Transcript Hot deserts and their margins

Learning episodes-1
Location and characteristics of hot deserts and their margins
(arid and semi-arid) – climate, soils and vegetation.
Causes of aridity – atmospheric processes relating to pressure,
winds, continentality, relief and cold ocean currents
Arid geomorphological processes: mechanical weathering.
The importance of wind – erosion: deflation and abrasion;
transportation; suspension, saltation, surface creep,
deposition.
Landforms resulting from – wind action: defletion hollows,
yardangs, zeugen and sand dunes –
1
Learning episodes-2
The importance of water – sources: exogenous, endoreic and
ephemeral. The role of flooding
Landforms resulting from water action: pediments, inselbergs,
mesas and buttes, salt lakes, alluvial fans, wadis and badlands.
Desertification: distribution of areas at risk, physical and
human causes, impact on land, ecosystem and populations.
Case study of desertification in the Sahel
Managing hot desert environments and their margins – to
consider and evaluate the strategies adopted with regard to
land use and agriculture in areas such as the Sahel and contrast
with the development of areas such as south western USA or
southern Spain. Implications and potential for sustainability. 2
JUN10: Distinguish between arid and semi-arid
areas. (2)
Arid areas receive less than 250mm of rainfall
per year (1), whilst semi-arid are those areas
receiving between 250 and 500mm rainfall per
year (1). The semi-arid areas are found on the
fringes of arid areas – the deserts (1).
Describe the distribution of arid and
semi-arid areas shown in Figure 7.
These areas mainly occur between 30 degrees
north and south of the Equator. With the
exception of an area in east Africa, they are not
found at the Equator. They are on the western
sides of continents and extend inland to varying
degrees – this is most limited in South America
and most extensive cross North Africa, across the
Middle East into Asia. The semi-arid areas are
generally on the edges of the arid ones. These
areas are most extensive in North Africa, Asia and
Australia. 3×1.
JAN 13: Describe the location of areas
of hot deserts (no map) (4)
Hot deserts are located near to the lines of the
Tropics of Cancer and Capricorn between
approximately 20° and 30° north and south. They
are usually found on the western side of
continents – such as the Atacama in South
America and Sonoran Desert in North America.
Sometimes, they extend the width of the
continent – as in the case of the Sahara in North
Africa. There are no deserts present in the vicinity
of the Equator. Hot deserts are present in all
continents, with the exception of Europe and
Antarctica. Inland if qualified. In lee of mountains.
Allow up to two for naming locations. 4×1
Negative water balance
Amount of rainfall not enough to define hot
deserts. Need to link hot deserts to the water
balance.
Key characteristic 2: very high rate of potential
evapotranspiration
Negative water balance since the amount of
precipitation received is much less than the
potential evapotranspiration .
Aridity Index
1.2 Characteristics of hot deserts and
their margins (arid and semi-arid) –
climate, soils and vegetation.
(a) Climate
Figure 3 Climate graph for
In Salah, Algeria
• Yearly Range is high (13 –
37 deg C°)
• Diurnal range is high
(daily) up to 30 deg C°
• Level of rainfall < 250mm
/ year
• Rainfall is sporadic
• Level of humidity is low.
JUN09 Describe the characteristics of
the hot desert climate. (4)
Temperatures are characterised by large annual ranges (1)
with figures to support related to location (1). Need to say
more than hot - needs an appropriate figure, e.g. approx 20
to 30 + degrees C (1). Diurnal range is also high (1). 15 - 30º
C (1). Very high temperatures have been recorded in the
shade - upper 50’s in Death Valley and Libya (1).Rainfall is
low - usually defined as less than 250mm p.a.(1), but often
much lower, e.g. Death Valley averages 40mm per year (1). It
is also unreliable - there may be a number of years without
any rainfall (1). Often very intense, and thunderstorms
when it does fall (1). Low relative humidity. Winds tend to
be local and seasonal (1) e.g. harmattan- a dry, dusty
easterly or northeasterly wind on the West African coast,
occurring from December to February-. (1). 4 x 1
Soils: key words
• Organic: Derived from living things eg plants,
animals.
