Lecture G102/201 Deserts
What do you think of when you think about a desert?
What is a desert?
Evaporation is greater than precipitation
Many arctic regions fall into this category
Net motion of water in a desert is up.
Water table feeds this upward migration
Leaves dissolved ions behind and creates salt and mineral deposits
caliche
Where are most of the deserts in the world located?
Why are many located between 20-30o latitude?
Humidity and rain
Absolute humidity: Actual amount of water present in an air mass.
Water Vapor capacity: the amount of water an air mass can hold at any given temperature
Relative humidity: amount of water present vs how much water the air can hold.
ex//
If an air mass has a temperature of 50oF, what is the H2O vapor capacity of the air mass?
If the air mass has 3 grams of water vapor in it what is the relative humidity
What is the Dew point of an air mass?
Dew point is the point where the amount of water the air mass is holding is the most it can hold…100%
Why are many located between 20-30o latitude?
Equator = warm air (can hold lots of H2O)
Warm air rises and cools—Reaches dew point
Air from equator sinks at about 20-30o
Cold sinking air warms as it decends. Retains water in air mass rather than releasing it as rain.
Air from 20-30o flows to equator or north to ~60o
Air rises and reaches dew point as it cools
Air moves north or south where it drops again
Principle Causes of Deserts
Tropical Deserts
Low latitudes (20-30o)
Created by descending air masses at subtropical highs
Examples: Sonoran, Chihuahuan, Mojave
Steppes: Semi-deserts adjacent to these regions
Orographic Deserts (any latitude)
Form on the leeward side of a mountain range.
Air mass is forced up and loses its water as it cools.
Examples?
Cool Coastal Deserts
Cool ocean air blowing over a warm land mass
Atacoma (chile), Kalahari (
Most erosion in deserts is done by water
Flash floods
High velocity carry lots of seds
Ephemeral Streams
Many deserts are intermountain basins
Located between mountain ranges
Internal drainage
Lots of valley fill
Basin and
Desert Features
Playas: Dry lakes
Often rich in salt deposits
Alluvial Fans
Slot Canyons
Bajada Slopes
Coalescing alluvial fans
Inselbergs
Wind
Moves particles by saltation and suspension
What are the size of particles moved by each?
Wind blown particles create frosting and abrasion
Features created by wind
Ventifacts: rocks polished and faceted by abrasion
Deflation: removal of soil by wind
Blowouts
Desert Pavement
(lag deposits)
Desert Varnish
Sand Dunes
Windward side
Lee side (slip face)
What is required for a sand dune to form?
Sand
Obstacle to grow on
Wind
Topography, Vegetation, quantity of sand, wind direction all determine type of dune that forms
Barchan Dune
Inland dune
Limited sand supply
Little vegetation
Arms in direction of the wind
Parabolic Dune
Inland dune
Vegetation present
Arms point against wind
Transverse Dune
Common along coasts
Sand abundant
Wind irregular
Longitudinal Dune
Slip face parallel to wind
Sand limited
Constant wind direction
Star Dunes (complex)
Inland dunes
Intermountain regions
Wind multidirectional
Don’t move around
Placement dependent on topographic features