Hot Desert Environments - Test 1 Flashcards
Desert
- A place receiving less than 250mm precipitation/year
Arid region
- A place receiving less than 250mm precipitation/year, which has high evaporation rates, poor soil, and little vegetation.
- Most deserts are also arid regions.
Semi-arid region
- A place receiving between 500 > 250mm precipitation/year.
- Often transition zone before you reach an arid region / desert. E..g. North East Brazil.
Hyper-arid region
- A place receiving less than 100mm precipitation/year, aridity index <0.05
Major hot deserts - N hemisphere
N America - Mojave, Sonoran, Great Basin, Chihuahuan
Africa - Sahara (North), Somali-Ethiopian (Horn)
Middle East / Asia - Iranian, Arabian, Thar
(Almost all located towards west of continents)
Major hot deserts - S hemisphere
S America - Atacama
S Africa - Namib, Kalahari, Karoo
Australia - Great Sandy, Great Victoria, Simpson
Open system
- System where matter and energy can be transferred from the system, across the boundary, and into the surrounding environment, or vice versa.
- I.e. matter is free to move in and out of the system to and from external sources
- E.g. deserts, as rivers can flow through deserts and out to the sea
Dynamic Equilibrium
- The point of balance between the inputs and outputs of a system where the stores remain the same.
- This is what all systems are trying to achieve
- If level of balance changes but inputs and outputs remain balanced,
Permanent Change
- Happens when one of the elements of a system changes without the others also changing to counteract this.
Positive Feedback
- When a system changes to a new state instead of returning to the original.
- Positive feedback often has a negative impact on hot desert environments bc it causes an irreversible change
Deserts as natural systems
PIC
Cycle of adaptations diagram
PIC
Desert Climate - Temperature
- Hot in day bc little/no cloud cover to block short-wave radiation (insolation) from entering, due to aridity
- Cold at night bc little/no cloud cover to trap in long wave radiation so it all returns back to space (re-radiation)
- So ^ diurnal range (difference in temp between day and night) - about 0 > 55 C
- But maritime (near the sea) deserts have lower diurnal range bc cold off shore current
Desert Climate - Rain
- Low annual rainfall (less than 250m / year)
- Unreliable - little to no rainfall for most of year, but extreme events e.g. flash floods can occur where there is a huge amount in a short space of time.
- Rapid surface run-off due to hard baked ground and lack of vegetation due to aridity causing low infiltration
The Water Balance
- P = Q (streamflow, result of run off & release of stress) + E (evapotranspiration) +/- S (Storage)
- LHS = inputs, RHS = outputs
Desert Climate - The Water Balance
- Negative, i.e. outputs > inputs, leaving deserts with a water deficit.
- Values for both sides will be very low due to lack of water in deserts, but equation is still useful for understanding where the water that is available is
- E is low bc not much water available in first place due to aridity, and small amount that there is is used up by plants
- (However potential E is high in deserts bc they are very hot + sunny in day so if there were lots of water available, lots of evapotranspiration would occur)
- Because E > P, deserts will have an aridity index of <1
- Storage is mainly in landforms, rather than soil or vegetation, bc of rapid surface run-off
Soil - Definition & Make-Up
= uppermost thin layer of Earth’s crust
- consists of: biota (living things), hummus (decomposing matter), air, water, and regolith (broken down rock from weathering
Soil - Formation
- Takes hundreds of years to develop, possibly more
Formation affected by: - Time period
- Human activity (i.e. have fertilisers been used, irrigation , or ploughing)
- Climate (bc affects moisture of soil & rate of weathering of parent material)
- Parent material from which regolith is created
- Topography
- Biota (bc affects biota & hummus elements)
Desert Soils
- Main type in desert = aridosol, but within this there is also solonetz, and solonchalks
- Aridosol takes a long time to form, is thin and low in organic content, has a high ph (7-8.5), can be fertile but only if well-irrigated, can be any colour from yellow/red to grey/brown, and has an low NPP (3-90)
NPP defintion
= Net Primary Productivity
- Is the growth rate of vegetation in a certain type of soil
- Measured by calculating the increase in dry weight of biomass / unit area / year
- So unit = g/m2/year
Hot Desert Soils - Aridisols
- Take a long time to form
- Are thin (usually less than 100cm deep) and low in organic content
- Have a high ph (7-8.5)
- Can be fertile but only if well-irrigated
- Range from yellow/red to grey/brown in colour, depending on nature of parent material
- Have a low NPP (3-90)
- PIC
Hot Desert Soils - Solonetz