• Inorganic: Derived from non living things eg
weathered rock.
• Humus: The amorphous, ordinarily dark-colored,
colloidal matter in soil; a complex of the fractions
of organic matter of plant, animal, and microbial
origin that are most resistant to decomposition.
The stuff that plants need to grow.
Main characteristics of a desert soil
(aridisol):
- yellow-red to grey brown
in colour
-very thin
-low organic content (under
2%)
-high pH value, 7.0-8.5
A profile is a vertical section cut through a soil
from the surface to the underlying bedrock. It
reveals a number of layers, or horizons, each with
its own characteristics.
Why?
Soil is a mixture of organic and inorganic
material.
Deserts tend to have very poorly developed
and thin soils, due to:
- lack of moisture (hinders production of
humus from dead vegetation)
- high rates of evaporation
- spare vegetation cover. (lack of organic
matter)
The climate is too dry and the vegetation too
sparse for any significant chemical weathering
of bedrock or the accumulation of organic
material.
In the few places where the water table is close
to the surface, moisture can be drawn
upwards by capillary action.
Salts and mineral bases (magnesium, calcium
and sodium) are also drawn upwards and
deposited in the upper layers to give a slightly
alkaline soil.
What is vegetation like in a hot desert environment?
Vegetation has to have a high tolerance to the moisture budget deficit,
intense heat and, often salinity
- Few species, sparsely distributed (sporadic cover)
Little can grow due to the low level of precipitation, the high
level of evaporation, high temperatures and strong
sunshine.
They are xerophytes
-: They are adapted to withstand drought.
Succulents are part of this group. They can
store water in their tissues. Many succulents
have fleshy stems and some have swollen
leaves. Cacti absorb large amounts of water
when it rains. Transpiration takes place from
the stems, but it is reduced by the stomata
closing during the day.
-Most plants have small, spiky or waxy leaves to
reduce transpiration. Some plants have silver
coloured leaves which reflect sunlight and so
plants stay cooler and minimise transpiration.
-Roots are either very long to tap groundwater
supplies- those of the acacia exceed 15 m- or
spread out over wide areas near to the surface
to take the maximum advantage of any rain or
dew, like those of the creosote bush.
- Seeds can lie dormant for months or several
years until the next rainfall.
Many plants are ephemerals
. They have a short life cycle. They come to life only when it
rains, they complete their life cycle by flowering and
germinating their seeds in up to 3 weeks. An example is the
Boerhavia found in the Sahara.
-Others, like the saltbush, are halophytic- adapted to salty
conditions – and can survive in salty depressions.
Oasis vegetation
Oasis, Sahel
Oasis vegetation is found
where there are sources of
water in the desert. The
classic oasis form where a
layer of permeable rock has
taken water from outside
the desert. The layer then
runs into an aquifer beneath
the surface, carrying water
between impermeable rocks.
The aquifer comes to the
surface because of erosion
or folding (like the picture).
Plant grow nearby. Most
oases are now settled and
cultivated.
• There are two types of vegetation clearly shown –
cover is sporadic. One type is a cactus (Saguaro)
that stands above the other vegetation. It is
mainly one large stem, with limited branches
emerging from the main trunk. It is dark green in
colour, but covered in white needles. The smaller
shrubs (creosote bushes) have many branches
which are quite thin. The leaves are plentiful, but
small and light green in colour.
• Allow up to 3 on either vegetation type, up to 2
for naming species shown.
• 4×1
• Explain how vegetation in hot deserts is
adapted to the climate. (7)
Small leaves (and their waxy nature) to reduce
evapotranspiration; the stomata on these plants close
during the day, minimising water loss.
The cactus is a succulent and water is stored in the fleshy
stem and branches to be used during periods of dryness.
Cacti have many long shallow roots allowing them to
maximise intake of water during periods of rainfall.
Other plants like the creosote bush have deeper roots and
search for water downwards and where there is permanent
groundwater.
Some plants have silver coloured leaves that reflect sunlight
and so plant stays cooler.
