The UNESCO index uses ratio values of P/ETP to delineate four broad arid regions.
semihumid zones (0.50 < P/ETP < 0.75)
semi-arid zones (0.20 < P/ETP < 0.50)
arid zones
(0.03 < P/ETP < 0.20)
hyper-arid zones
(P/ETP < 0.03)
5 major regions of aridity
Desert Climate
Have Arid
Conditions Been
Continuous?
SAHARA DESERT EXAMPLE
During the Ordovician time period (505 to 438 mya)
the Sahara was actually located at the South Pole
(indicated by paleomag evidence). Well preserved
striations, crescentic gouges, and boulder erractics.
In the early Tertiary Period (66 to 50 mya) there was
a long interval of intense weathering which probably
marked a period of much more humid conditions.
In the Late Cenozoic Era (40 mya) is when we
begin to see the onset of aridity in the Sahara.
STS036-152-252 Tassili-N’-Ajjer (Plateau), Algeria March 1990
The Tassili-N’-Ajjer, located in southeastern Algeria and considered part of the northern Sahara
Desert, is a series of eroded sandstone plateaus (darker gray areas). Both the Tifernine Dune
field and the larger area of dunes to the east result from complex wind regimes that prevail
throughout this arid region that receives less than 4 inches (10 cm) of precipitation per year.
Have Arid Conditions Been Continuous?
Sahara Desert Example
During the Pleistocene (last 1.8 millions
years) we have seen patterns of climatic
oscillation.
90,000 years of aridity
Dust
Senegal River Deposits
The last 10,000 years experience:
1. moister conditions (glaciers recede)
2. Around 4 to 5 kya there are drastic
change back to more arid and semi-arid
plant species in the fossil records.
Have Arid Conditions Been Continuous?
Sahara Desert Example
•
Take home message:
–
–
–
–
Present day arid landforms and landforms of the
inheritance (relict);
Temporal scales of temperature fluctuations long and
short have impacts on aridity;
Multiple environmental components leading to the
present day formation of arid conditions; and
Regardless of the desert environment, there have been
similar environmental change, but not exactly like the
Sahara Desert.
Causes of Present Day Aridity
1. Trade Winds
The major low
latitude deserts
consist of parts
of the Sahara,
Kalahari and
Arabian Deserts
and all of the
Great Sandy
Desert of
Australia.
Causes of Present Day Aridity
2. Semi-permanent
High Pressure
Zones
Deserts commonly
associated with the
subtropical highs
are major portions
of the Sahara,
Kalahari and
Arabian Deserts,
Thar (Pakistan),
Syrian, Lut (Iran),
Gibson, Great
Victoria and
Simpson Deserts
of Australia.
Causes of Present Day Aridity
3. Rainshadow -- Examples of deserts created by rainshadows are the Great Basin
(Southwestern U.S.), Takla Makan (Xinjiang Uygur, China) and the Gobi Desert.
Tarim Basin
Kunlun Mtns
4. Distance from
a moisture
source
the Kara
Kum
(Turkmen),
Kyzyl Kum
(Uzbek) and
Sary
Ishikotrau
(Kazakh)
deserts of
central Asia.
Causes of Present Day Aridity
5. Proximity to Cold Ocean Currents -- The currents that create some of these deserts are the
Humboldt Current, which has its origins in the Antarctic region, helps to form the Atacama Desert (the driest
desert in the world) the California Current, which has its origins near the Aluetian Islands, helps propagate the
fog deserts of Baja California and finally the Benguela Current originating in the Antarctic, it flows along
southwestern Africa where the Namib Desert is located.
DESERT MORPHO-STRUCTURAL
CHARACTERISTICS
Pediment and Inselberg
Reg
Reg from Libya
1.
2.
3.
Gibber from Australia
Why should we care?
Important from an archeological
perspective (pictograms and tools)
micro-laminations in rock varnish indicator
of paleo-climate, and
soil protection in arid environments.
Ergs
Ergs
Cuesta-Form Landscapes
Cuesta-form Landscapes
Cuesta-Form Landscapes
Hoodoos
Cuesta-Form Landscapes
Arches
Basin and Range
Zagros Mtns., Iran
Alluvial Fans
HORST
ALLUVIAL
FANS
GRABEN
PLAYA
HORST
SPACE SHUTTLE RADAR IMAGE OF DEATH VALLEY
Playa