Transcript Deserts - physical processes and landforms

Desert environments in Namibia
The physical processes and landforms of deserts
The example of the Namib and Kalahari Deserts
Key Question 1.1 What are the characteristics of a desert environment that make it
extreme?
*
The climatic, biotic and soil characteristics of a desert environment.
*
The links between climate, biotic and soil characteristics.
Key Question 1.2 How is human activity causing pressures on the desert
environment?
*
The threats that are posed by
(i)
population growth;
(ii)
mineral exploitation;
(iii)
farming;
(iv)
tourism.
*
The positive and negative outcomes of human activity.
Key Question 1.3 What are the strategies that can be used to manage human
activity in deserts?
*
Strategies that attempt to
(i)
conserve the desert environment;
(ii)
alleviate the impacts of human activity;
(iii)
control the use of the desert environment
(iv)
monitor the impacts of human activity.
*
The role of local, national and international groups in the
management of the desert environment.
LIST OF LANDFORMS IN DESERT AREAS
Sandy desert
- ergs/sand seas
Gravel desert
- lag deposits/reg
Rocky desert
- hamada
Desert pavements
Ventifacts (dreikanter)
Yardangs
Zeugens
Rock lattice (tafoni)
Pedestal rocks (mushroom blocks)
Deflation hollows
Gibber plains
Duricrusts
Mesas and buttes
Canyons
Wadis
Pediment
Bajadas
Dunes
•
Ripples
•
Draa (megadune)
•
Barchans
•
Star dunes (rhourd)
•
Seif dunes (linear dunes)
•
Lunette dunes
•
Nebkha dunes (shrub coppice dunes)
•
Lee and fore dunes
•
Echo dunes
•
Rhourds
•
Zibar
Pans (playas/sebkas/chotts)
Inselbergs
Bornhardts
Kopjes
Badlands
Hoodoos
Natural arches
Piping
The main processes at work on
landforms in arid areas
Mechanical weathering
Thermal fraction, i.e. The breaking
down of rocks as a result of
temperature change is the most
important type of mechanical
weathering in arid environments.
Diurnal temperature ranges may be
as much as 25° - 30°C in deserts
during summer months and rock
temperatures may reach 75°C, The
expansion and contraction so
produced help disintegrate rocks.
Geological structure (joints and
bedding planes), surface colour and
chemical composition of rocks all
influence the way they disintegrate.
The four main processes are as
follows:
• Granular disintegration. Grainy rocks such as coarse sandstones and
granite break down into grains of sand. Granite disintegrates quickly
because it contains both black mica and white quartz crystals which heat
up and cool down at different rates because of their colour
• Block separation. Well-jointed sedimentary rocks, particularly limestone
and types of sandstone frequently break down along their joints and
bedding planes which are their main lines of weakness. Blocks can
commonly be seen breaking off the hard cap-rocks of features like mesas
and buttes.
• Shattering. Some rocks have neither coarse grains nor a blocky
structure, and so shatter into irregular fragments with sharp edges.
Basalt, which is black and highly metallic, absorbs heat and expands
rapidly. It is the most common rock which disintegrates this way.
• Exfoliation. The surface of any rock is more exposed to temperature
changes than the underlying rock. As a result some massive rocks,
especially if they have a rounded profile, tend to peel off in layers.
Granite and sandstone frequently disintegrate this way.
Other mechanical weathering processes.
Frost shattering / congelifraction
This may occur in higher altitude areas during
winter time when the temperature goes below
freezing point. Water, especially from dew, is
trapped in cracks, expands when freezes and
thaws and contracts.
Wetting and drying
Some softer rocks are very vulnerable to being
wetted and then drying lout. Clays, mudstones
and shales expand when wetted, then dry out
and contract. Fragmentation then results.
Chemical weathering
• Crystal growth – as different minerals are evaporated out of the
groundwater or as salts are deposited by evaporation following
rainstorms their crystals grow, leading to rocks being prised apart
until
they fragment. This depends on the solubility of the mineral and
the rate of
crystal growth.
•
Hydration - this process involves the absorption of water by an anhydrous
mineral such as gypsum (calcium sulphate) which then causes
expansion of the mineral and the break up of the rock.