There may be reference to plants that have a short growing
season following rain to avoid the lack of water – these may
be annuals that germinate, flower, seed and die quickly or
perennials that have limited bursts of activity before
becoming dormant again.
• Marks here are for explanation only with reference to
vegetation.
3- Causes of aridity. Atmospheric processes
related to
a) Atmospheric pressure and winds (global
atmospheric circulation)
b) Continentality
c) Relief
d) Cold ocean currents
a) Global atmospheric circulation
Annotate Figure of global atmospheric circulation to
explain why most deserts are found around 30° North
and South of the Equator
Stable, clear skies occur here, with few clouds. It is
dry because most water vapour was dropped as
convection rainfall near the equator. This creates
regions known as sub-tropical anticyclones.
The air that has risen in the convection currents stops when
it reaches a layer in the atmosphere called the tropopause.
It cannot continue to rise, so is forced to flow Pole wards.
As air flows away from its
main heat source, the
equator, it sinks.
Cooling makes the
air denser, and
much of this
denser air sinks
back towards the
earth and 30˚
north or south of
the equator.
(Descending limb
of the Hadley Cell.)
The ITCZ - Inter Tropical Convergence Zone
- is an area of low atmospheric pressure that
forms where the Northeast Trade Winds meet
the Southeast Trade Winds near (actually just
north of) the earth's equator.
This causes thick
clouds and
torrential rain.
(Rising limb of the
Hadley Cell.)
The hot air contains large
amounts of water vapour,
evaporated from the
ground or transpired from
vegetation. This heated air
rises in the form of
convection currents and
then cools, creating an
area of low pressure
known as the ITCZ.
The sun’s rays are most concentrated here – leading to the
heating of the air lying above the ground surface.
Booklet page 29
• Outline how the atmospheric circulation is
responsible for the aridity in the Sahara. (4)
• The Sahara Desert is found in an area of high
pressure (1).
• Here air that has risen near the Equator/further
north in temperate low (1)
• has cooled/become denser (1)
• and so sinks to the surface (1).
• It is dry and its descent makes rainfall improbable
(1).
• The North East Trades blow offshore from the
continental interior (1),
• these have blown across large extents of land so
rainfall is improbable (1).
Sinking / high pressure interchangeable. 4 x 1
Winds
Winds are affected by the
presence of the high
pressure. Winds blow from
areas of high pressure and
so winds blow out from
these areas of land –
receiving no moisture- this
contributes to the aridity.
For example, the North East
Trade winds blow across
north Africa across the
Sahara from about 30
degrees north.
Explain how continentality is a cause
of aridity. (3) JUN12
Continentality refers to areas within the interiors of
land masses away from the influence of the sea.
Central areas of continents more arid than
coastal areas. Why?
Moist wind from the sea move inland and the
moisture held is dropped as precipitation. So
when the wind reaches the centre of a large
continent it’s carrying very little moisture, so very
little rain falls.
Relief
Some very dry areas lie in the rain shadow of
high mountains ranges. As the prevailing winds
in the sub-tropics are the trade winds, blowing
from the north-east in the northern hemisphere
and the south-east in the southern hemisphere,
then any barrier, such as the Andes, prevent
moisture from reaching the western slope.
Cold currents
In some places cold ocean currents run along the
coastline.
Wind is cooled as it travels over the cold water and
its ability to hold moisture is reduced.
Moisture that’s stored in the atmosphere is
released as precipitation over the ocean before
reaching the land.
So when the wind reaches the land there’s very
little moisture left so very little rainfall falls.
For example, the Namib Desert in Africa exists
because of the Benguela Current, a cold ocean
current that rubs up the west coast of Africa.
Atacama DesertRain shadow and
cold current
Booklet page 28
Outline how the cold ocean current
and the mountains are responsible for
aridity in the Atacama desert. (4)
The cold current (Humboldt/Peruvian) resulting from
water being transferred from Antarctic (1). This causes air
above the Pacific off the coast to cool and condensation
to occur (1). As air drifts in over land, it is warmed and
moisture is evaporated (1). Any winds crossing the cold
water will cool, rainfall may occur over water (1); its
capacity to hold moisture is limited and it too will warm
up over the land (1).