- Clay-like
- No surface crust
- Enough water for some leaching, but not enough to wash away base
Hot Desert Soils - Solonchalks
- Infertile
- Thick, salty, surface crust
- High evaporation rates so high concentration of mineral salts at surface
Vegetation in Hot Deserts - Xerophytes
- E.g. Cactus - adapted to arid conditions by having thin, tough, spiky leaves, and thick bark and silvery hairs to reduce water loss by transpiration
Silvery hairs also cut wind speed and keep plant cool by reflecting light
Vegetation in Hot Deserts
- Most deserts will have some, but sparse + scattered
- To survive, must be adapted to: prevent water loss, store moisture in their stem + leaves, have deep/wide roots, and a short life cycle after it rains
Vegetation in Hot Deserts - Ephermals
- E.g Boerhavia - adapted to unreliable rainfall by producing seeds that can lie dormant for long periods between rainfall, but germinate and flower quickly when rain does arrive
Halophytes
- E.g. Pickleweed - adapted to tolerate highly saline conditions by storing salt in its glands and then secreting it on leaves
Causes of Aridity - Formation of subtropical high pressure cells
At equator:
- High angle of i + concentrated insolation > surplus of heat energy
- Humid air at ground level becomes warm and rises > low pressure over equatorial latitudes
- Temp. falls with ^ altitude so humid air condenses > thick layers of cloud
At tropics:
- But dry air continues to rise until reaches tropopause, where it can’t rise any further and so is forced polewards
- Temp. falls with distance from equator so at about 20-30N/S (sup-tropical regions) dry air is so cool it starts to sink
- Dry descending air causes high pressure which inhibits cloud formation
- So sub-tropical areas have clear skies & therefore little precipitation
Phreatophytes (water seekers)
- E.g. Cresote Bush - adapted to arid conditions by having long roots which are sent out to tap water from underground, are always in contact with water to maximise water supply to plant.
Causes of Aridity - Rain Shadow Effect
- When prevailing winds that have gained moisture from traveling across oceans, are forced to rise over mountains, the water condenses as temp. falls with ^ altitude and precipitation occurs on windward side.
- So when air reaches leeward side, has lost all its moisture
- Dry air descends and warms on leeward side > clear skies + little precipitation
- Some ‘rain shadow’ areas are so dry they are considered deserts e.g. Atacama is in rain shadow of Andes, Mojave + Sonoran in shadow of Rockies
Succulents
- E.g. Cactus - stores water in roots + stems
Weathering
The breaking down of rock, soils, minerals, and artificial materials through contact with the world’s biota, atmosphere, and water. It occurs ‘in situ’ (aka with no movement) which is what makes it different to erosion.
Sediment system
- inputs - e.g. weathering of underlying rock or parent material, fluvial sources (e.g rivers / floodwaters bringing and depositing sediment), aeolian sources (i.e wind blowing and depositing sediment)
- stores - e.g. deposits and landforms
- outputs - e.g. wind + water erosion and transportation - haboobs, the huge dust storms in Sahara, are an output of a sediment system
Causes of Aridity - Cold Current Effect
- When prevailing winds travel across oceans, lower layers are cooled by contact with water
- So if travel over cold ocean currents, effect is magnified > very cold wind
- This means the moisture they have picked up by travelling across oceans condenses > fog / mist
- But when wind reaches land, this is straight away burnt away by sun
- Also very cold air inhibits cloud formation
- So clear skies > little precipitation
- Some areas where there are nearby cold currents are dry enough to be considered deserts e.g. Atacama.
Causes of Aridity - Continentality Effect
- As air moves over a continent is gradually loses its moisture > air in areas in the centre of continent is very dry
- So maritime locations have ^ precipitation than those inland
- Some inland areas are so dry they are considered deserts e.g. Sahara (some areas are even 200km found a coast)
Sources of energy in the desert - Insolation
- Insolation = a measure of the amount of solar energy that a point on the Earth’s surface receives.
- With distance from the equator, insolation tends to decrease, bc of the shape and tilt of the Earth
- This is bc: the solar radiation has to travel through more atmosphere, which means more is reflected back to space and never actually hits the earth
- And bc there is a lower angle of incidence so the solar radiation is spread over a larger area, so there is less at one point.
- But because there is a high angle of i AND little/no clouds to block the incoming solar radiation, insolation in deserts is very high, even than that at the equator (bc here there are lots of clouds to block it).
