ARID ENVIRONMENT AND ASSOCIATED PROCESSES

1. ARID ENVIRONMENT
Content Overview
1. Definition and arid environments
2. Causes of aridity
3. Sediment source and transport
4. Significance of weathering in arid regions
5. Insolation
6. Moisture
7. Salt weathering
8. Zone of net erosion
9. Slopes. Exposed lake sediment
Prepared by; Rejina Karki

2. 1. Definition and arid environment
 Arid environments normally have very low rainfall, such as deserts. Many
arid environments are hot, causing water to evaporate faster than it can be
replaced.
 A place that receives less than 25 centimeters (10 inches) of rain per year
is considered a desert. Deserts are part of a wider class of regions
called drylands.
 Deserts cover more than one-fifth of Earth's land area, and they are found
on every continent.

3. Arid
environments enc
ompass more
than 35% of the
world’s land area,
nearly 60 million
square
kilometers, and
represent the
most common
habitat on Earth
after oceans
(Mares, 1999)
Figure 1: Aridity and humidity
1. Definition and arid environment

4. 1. Definition and arid environment
 Humidity index(Ih)=
𝑤𝑎𝑡𝑒𝑟 𝑠𝑢𝑟𝑝𝑙𝑢𝑠
𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝐸𝑣𝑎𝑝𝑜𝑡𝑟𝑎𝑛𝑠𝑝𝑖𝑟𝑎𝑡𝑖𝑜𝑛
×100%
 Aridity index(Ia)=𝑤𝑎𝑡𝑒𝑟 𝑑𝑒𝑓𝑖𝑐𝑡
𝑝𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝑒𝑣𝑎𝑝𝑜𝑟𝑎𝑡𝑖𝑜𝑛×100%
 Moisture index(Im)= Ih- Ia
Classification Aridity index
Hyperarid AI < 0.03
Arid 0.03<AI < 0.20
Semi arid 0.20<AI < 0.50
Dry subhumid 0.50<AI < 0.65
Moisture Index % Climate Type
Above 100 Pre humid(A)
100-80 Humid(B4)
80-60 Humid(B3)
60-40 Humid(B2)
40-20 Humid(B1)
0-20 Moist sub-humid (C2)
0 to -33.3 Dry sub-humid (C1)
-33.3 to -66.7 Semi-arid (D)
Less than -66.7 Arid (E)
Table 2: Classification of climates according to moisture
index(%) climate type(Symbol)
Table 1; UNESCO aridity classification;

5. Figure 2; World wide distribution of Deserts
1. Definition and arid environment

6. 2. Causes of aridity
 Aridity is basically a comparison between water supply and water
need.
 Aridity may be considered as an expression in a qualitative or
quantitative manner of the dryness of an area.
Causes of aridity
Climate Tectonics Human

7. Climate
 Climate plays major role in aridity
 Arid environment are typified by variable rainfall
 Four main individual climate causes are;
1. Global atmospheric circulation
2. Continentality
3. Rain shadow created by high
mountain area
4. Cold upwelling ocean current
2. Causes of aridity

8. 1. Global atmospheric circulation
 Areas dominated by high-
pressure cells and hot
(such as the tropics of
Cancer and Capricorn) or
cold dry subsiding air (the
North and South Poles)
will have limited rainfall
capability.
Figure 3; Global atmospheric circulation
Climate
2. Causes of aridity

9. 2. Continentality
 Continentality refers to areas within the interiors of land masses away from
the influence of the sea.
 Central area of continents are dry because air moving over landmasses
doesn’t absorb large amount of water vapour.
3. Rain Shadow created by high mountain area
• Tall mountain ranges, such as the Andes, force air to rise on the windward
slopes of the mountains, as pressure decreases and the air cools,
condensation and cloud formation occurs and this produces rain.
• This results in windward slopes being much wetter than leeward slopes
and can cause arid areas to form on the leeward side.
Climate
2. Causes of aridity

10. 4. Cold upwelling ocean currents.
 Winds blowing across the ocean surface often push water away from an
area. When this occurs, water rises up from beneath the surface to replace
the diverging surface water
 Several deserts lie on the western coasts of continents where there is an
upwelling of cold water due to the circulation of wind currents.
Figure 4; showing cold upwelling ocean currents
2. Causes of aridity
Climate

11. Tectonics
 Arid environments can be found in a range of tectonic settings. These
include currently stable intraplate settings such as cratons (e.g. the
Australian Desert) and passive margins (e.g. the Namib).
1 Orogeny:
2 Continental drift:
Anthropogenic agents
• The mismanagement of land through overgrazing and soil salinization has
occurred since the Holocene increase in human population.
• Desertification, also called Desertization, the process by which natural or
human causes reduce the biological productivity of drylands(arid and
semiarid lands)
2. Causes of aridity