The Andes are the cause of a rainshadow effect (1). The
air coming from the east has to rise over the mountains
(1), will cool and condensation will occur (1). Thus rain
will fall as the air ascends; as it descends the air will be
drier and will be warmed, increasing its ability to hold
moisture and so rain will not fall.
4 x 1 per basic point, 2 x (1+1) per developed point. Any
combination. Maximum 3 on either mountains or cold
current.
• Explain the distribution of arid and semi-arid
areas. (4)
• Explain two or more causes of aridity. (15)
Explain the distribution of arid and
semi-arid areas. (4)
There are a variety of reasons that might be considered –
1-the presence of high pressure as a result of the areas being
on the falling limbs of the Hadley and Ferrel cells and the
stability of the air, thus the warming air means precipitation
will not occur;
2- Prevailing winds are out-blowing from high pressure belt
and so as they are land based do not contain moisture;
3- The presence of cold currents off the western coasts results
in winds coming across these precipitating over the sea;
4- Many of the areas are in rainshadow as a result of
mountains elsewhere that have been crossed by prevailing
winds earlier and
5-many are substantial distances from the sea – the influence
of continentality.
4x1. Allow 1 mark for naming 2 factors.
3- Arid geomorphological
processes: mechanical weathering.
Define mechanical weathering. (2)
Mechanical weathering involves the physical
breakdown/disintegration of rock in situ. The
pieces of rock are smaller, but they are the same
as they were when attached to the main part of
the rock – no chemical change or change in
appearance has taken place. Direct link to
climate. 2 x 1
Thermal expansion  Exfoliation and
granular disintegration
Booklet page 20
4 (a) (ii) Study Figure 7 which shows the result
of mechanical weathering in Death Valley,
California.
Describe evidence of mechanical weathering
shown in Figure 7 and explain the processes
involved. (4)
D=Layers can be seen on the large boulder,
partial layers appear to have peeled away. There
is debris around the boulder, evidence of rock
being weakened and pieces falling off.
E= This is the result of temperature fluctuation
on a daily basis. During the day, the surface
layers expand in response to the high
temperatures to a greater extent than those
concealed beneath the surface. At night, cooling
and contraction occurs at similarly different
rates. This causes the outer layers to peel away
from the lower layers – the process of
exfoliation.
Block separation
This happens to rocks
such as limestone,
which are well jointed
with bedding planes.
The surface breaks
down into block
structure along the lines
of weakness.
The Importance of wind
Erosion, transportation and
deposition
Transportation
Movement of particles determined by
several factors
1- Greatest where winds are strong (usually over
20km/hr).
2- Turbulent wind
3- come from a constant direction and blow steadily
for a lengthy period of time
4- Nature of the regolith- the layer of
unconsolidated rocky material covering bedrockmore likely to be moved if:
- there is no vegetation to bind it together
- particles are small enough to be transported.
Mystery of Death Valley's Moving
Stones Solved
Booklet page 26
The largest material – between 0.25 and 2mm
rolls along the surface in a process known as
surface creep. Intermediate size of 0.25 –
0.15mm hops along the surface at heights of
up to a metre – saltation whilst the finest
material of under 0.15mm is carried within
the atmosphere – up to 300m via suspension.
3×1. Allow 1 mark for identifying 2 processes.
Erosion: Deflation and abrasion
Deflation: the wind removes loose material and
carries it away. As the fine material is
transported, deflation leaves behind a rocky
surface called a desert pavement.
Desert pavement
Deflation hollows
Formed when the wind removes vast amounts
of surface material. They form when there is
some hollows. On cold nights the cold air sinks
into these hollows and dew forms. The dew
speeds up the weathering of the rock
(chemical) and makes particles available for
removal by wind. These processes can lead to
depressions that are many metres deep.
The Qattara Depression (Egypt)
Abrasion
Sandblasting action by materials as they are
moved by saltation. It can pit the surface of
rocks or it can wear then smooth. Since sand
particles cannot be lifted very high, the zone
of maximum erosion tends to be within 1.5 m
of the earth’s surface.