•
Solution – this is the dissolving of soluble minerals in water leading to the
wearing away or disintegration of rock surfaces.
LANDFORMS OF CHEMICAL WEATHERING
Tafoni - cave like holes in a rock surface
Alveoles – smaller pits like a honeycomb in a rock surface
Rock pillars
Duricrusts such as gypcrete
Desert varnish – a thin hard shiny dark red layer of evaporated iron and
magnesium oxides
Mass movement
Water is not usually available so mass movement processes which involve water
and lubrication are not active at these times. When rain falls they come into play.
The lack of vegetation cover and thick soils make mass movement more likely
There are two main processes of aeolian erosion in deserts:
[1]
[2]
deflation – loose surface material is picked up and carried away
if fine material is all carried away only gravel etc is left forming a
gibber plain or reg
abrasion – surfaces are sand-blasted by particles carried in the wind. Its
effectiveness depends on the wind speed, frequency and direction,
the particle size and the lithology of the rock being eroded
Abrasion creates such landforms as
pedestal rocks, ventifacts, yardangs and
zeugens.
Aeolian
erosion
Aeolian transport and deposition
Deserts are often very windy. There is little vegetation to slow the wind down.
Material eroded from one area will be transported and deposited in another.
Material can be transported by being
[1]
bounced (saltation) – sand grains are carried a few centimetres above
the surface and occasional return to the ground to bounce
into the air again.
[2]
suspended (suspension) – lighter material is picked up and carried in
the air for hours or even days. This process is at work in
sandstorms. Material may transported for thousands of
miles, quite often to the UK.
[3]
rolled (surface creep) – heavier material is rolled along the surface but
they are too heavy to be lifted into the air.
Aeolian deposition creates various forms of sand dune.
A sand storm approaching Gobabeb in the Namib Desert
Fluvial erosion
Water is surprisingly important in the world’s deserts because when rain arrives it is often
intense and there is no surface coverage of soil to protect the surface of the land. The
exception is the sand sea of the Namib, because of high surface permeability.
In highland areas water is dominant as an erosional agent, creating wadis and canyons,
whilst in the lowlands deposition is more important making pans and bajadas.
Water erodes where it flows quickly, e.g. on steep slopes.
It erodes by means of 5 processes:
[1]
cavitation – bubbles of air implode against the rock creating shock waves
[2]
corrosion – water may dissolve some rocks, e.g. limestone and chalk
[3]
corrasion – material rubbed against the banks and bed may scrape away
particles
[4]
hydraulic action – the force of the water washes away loose material
[5]
attrition – particles may rub against each other, getting smaller and
rounder
Fluvial
transport and
deposition
Material may be washed away in rivers,
perhaps in flash flood on the rare occasions it
rains in the desert until the energy level of the
river drops. This is usually at the base of the
slope.
Here an alluvial fan may be formed. If alluvial
fans join together along the base of a slope a
pediment is formed. Further away and with an
even lower angle of slope are the bajada and
playa (or chott).
Gravel
desert
Sandy
desert
Types of
desert
landform
Rocky
desert
Location of Kalahari Desert
Erg
An erg (also sand sea or dune sea) is a large, relatively flat
area of desert covered with wind-swept sand with little to no
vegetation cover
The dune sea of the Namib Desert. Note how the crests of the dunes are aligned in a
marked northwest-southeast orientation. These crests form transverse to the formative
wind (i.e. crosswise). The dunes act as obstacles, and obstacles cause winds to be deflected
significantly to the right, in the southern hemisphere—in effect reorienting the southerly
wind as a southwesterly wind. A strand plain lies between the dunes and the ocean.
Linear (seif
dunes)
Pan
Rhourds (star
dunes)
Barchans
Inselberg
Reg (gravel desert / gibber plain)
Many deserts are covered with desert pavement, created when wind blows fine
material away, leaving coarse material behind. A deposit of heavy material left behind
when light material is carried away is called a lag deposit. Lag deposits can be created
by wind, running water, and wave action.
