular rock materials
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
Running water is responsible for creating many of these landscapes
Alluvial fan + Alluvial fan = Bajadas
is the Arabic term traditionally referring to a valley or channel.
A dry riverbed that contains water only during times of heavy rain
Heat evaporation salt deposits
Present when playas fill with water only rainstorms causing flash floods
Deep, steep-sided valleys
E.G. Grand Canyon
Mesa:an elevated area of land with a flat top and steep sides
Buttes: isolated hill with steep, vertical sides and a small, relatively flat top
PLEATEAU > MESA > BUTTES
Sometimes also known as stone or rock pavement
Wind removes all sand from the basin (deflation) leaving behind only the underlying rock layer
Semiarid, flat-floored arid valleysurrounded by hills or mountains
A large desert basin that is slowly filling up with debris
a large, relatively flat area of desert covered with wind-swept sand with little or no vegetative cover
Formed by the accumulation of wind-blown silt