Abrasion produces a number of distinctive
landforms such as rock pedestals, yardangs
and zeugen.
White Desert @ Egypt - Planet
Earth (2006)
Question
Explain how methods of wind transportation
affect the process of deflation and abrasion.
(5)
There is a clear link between suspension and
deflation. Deflation occurs when the fine material
is removed by the wind via suspension to leave
behind a pebble strewn flat surface known as a
desert pavement. Deflation hollows also result as
material is picked up to create a basin shape.
There is an equally clear link between saltation,
suspension and abrasion. Material carried by the
wind has a sandblasting effect on exposed rock
surfaces, especially near the surface (1.5m). The
material is carried and responsible for the
abrasion process eroding areas of weakness –
such as layers of soft rock to create landforms
such as rock pedestals, yardangs and zeugen.
Level 1 (1-3 marks) Describes the erosion
processes. Shows some awareness of what
wind transport does. Implicit link. Some use of
appropriate terminology present at the higher
end.
Level 2 (4-5 marks) Explains clearly how
transportation processes affect erosion
processes – links explicit.Both erosion
processes needed for top of level. Appropriate
geographical terminology is used.
5- Landforms resulting from – wind
action: defletion hollows, yardangs,
zeugen and sand dunes –
JUN13: Draw a labelled sketch to show the
characteristics of yardangs. (4)
Marks should be awarded for appropriate shape,
reference to nature of rock layers and
orientation, size – e.g. yardangs can be 100m high
and continue for a number of km.4 x 1, including
1 mark for sketch. Dimensions – max 2.
1 mark for showing parallel ridges of appropriate
shape linked to layers of hard rock. In addition,
mark for labelling different rock layers/alignment,
shape indicating widening near top, undercutting
at base.
Homework
Draw a labelled sketch to show the
characteristics of zeugens. (4)
Describe the role of wind action in the
formation of yardangs and zeugen. (7)
JAN10: Describe the role of wind action in the
formation of yardangs and zeugen. (7)
Wind action causes yardangs – ridges of hard rock
that stand above eroded lines of soft rock –
etched out by prevailing wind, zeugen – ridges
composed of a hard cap rock with underlying soft
rock that is eroded more. Abrasion is the key
process in forming both these landforms as
material (sand) carried by the wind via saltation
and suspension hits the masses of rock and wears
it away – similar to sand blasting as is used to
clean old buildings. This effect is most effective
within 1.5 metres of the surface and causes rock
pedestals/mushroom rocks to occur in a less
extensive fashion than zeugen. The formation of
such features is dependent on the geology of the
area also.
Wind deposition: sand dunes
Form when sand grains carried in suspension are
deposited as the wind slows down.
May form around an obstacle as the wind is
slowed down on the downward side of the
barrier.
As the wind velocity decreases, some of the
sediment in suspension can no longer be held
so it falls to create some form of deposit.
The barchan is a small,
crescent-shaped dune,
about 30m high, which is
moved by the wind.
Booklet page 24
• The photograph shows sand dunes. They appear
to occur in lines. One slope seems to be steeper
than the other/asymmetrical. The middle part of
the dune seems to be higher than the sides. They
are crescent shaped/barchans/seif.
• In the foreground, it is clear that the sand is not
smooth, but is rippled. There is very little sparse /
low lying vegetation present on the dunes.
• Allow 1 mark for reference to Zone B and 1 mark
for reference to vegetation.
• 4x1
1- Explain the formation of the landscape
shown…
2- 15 mark question
Seifs
• Seifs are longitudinal sand dunes. In places,
they can stretch for several hundred metres.
They form parallel to the prevailing winds.
Barchans can develop into Seifs,
6- The importance of water –
sources: exogenous, endoreic and
ephemeral. The role of flooding
Booklet page 22
• Rain is heavy and intense and so there is no
time for infiltration. This gives rise to rapid
surface runoff, meaning water reaches
streams very quickly. Ground will be dry and
baked, reducing the likelihood of infiltration
further. After rain, evaporation rates high,
reducing discharge quickly. Infiltration will
begin to occur.