Reg
A vast stony plain in a desert. (also called a gibber plain or gobi)
Hamada (rocky desert)
A hamada (Arabic, ‫ حمادة‬ḥammāda) is a type of desert landscape consisting of
largely barren, hard, rocky plateaus, with very little sand.
Hamada
Barren rocky highlands in a desert.
Ventifacts (dreikanter)
Wind-abraded
rocks or
ventifacts are
often called
dreikanters from
a South African
word meaning
"three corners."
Hunks of bedrock sticking up above the
average surface level tend to be eroded into
characteristic shapes called yardangs. A
yardang is generally sloping on its stoss side
and steep on its lee side.
Wind direction
Yardangs
Zeugens
Zeugens occur in areas with
resistant and less resistant
rocks occurring in horizontal
layers. Joints are exploited
and the less resistant rock is
more eroded than the more
resistant rock producing a
ridge and furrow landscape.
Tafoni or rock lattice
Tafoni are holes or, if larger,
cave-like features found in some
deserts; of uncertain origin, they
have arch-like entrances,
smooth, concave walls, and flat
floors covered with rock debris.
They often occur in groups that
can riddle a hillside, cliff, or
other rock formation. The
currently favoured explanation
for their formation is salt
weathering, which would be
facilitated by the slightly cooler
and more humid environment
inside them. Sometimes the
smaller holes are called alveoles.
Pedestal rocks
pedestal rock
An isolated, residual or erosional mass of
rock supported by or balanced
on a pedestal. Isolated masses or rock
above and resting on a smaller base or
pedestal.
The Mukurob (Finger of God), near
Asab in Namibia, collapsed on 04
December 1988 as a result of earth
tremors produced by the
earthquake in Armenia.
Deflation hollows
Deflation basins, called blowouts, are hollows formed by the
removal of particles by wind. Blowouts are generally small,
but may be up to several kilometres in diameter.
Dunes
Sand dunes are formed by
aeolian deposition (by the wind).
They take on a wide variety of
forms and sizes depending upon
the strength and variety of wind
directions.
Sand ripples are the smallest
scale of sand dune measuring
only a few centimetres in
height
Draa (megadunes)
Dune 7 at Sossusvlei, 383 m
high
An area of
megadunes in
the Eastern
Namib
The largest size of sand dune,
although often without a clear
shape (then called complex
dunes). The pattern of winds is
variable.
Barchan
Crescent-shaped mounds are generally wider than they are long. The slipface
is on the dune's concave side. These dunes form under winds that blow from
one direction, and they also are known as barchans, or transverse dunes.
Some types of crescentic dunes move faster over desert surfaces than any
other type of dune
They are crescent in shape with their arms called horns.
Horns always point along the direction of wind where
they move. Ripples form as usual at their backs where
winds blow. Barchans form when wind blows in one
direction all year long with no change.
Star dunes (rhourds)
Star
Radially symmetrical, star
dunes are pyramidal sand
mounds with slipfaces on
three or more arms that
radiate from the high centre
of the mound. They tend to
accumulate in areas with
multidirectional wind
regimes. Star dunes grow
upward rather than laterally.
They dominate the Grand Erg
Oriental of the Sahara. In
other deserts, they occur
around the margins of the
sand seas, particularly near
topographic barriers
Seif dunes (linear dunes)
A linear dune near
Gobabeb
Linear
Straight or slightly sinuous sand ridges
typically much longer than they are wide
are known as linear dunes. They may be
more than 160 kilometres long. Linear
dunes may occur as isolated ridges, but
they generally form sets of parallel ridges
separated by miles of sand, gravel, or rocky
inter-dune corridors. Some linear dunes
merge to form Y-shaped compound dunes.
Many form in bidirectional wind regimes.
Mostly the wind blows from one direction
but there is a secondary direction. Which
helps create the landform, The long axes of
these dunes extend in the resultant
direction of sand movement
Lunette dunes
U-shaped mounds of sand with convex noses trailed by
elongated arms are parabolic dunes. Sometimes these
dunes are called U-shaped, blowout, or hairpin dunes, and
they are well known in coastal deserts. Unlike crescent
shaped dunes, their crests point upwind. The elongated
arms of parabolic dunes follow rather than lead because
they have been fixed by vegetation, while the bulk of the
sand in the dune migrates forward.