Sources of energy in the desert - Wind
- Source of energy, driver of processes, and
acts as an agent of erosion and transportation > important input to hot deserts - Common in arid regions bc:
the creation of high pressure cells generates strong winds blowing out of the desert
and there is a lack of vegetation, surface features, etc. to create friction to reduce speed or provide shelter
Sources of energy in the desert - Run-off
- Lack of frequent rainfall > hard baked ground + little vegetation > low infiltration
- So when is does rain, lots of surface run off created > lots of erosion + transportation of sediment
Mass movement definition
A large scale movement of the Earth’s lithosphere (sand, soil, rock, and other material) down a slope under the influence of gravity.
Factors affecting slope stability
- Gravity + slope angle
- Weight of material
- Water content & make up of material
- If there are triggering events e.g seismic activity, top loading, etc.
What causes mass movement?
- mass movement is caused by slope failure
- slopes are a dynamic system subject to a number of forces, most notably gravity
- most of time, all these forces are in balance, but slope failure occurs if there is an imbalance (e.g. downslope forces exceed upslope forces due to one of factors above).
- in other words, slope failure occurs when there is a reduction in the shear strength of the slope, or an increase in the shear stress acting on the slope, or a combination of the two!
Risk of mass movement is higher when…
- Ground is saturated, i.e during / after heavy rain
- Slope is on a steep hill
- There is lots of loose soil / fractured rock already (perhaps from previous landslides there)
- There have been triggering events e.g heavy rain, a fire, an earthquake.
Type of mass movement in deserts
Mainly rockfalls (bc these occur when ground is dry) but also some landslides & slumps (which occur when the ground has a bit more moisture.)
Shear strength
The internal resistance of a body to movement
Shear stress
The force acting on a body that causes movement of the body down the slope.
Mechanical weathering
Rock is physically broken down
Mechanical Weathering - Differential Expansion
- Lots of different minerals in rock > differential heating and cooling within rock (as different minerals are different colours and therefore absorb different amounts of solar energy)
- Minerals that absorb lots of solar energy expand and contract as they are heated and cooled
- This creates stress on rock in various places
- Cracks form and rock splits apart in lots of different places.
(Form of granular disintegration)
Mechanical weathering - Shattering / Thermal Fracture
- Repeated cycles of heating + cooling over time in a homogenous piece of rock
- This causes stress to build up throughout rock and eventually rock splits down the middle.
(Form of block disintegration)
Chemical weathering
Chemical processes react with the minerals in rock, causing it to break down
Biological weathering
The actions of plants and animals break down rock.
Important in understanding mechanical weathering
Idea of differential cycles creating stress on rock
Chemical Weathering - Crystal Growth
High temp in day > rock is heated
Water evaporates & salt deposits are left behind > crystals form
These prise apart the rock, causing it to be broken down
Mechanical weathering - Exfoliation (Onion skin weathering)
High diurnal range > differential heating + cooling (as outer layers are heated + cooled very quickly, whereas this happens a lot slower in inner layers).
Outer layers expand and contract as they are heated and cooled > stress on rock in outer layers but not inner layers
Cracks form in outer layers and they break off.
(Form of block disintegration)
Chemical Weathering - Hydration (another type of crystal growth)
Rain > salt minerals in rock absorb water water > crystals form & rock swells
Stress of this expansion causes rock to break apart.
Chemical Weathering - Hydrolysis
Slightly acidic rain > clays + dissolvable salts contained in some rock dissolve
Some of rock dissolves
Chemical weathering - oxidation
rain > minerals of iron contained in some rock oxidise (bc of presence of water + oxygen)
rock becomes weaker and so collapses more easily, which causes breakdown
Important in understanding chemical weathering
Presence of water is vital
Aeolian Landforms
Formed from abrasion - Rock Pedestals, Ventifacts, Yardangs, Zeugens
Formed from deflation - Deflation Hollows, Desert Pavement
Formed from deposition - Sand Dunes (Barchan and Seif)
Wind transport / erosion is more effective when
- less shelter
- more weathering + erosion has occurred (bc more loose material)
- loose material is smaller (bc can easily be picked up by wind)
- high wind speed
- wind blows for a long period (bc more time for transport/erosion)
Saltation
- Happens when wind speed exceeds threshold velocity (speed requires to move grain particles), so both coarse & fine particles can be picked up by wind
- Particles rise for several cm, then fall to ground, following a flat trajectory that sees them fall a short distance from where they were picked up, but not in the same place
- Process repeats > particles moved by wind in leap-frogging / bouncing manner
Ventifacts
- Fin shaped rocks lying on desert surface
- Smooth and flat windward side
- Sharp & narrow edges
Surface Creep
- Happens when particles are more than 0.25mm in diameter, so are too heavy to be lifted off ground by wind
- But due to continued bombardment by smaller particles being transported by saltation / suspension, can dislodge & be pushed slightly forward
- Process repeats > slow creeping motion of particles along desert floor
Suspension
- Happens when materials are less than 0.15mm in diameter, so can easily be picked up by wind
- Material raised to a considerable height + carried in air for great distances
- Can create dust storms that reduce visibility to less than kilometre
Abrasion
- Process of wind erosion
- Sand carried in wind, mainly by saltation
- Sandblasts rock, eroding it.