• 2x1
Booklet page 18
• Question 1
Sources of water
• The specification notes exogenous sources –
those that originate outside the desert and
continue through it without drying up, such as
Colorado and Nile; endoreic – those that
originate outside the desert but dry up or flow
into inland lakes, such as Jordan and ephemeral –
these are intermittent streams, present after
rainfall. There may also be reference to aquifers
and mist and fog, as well as direct precipitation.
• 1 mark for list of two or more sources.
• There must be reference to two sources for 3
marks.
Landforms resulting from water
action
Booklet page 18
Booklet page 18 Impact of flooding on
landforms
Figure 5 suggests that water can have a significant
effect and that it is present in substantial
quantities at times. As such, the water has
washed away a substantial depth of the material
at the side of the road and has undercut it. 1
mark for description. This indicates a clear role
for water in the desert in eroding and
transporting material, and therefore, in shaping
landforms – such as wadis, canyons. Material that
has been transported will ultimately be deposited
and such a substantial amount that has been
moved will explain the subsequent formation of
landforms such as alluvial fans.
• Comment on the role of water in hot desert
areas. (4)
• Assess the role of water in the formation of
desert landforms. (15)
JAN12: Draw a labelled sketch to show characteristics
of a wadi and explain its formation. (7)
There should be a recognisable sketch of wadi likely showing a
narrow, steep sided valley, perhaps with much material within it
and no stream or the presence of an intermittent/ephemeral
stream. A wider, more gently sloping valley in lower course is
also acceptable.
Explanation should refer to the significance of the intermittent
stream and the power that it has to erode due to its flashy
nature. The energy generated allows large items of material to
be eroded and subsequently transported by traction and a huge
amount of material to be carried in suspension. As the river
cannot be sustained due to the limited duration of the rainfall,
the capacity and competence of the stream reduces and it
becomes choked with the sediment it is transporting. This is then
deposited, strewn along the bed to await the next rainstorm.
Pediment
Inselberg
Mesa and butte
Wadi and alluvial fan
Playa (Death Valley)
Badlands
https://plus.google.com/photos/+SpanishhighsC
oUk/albums/5937574357394013201/5937575
358104020530?pid=5937575358104020530&
oid=109502457382781256798
8- Desertification: distribution of
areas at risk, physical and human
causes, impact on land, ecosystem
and populations.
Booklet page 23
Areas at greatest risk from desertification are
generally located adjacent to existing deserts. This
is clearly the case adjacent to the Sahara – where
there is the largest continuous area at risk to the
south (the Sahel) – and Namibia, Western Australia
and those in USA. Some adjacent areas appear not
to be at risk – e.g. southern side of the Arabian
Desert. The main areas at risk are around the
Tropics, but desertification risk is clearly not
confined to these areas in USA.
• JAN10: To what extend do you agree that the
human causes of desertification are more
important than the physical causes? (15)
Video: Desertification - A Visual Disaster (1.34m)-End: management
Consequences: Video: Explaining the Sahel drought problem
9- Case study of desertification in the Sahel –
(i) the struggle for survival to include the
energy/ fuelwood crisis, water supply, the
impact on food supply/farming and
livelihoods and coping/
(ii) management strategies including external
aid.
Handout: JUN11: Describe the difficulties faced
by people living in the Sahel and comment on
the extent to which they result from
desertification. (15)
Management strategies including
external aid.
• Re-greening in Niger, a road trip with Dr Chris
Reij
Classwork
JUN15: Life in the Sahel needs not be a struggle
for survival; management strategies give
hope for the future. To what extent do you
agree with the statement? (15)
-adverse conditions in the Sahel and issues such
as removal of trees
-impact of desertification;
-various strategies aimed at improving life in the
Sahel.
Comment on question
• Booklet page 21
Southern Spain
Reasons for desertification and
management
JUN14: To what extent is development
sustainable in areas such as South-western USA
or Southern Spain? (7)
JUN10: Assess the sustainability of different
strategies used to manage land use, including
agriculture in contrasting arid areas of the
world. (15)
Extra question on landforms
• Booklet page 30