Lunettes are accumulations of semiconsolidated fine sand, silt, and claypellet aggregates that form rounded,
low (meters high) dunes on the
downwind sides of playas. Like
ordinary parabolic dunes, these
dunes are transverse to the effective
wind, and their arms point upwind.
The arms, however, are much shorter
than those of parabolic dunes
because lunette dunes are composed
of cohesive materials deflated from
the adjacent playa floor.
Shrub coppice dunes (nebkha dunes)
A small dune formed on the
leeward side of bush-and-clump
vegetation.
The strength and direction of
desert winds and the amount of
sand available determine the
shape and size of dunes. Apart
from dunes anchored to
vegetation, known as shrubcoppice dunes, all dunes are
mobile to some extent, the
crescent-shaped barchans most
of all. Shrub-coppice dunes,
usually only a metre or two high,
are common near the sea. They
form around a bush or clump of
grass when windblown sand
lodges among the stems and
leaves to form a hump-like
mound. As the mound grows the
roots lengthen to keep the plant
above the surface.
Zibar dunes
WHALEBACKS OR ZIBAR DUNES
They've no slipface and look from far exactly like a
whaleback. They're the gigantic in size like typical
dunes and are the most suited to cruise on with
your 4x4. If you ask a British how to describe them
they'd say, they're 'rolling' hills of sands because of
their smooth surfaces with no crests.
They're called whalebacks by the famous British
Sahara expert Bagnold active in Egypt at the first
half of the 20th century (and the father of the
scientists working in desert geomorphology) but
Zibar now is becoming more used in science. The
word Zibar is 'borrowed' from Arabic by D. A. Holm
in 1953 when he was discussing dunes in Nejd,
Saudi Arabia.
Amorphous dunes with no major
recognisable shape
Duricrusts
Duricrust refers to a thin hard layer on or
near the surface of soil, usually a few
millimetres to a few centimetres thick.
It is a general term for a zone of chemical
precipitation and hardening formed at or
near the surface of sedimentary bodies
through pedogenic and (or) non-pedogenic
processes.
It is typically formed by the accumulation of
soluble minerals deposited by mineralbearing waters that move upward,
downward, or laterally by capillary action,
commonly assisted in arid settings by
evaporation. Minerals often found in
duricrust include silica, iron, calcium, and
gypsum.
These duricrusts are known as silcrete,
ferricrete, calcrete and gypcrete. Gypcrete is
especially common in arid environments.
Mesas and buttes
The Waterberg plateau
A mesa (Spanish and Portuguese
for "table") is an elevated area of
land with a flat top and sides that
are usually steep cliffs. It takes its
name from its characteristic tabletop shape. It is a characteristic
landform of arid environments.
A butte is an isolated hill with steep sides
and a small flat top, smaller than mesas
and plateaus.
Canyons
Fish River Canyon
A canyon (rarely cañon) or
gorge is a deep valley
between cliffs often carved
from the landscape by a
river. Most canyons were
formed by a process of longtime erosion from a plateau
level. The cliffs form because
harder rock strata that are
resistant to erosion and
weathering remain exposed
on the valley walls. Canyons
are much more common in
arid areas than in wetter
areas because weathering
has a lesser effect in arid
zones. Canyon walls are
often formed of resistant
sandstones or granite.
(Fig 8.10).
Wadis
(Arabic: ‫ وادي‬wādī) is
traditionally a valley. In some
cases it can refer to a dry
riverbed that contains water
only during times of heavy
rain. Compare arroyo (creek),
canyon, gulch, wash and gully.
Wadis tend to be associated
with centres of human
population because subsurface water is sometimes
available in wadis.
Crossing wide wadis at certain
times of the year can be
dangerous, because of
unexpected flash floods.
Usually a lot smaller than a canyon without the stepped sides.
Pediment
The pediment area is at the foot of the mountains. It is composed of alluvial fans and
bajadas. An alluvial fan is a fan-shaped deposit formed where a fast flowing stream
flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A
convergence of neighbouring alluvial fans into a single apron of deposits against a slope is
called a bajada, or compound alluvial fan.
