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EA1300 World of Dynamic Environments > Sahara Desert > Flashcards

Sahara Desert Flashcards

1
Q

What are the different types of Saharan landscapes?

A
  • ERGS: seas of sand (~20% of Sahara Desert).
  • REGS: stony desert plain (~70% of Sahara Desert).
  • HAMADAS: elevated, barren rocky plateaus, most finer sediment removed by deflation processes.
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2
Q

What is physical weathering?

A

Debris forms sediment grains (clasts) of clastic sediments and sedimentary rocks.

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3
Q

How are clastic sedimentary rocks formed?

A
  • Weathering (disintegration of pre-existing bedrock).
  • Erosion (removal of grains from bedrock).
  • Transportation (by wind, water, ice).
  • Deposition.
  • Lithification (turned into hard rock).
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4
Q

How is the roundness of sediment grains determined?

A

Pettijohn et al, 1987

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5
Q

How are sediment grain sizes classified?

A

The Wentworth Scale:
- Gravel (>2mm in diameter).
- Sand (63 micrometres - 2mm in diameter).
- Silt (4-63 micrometres) and clay (<4 micrometres).

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6
Q

Classifying sediments: sorting

A
  • Measure spread of grain size.
  • Tells us about transport/depositional environment.
  • Can influence porosity and permeability of rocks.
  • Wind very effective at producing sorted sediments in deserts.
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7
Q

What is the Sahara sand like?

A
  • Fine to medium sand grains.
  • Very well sorted.
  • Colourless to reddish brown.
  • Frosted.
  • Rounded grains.
  • High sphericity (mostly quartz).
  • Wind-blown (sandblasted) in hot, arid environment = desert.
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8
Q

What are aeolian landforms?

A
  • Ripples form with ridge crests at right angles to prevailing wind.
  • Dunes tend to form either as ‘free dunes’ or ‘impeded dunes’.
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9
Q

What are impeded dunes?

A

Lower wind speeds in lee of obstacles –> accumulation of sand deposits –> enlarged into dunes.

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10
Q

What are desert pavements?

A

Sometimes rocks in REGS are very closely packed. Different theories: deflating mechanism and silt/clay accumulation.

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11
Q

Desert pavement formation: deflation mechanism

A
  • Air = 1/1000x as dense as water so very strong wind required to transport sand size grains.
  • Usually coarser grains are left behind.
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12
Q

Desert pavement formation: silt/clay accumulation

A
  • Weathering forms rocky fragments on surface of exposed bedrock.
  • Silt/clay blown in accumulate between rocks.
  • Rainwater causes fine particles to percolate rocks, rocks remain on surface.
  • Over time, rocks break apart, forming tightly fitted stone surface.
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13
Q

When was the ‘African Humid Period’ (the green Sahara)?

A

Across North Africa, peaking 9000-6000 yrs ago.

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14
Q

What is the difference between a dry Sahara and a wet Sahara?

A

Dry - lots of dust blown into Atlantic.
Wet - little dust blown into Atlantic.

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15
Q

What is the West African Monsoon?

A
  • In summer, African land mass heats up faster than oceans.
  • Warm air rises, creating low pressure zone over continent.
  • Drives an inflow of moist air from Atlantic Ocean.
  • Brings rains to Sahel.
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16
Q

Why was the Sahara greener 9000-6000 years ago compared to today?

A
  • Perihelion: Earth’s closest pass to the Sun.
  • Perihelion today occurs in northern hemisphere winter.
  • 10,000 years ago, it occurred in northern hemisphere summer.
  • Summer radiation over North Africa was around 7% higher than today.
17
Q

What was the green Sahara’s impact on the human development and civilisation?

A
  • Before 8500 BCE: Late Pleistocene, hyper-arid desert with no settlements.
  • 8500-7000 BCE: Monsoon rains, Savannahs and prehistoric settlers.
  • 7000-5300 BCE: Human settlement well established, and cattle pastoralism.
18
Q

Is there still water leftover from the monsoons?

A

Yes - stored under Sahara in pore spaces between sand grains of Nubian Sandstone Aquifer System.

6,500,000 cubic metres extracted every day for use in Libya, but this water is not replenished.

19
Q

What are the two places in the layers of a rock water is stored in?

A
  • Pore space (intergranular/primary porosity): some rocks/unconsolidated sediments have space between individual particles which fill with water.
  • Fractures/cracks (fracture/secondary porosity): some rocks have cracks and fractures where water is stored.
20
Q

How is porosity calculated?

A

Volume of voids / total volume

21
Q

What is permeability?

A

How easily a fluid passes through a material.
e.g. low permeability = hard for fluid to move through material.

22
Q

What helps increase/decrease permeability?

A

Increase - connections between pore spaces, fractures.

Decrease - mud and clays (gaps between particles small).

23
Q

What are aquifers?

A

Geologic unit (e.g. rock/sediment) that can store and transmit water.

24
Q

What is a confining layer?

A

A geologic unit that doesn’t allow water to move through it quickly.

25
Q

What are the different types of aquifers?

A
  • Unconfined
  • Confined
  • Perched
26
Q

What is the recharge area?

A

Where water enters aquifer.

27
Q

What is the potentiometric surface?

A

Level that water in confined aquifer would reach if depressurised.

28
Q

What is an Artesian Well?

A

Water rises to ground surface without needing to pump it, after drilling into confined aquifer.

29
Q

What is the water table?

A
  • Loosely follows topography, generally subdued image of surface.
  • Rises under high ground, close to ground surface in valleys.
  • Can change over time (e.g. between end of rainy season and end of dry season).
30
Q

What is the movement of water?

A

Moves from point of high pressure to points of low pressure.

31
Q

What is the hydraulic head?

A
  • Measure of water pressure above specific point.
  • If ‘specific point’ is sea level, hydraulic head is measurement of pressure of water in rocks, sand, or gravel at any point above sea level.
  • Water will move from areas where this measurement is higher to areas where it is lower.
32
Q

Pressure = ?

A

force / area

33
Q

Force = ?

A

mass x acceleration

34
Q

What happens during groundwater abstraction?

A
  • During pumping, cone of depression is formed around well, lowering water table.
  • Where abstraction > recharge, groundwater is depleted.
35
Q

What are some problems caused by groundwater depletion?

A
  • Wells stop being functional.
  • Land subsidence: before pumping, stresses are in equilibrium. During/after pumping, stresses not in equilibrium, pressure reduced, stress from overlying material > opposing stresses = equilibrium restored by increasing effective stress = compaction = reduces aquifer thickness = lowers land surface.
  • Water quality (saline intrusion): abstraction in coastal regions results in saline water moving into freshwater aquifers.
  • Cost of drilling borehole expensive, poor cannot access water.

EA1300 World of Dynamic Environments (4 decks)

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