12. 3. Sediment source and transport
 The dominant transport processes and in situ
modification (weathering) that can occur after
deposition.
 Water is the main agent of weathering, and
lack of water slows weathering. Precipitation
occurs in deserts, only less than in other
climatic regions.
Figure-5: Simplified flow diagram of sediment
production and routing in arid environments
 Sediment in arid environment mainly transport
through following processes;
 A. Suspension
 B. Saltation
 C. Creep

13. • Physical weathering processes like thermal expansion and contraction
caused by daily temperature variations, freeze-thaw cycles (in cold
deserts), salt crystal growth, and wind abrasion are prevalent.
• Weathering in arid regions contributes to the development of unique
landforms. Features such as inselbergs (isolated rock hills or mountains),
mesas, buttes, etc.
• In arid regions, weathering can lead to changes in mineral composition.
For instance, the breakdown of feldspars and other minerals can lead to
the formation of clays and oxides, which are more stable under surface
conditions.
4. Significance of weathering in arid region

14. Split stones are commonly occurring and widely reported
features in arid regions
Figure 7-Small cobble-sized clasts that exhibit a
regular form of in-sit parallel/subparallel splitting that
cuts across geological structures such as bedding
5. Insolation
 Large temperature ranges (which can exceed
50°C, Goudie 1997) associated with the diurnal
temperatures of deserts have been attributed with
expansion and contraction of rocks insitu.
Figure6 -showing the theoretical
effects of differential heating of rock
surface and subsurface layers within
a rock and how this notionally varies
in response to diurnal patterns of
heating and cooling

15. Moisture
 Arid areas have limited water availability from rainfall, moisture from other
sources may be significant. Such sources of moisture tend to be alkali as
they are sourced from salt-rich waters such as the sea or salt lakes.
 This high pH greatly increases silica mobility during rock weathering.
Where moisture is trapped within rocks the thermal expansivity of the rock
(Goudie 1997) is increased.
6. Moisture

16. In arid environments salt can be sourced
from a number of potential areas. These
may include:
1. sea water or relict sea water which
may be found in bodies of water (e.g.
lakes) formerly connected to the sea.
2. the release of salts through rock
weathering, especially where the
bedrock is evaporitic in origin (e.g.
halite).
Salt weathering can occur through
number of processes;
Figure 8-Death Valley saltpan showing well-developed
evaporite deposits of salt with patterned ground
phenomena evident in the form of salt polygons with
clay pinnacles.
7. Salt Weathering

17. 1. Salt crystal growth:
This may result from changes in solubility with
temperature , evaporational concentration of solutions,
and mixing of salt solutions with the same ion.
Figure-9 Scanning electron
microscopy image showing
sodium chloride salt crystals
between the cleavage planes of
mica.
7. Salt weathering
2. Hydration
 Some of the largest hydration pressures occur during
anhydrite (dehydrated calcium sulphate) to gypsum
(hydrated calcium sulphate) transformation
3. Thermal expansion;
 When halite (sodium chloride) is heated from 0°C to
60°C it expands by 0.5%, whereas granite minerals
only expand by up to 0.2%. This differential expansion
can contribute to rock disintegration (Goudie 1997).

18. • Within arid environments sediment production will be dominated by source
areas susceptible to erosion. These most commonly include areas that
generate conditions of high erosivity (slopes) or areas of high erodibility
reflecting lithologies less resistant to erosion, such as lake-bed sediments.
8. Zone of net erosion
Slopes;
• These caprock may be a part of geologic sequence or it could be developed
in insitu.
• Many arid areas are associated with landforms such as pediments, cuestas
and mesas that are protected by a caprock.
• Rates of scarp retreat, and thus sediment production, vary according to the
local balance between the geological characteristics and the climate

19. Exposed lake sediment;
• Lake-bed deposits in arid regions range in thickness from a thin veneer
to hundreds of meters.
• Lake sediments contain a range of grain sizes, depending on the extent
and depth of the lake system, and the characteristics of the surrounding
catchments feeding the lake with water and sediment
• Lake sediments may have been sourced originally from terrigenous,
allochthonous sediments external to the water body.
• The finest of these lake sediments will be prone to dust erosion.
8. Zone of net erosion

20.  This is currently occurring in Lake Burdur in central Turkey, where
tectonically elevated Pliocene lake sediments are being eroded by gullying,
and redeposited in the smaller, and constantly shrinking, modern lake
system.
Figure -10 Satellite images of the Lake Burdur over time showing the water surface areal
8. Zone of net erosion
Exposed lake sediment;

21. Thank You