the sahara Flashcards
what are ergs?
seas of sand, making up around 20% of the sahara. mostly in the northern sahara. a dune field
what are regs?
stony desert plains, making up around 70% of the desert
what are hamadas?
elevated rocky plateaus
describe physical weathering and the sahara
breaks rocks intomsmaller debris. debris forms sediment grains.
weathering, erosion, transportatio, deposition, lithification
describe transport and the sahara
once eroded, sediment grains may be transported by water, ice and wind
grains become smaller and rounded during transport
how can you descibe sediment grains?
roundness - angular, subangular, subrounded, rounded, well rounded
grain size - the Wentworth Scale
describe the classifications in the wentworth scale
gravel - sediment more than 2mm in diameter
> conglomerate = rounded pebbles
> breccia = angualr pebbles
sand - sediment 63 micros to 2 mm
slit - 4 to 63 micros, form siltstones
clays - less than 4 micros, forms claystones
descibe saharan sand
fine to medium sand grains
very well sorted
colourless to reddish brown (iron)
texture = frosted
rounded grains
high sphericity
windblown in a hot arid environment
what are aeolian landforms?
ripples that form with ridge crests and right angles to the prevailing wind
dunes tend to form either as free dunes (enlarged ripples or as impended dunes
what are sand dunes and what are the different types of dunes?
windblown accumulation of sand
brachan, star, transverse, parabolic and longitudinal
what is desert pavement?
a deflation mechanism, forming a hamada
describe the mechanism of desert pavement
air is less dense than water, so a very strong wind is required to transport sand sized grains. usually coarser grains are left behind.
describe porosity
water can be stored in 2 places in a layer of rock
1. pore space - the space between induvidual sediment particles that can fill with water ans store it
2. fractures/ cracks - water stores in cracks
different rock types have different porosity
can be expressed as a number
porosity = volume of voids/ total volume
descibe permeability
how easily a fluid can pass through a material
low = hard for a fluid to move through
connections between pore spaces and fractures help to increase permeability
mud and clays help to decrease permeability as the gaps between them are very small
a well sorted sediment has a higher permeability
what is an aquifer?
a geological unit of rock or sediment that can store and transmit water
what is a confining layer?
a geological unit that does not allow water to move through it quickly
what are the 3 different types of aquifers?
- unconfined aquifer - above the water table, pore spaces are filled with water (saturated zone)
- confined aquifer - has a confining layer e.g. clay which prevents the movement of water. has a recharge are where water enters the aquifer
- perched aquifer - above the water table, has springs connecting
describe the water table
loosely follows the topography, generally as a subdued image of the surface. Is close to the ground in valleys, where rivers and streams often form
can change over time e.g. between the beginning and end of the rainy season
how does water move?
from points of high pressure to points of low pressure
in a typical aquifer, groundwater moves 5 - 30 cm a day
describe groundwater abstraction
during pumping, a cone of depression is formed around the well, lowering the water table
where abstraction id greater than the recharge of water, groundwater id depleted
what are some probelms of groundwater abstraction?
- wells can stop being functional
- reduced surface water supplies
- land subsidence - surface of the land is lowered
- water quality (saline intrusion) - saltwater invading freshwater aquifers