Namib Sand Sea (looking south) from the
NASA Space Shuttle. Atlantic Ocean is on right.
The north end of the Namib Sand Sea (nearest
the camera) is distinctly marked by the course
of the ephemeral Kuiseb River, which flows
out of the Namibian highlands (left) and down
to the Atlantic Ocean (right). The general
overall sand movement in the Namib Sand Sea
is to the north. Yet the dunes do not succeed
in crossing the Kuiseb, though they do
encroach into the river bed between flows.
This shows that, even though floods in the
Kuiseb are infrequent (once a year or less),
water action is dominant over wind-driven
sand movement. The exception is right near
the coast (lower right of picture), the part of
the Kuiseb which receives the least water, and
the most infrequent flow (the Kuiseb reaches
the sea roughly once every 8-10 years).
Not all rivers can achieve this - you can see
south of the Kuiseb two more epehemeral
rivers entering the Namib Sand Sea from the
east (left), but their paths are blocked by
dunes. Their drainage catchments are much
smaller than that of the Kuiseb.
Perfectly planar pediment surface north of Gross
Spitzkoppe, with episodic watercourses marked by linear
stripes of vegetation.
Pans / playas
A playa; also known as an alkali
flat, sebkha, chott, saltpan or flat;
is a dry lake bed. Such flats consist
of fine-grained sediments infused
with alkali salts. Their surface is
generally very dry, hard and smooth
in the summer months, but wet
and very soft in the winter months.
While the playa itself will be devoid
of vegetation, they are commonly
ringed by salt-tolerant plants that
provide critical winter fodder for
livestock and other herbivores
Natural salt pans are flat expanses
of ground covered with salt and
other minerals, usually shining
white under the sun. Salt pans can
be dangerous. The crust of salt can
conceal a quagmire of mud that can
engulf a truck.
Etosha Pan
Inselberg
A monadnock or inselberg is an isolated hill, knob, ridge, or small mountain that rises
abruptly from a gently sloping or virtually level surrounding plain. Inselberg means island
mountain
Spitzkoppe
Kopjes
In southern and southern-central Africa, a similar formation of granite to an
inselberg is known as a kopje from the Afrikaans word: koppie. Kopjes are often
topped by a jumbled mass of rounded boulders
Bornhardts
Mirabib near Gobabeb
bornhardt - A dome-shaped rock outcrop more than 30 m high, and sometimes
several hundred metres in width. They commonly rise above erosional plains in the
tropics, but also develop in unglaciated uplands in high latitudes. Most are formed in
granites and gneisses; some in sandstones and conglomerates.
Badlands
Badlands are a type of arid
terrain with clay-rich soil
that has been extensively
eroded by wind and water.
Canyons, ravines, gullies,
hoodoos and other such
geological forms are
common in badlands. They
are often difficult to walk
through. Badlands usually
have a spectacular color
display that alternates
from dark black/blue coal
stria to bright clays to red
scoria.
Valley of the Moon, Swakop River area
Natural arches and Hoodoos
A natural arch or natural bridge is a natural formation (or landform) where a
rock arch forms, with a natural passageway through underneath. Most natural
arches form as a narrow ridge, walled by cliffs, become narrower from erosion,
with a softer rock stratum under the cliff-forming stratum gradually eroding out
until the rock shelters thus formed meet underneath the ridge, thus forming
the arch. Natural arches commonly form where cliffs are subject to erosion
from the sea, rivers or weathering (sub-aerial processes); the processes "find"
weaknesses in rocks and work on them, making them bigger until they break
through.
Hoodoos are tall thin spires of rock that
protrude from the bottom of arid
basins and badlands. They are
composed of soft sedimentary rock,
and are topped by a piece of harder,
less easily-eroded stone that protects
the column from the elements
A natural arch at Spitzkoppe
Development of a natural arch
1. Rock is faulted or jointed
2. Chemical weathering occurs along the joints widening them
3. Weathering continues completely eating away some areas of rock and creating
holes on other areas.
4. The holes and caves are exploited further to form a natural arch