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

Tropical desert characteristics  Sparse 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 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.

 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)

 Deserts occurs in 2 broad belts: at 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 dries out  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

How does wind affect deserts?

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

What is a toadstool?

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 2 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

 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.

 Dunes are the most common depositional landform of the desert  Windward slope: long, gentle (saltation)  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

There are four types of domes…

 Dunes are influenced by the strength and direction of the wind  The vegetation  The landforms in the area

 Abundant sand is available  A continuous sand ridge  Right angles to the wind

 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

 Crescent-shaped  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

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,…  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

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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

 Shallow lakes  Heat  evaporation  salt deposits  Present when playas fill with water only rainstorms causing flash floods

 Deep, steep-sided valleys  Sedimentary rocks  Vertical erosion  E.G. Grand Canyon

 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

 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 valley surrounded 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