Amount of abrasion depends on
- velocity, frequency, direction of wind
- rock lithology
- size of loose particles
Yardangs
- Form where alternating bands of hard and soft rock run vertically, parallel to direction of p.v.
- So softer rock eroded by abrasion, leaving ridges called yardangs.
- However can become discontinuous if e.g ^ weathering, period with lots of wind from another direction, but will still be very prominent in landscape.
- Possible that the Spinhx at Giza is/was one
Rock pedestals
- Bc sand particles in saltation are too large to be lifted very high, abrasion mainly takes place less than 1.5m above the surface
- So lower down rock is eroded more than higher up rock
- Creates rocks with narrow bases, known as rock pedestals
Zeugens
- Very similar to yardangs, which causes dispute as to whether they are actually present in H.D.E.s or not.
- But formation is different, bc alternating bands of hard and soft rock run horizontally, perpendicular to p.v.
- So often form where hard rock is just duricrust (i.e. brittle layer of mineral salts on surface) as once this is broken, underlying softer rock will have lost its protection & will be stripped away easily.
Deposition occurs when
Wind velocity decreases, meaning sediment can no longer be transported.
Landforms of deposition in H.D.E.
- Sand dunes - large deposits of sand
- 2 main types - Barchan and Seif
Barchan Dunes Formation
- Flow of wind changes to go around the mound, gradually shaping it into a crescent shaped mound
- Once established, reaches equilibrium, whereby sand moves up back slope by saltation or surface creep, and is deposited on leeward crest
- This forms slope of angle 32
- But when more sand is added to crest, angle of repose is exceeded, so leeward face collapses, and dune moves forward
- This process then repeats over and over is why the dune is so mobile
- Horns move faster than rest of dune, so over time crescent becomes stretched out / elongated
Types of river - Endoreic
- Formed when a water body’s journey is ended before its surface water reaches the oceans
- This forms highly saline lakes, as there is a continuing input of minerals but little input of water.
- Exist in belts between the N + S margins of desert zones in both hemispheres
Seif Dunes
- Sinous longitudinal dunes with 2 slipfaces, a sharp chest, and a pointed downwind terminus
- Develop parallel to average wind direction, & are shaped by diurnal / seasonal slight changes in wind direction.
- Develop in rows with secondary dunes in between, which over time can merge to form Y-shaped junctions going downwind
- Can grow up to 200m tall, 100’s of km long
- Gradually elongate but this is very subtle, so well established and stable that are used as navigational tools
Seif Dunes Formation
-
Types of river - Ephemeral Streams
- Short lived rivers that flow intermittently or seasonally after rainstorms
- Occur as a result of a huge amount of rain being deposited, which exceeds the infiltration capacity of the ground (bc hard baked ground is impermeable and lack of vegetation to intercept the precipitation)
- So as a result these streams generate lots of discharge
- Often help in the creation of fluvial landforms such as alluvial fans
Types of river - Exogenous
- Permanent rivers
- Begin at a permanent water source in an area of water surplus, often the highlands beyond the desert, then flow through desert to ocean.
- Discharge must always be > POTET
- E.g. River Nile begins in East African Highlands, flows through Sahara, and out to Med. Sea.
- Spend lots of their energy on vertical erosion, and so often flow through valleys with steep sides, formed as a result of this erosion happening repeatedly over a large time period
- Can also form canyons, e.g River Colorado formed the Grand Canyon.
Episodic Flash Floods
- Infrequent but heavy rainfall events that unleash huge amounts of water in a very short space of time
- Often there is so much rain that it doesn’t stay in channels but spreads out over a large surface area.
Barchan Dunes
- Crescent shaped mounds with gentle backslope (32 degrees) and steep slipface
- Develop around obstacles e.g. vegetation, rock, sand patches.
- Form perpendicular to p.v. with backs (i.e. gentle slope) to the wind
- Can grow up to 30m tall, 100’s of m long
- But growth by disrupted by dramatic changes in wind speed / direction or sand supply
- Highly mobile, move 5-50m per year
Mesa
- A plateau which is the remnant of the former land surface
- The land has probably been uplifted
