Silt and sand Flashcards
size of coarse dust
20-70 microns
size of fine dust
less than 20 microns
3 sources of silt
from the bedrock, glacial grinding, regional sources
name 3 regional sources of silt
river channels, lake basins, agri land
what does cloudy blueish water in valleys indicate
high silt content due to glacial grindign
what can indicate the weathering process that released the grain
study of the grain itself
what is flocculation
silt particles joining together to form sand sized grains
name 3 high yielding sources of dust
floodplains, playa lakes and agri land (due to ploughing practices0
dustiest place on earth
Lake Chad
how much dust transported annually in the troposphere
1-3 billion tons
3 functions of dust transport
- source of nutrients for rainforest system- millions of tons from Africa to the Americas
- primary productivity of the oceans (taken in by phytoplankton)
- radiative transfer- scatter or absorb radiation
give two possible effects of dust on rainfall levels
could interfere with cloud formation by lowering the adiabatic cooling of the atmo.
OR could act as cloud condensation nuclei
what percentage of earths surface is covered in loess
10%
how is loess good for agri
high calcium content, fertile, moisture-holding, good for ploughing
where is most loess derived from
glacial deposits
4 sources of sand
weathered rock outcrops, river channels/floodplains, coastlines, lake basins
what is the composition of a dune dependent on
the source of the sediment
where is sand primarily sourced from
river deposition
what is the most common mineral in sand
quartz
what processes are effective at creating sand sized particles
fluvial and coastal movements of sediment
example of a direct source of sand from a river basin
if the river basin is directly adjacent to the desert, sand can be transported by wind onto the desert
example of an indirect source of sand from a river basin
river sediment dispatched into the sea which carries it by longshore drift to be blown onto the desert
how are sand particles transported from a lake basin
usually clay size particles are transported, but they can be bound into sand sized particles by salt-> deposits “clay” dunes as the bonds dissolve
how are the bright white sand of the White Sand Dunes National Monument formed
sourced from a playa where gypsum crystals precipitated out of the evaporating water
what are sand sheets
extensive accumulation of sand in thin low relief deposits, 40% of sand seas covered by sheets
what are zibars
A low-relief, rounded, coarse-grained, sand dune with no slipfaces…. usually on sand sheets
what are sand stringers
focused pathways of sand transportation
what percentage of sand seas are covered by dunes
60%
what is a sand dune
hill or ridge of loose, well sorted sand shaped by wind and usually capable of downwind motion
other word for the windward slope
stoss slope
other words for the leeward slope
avalanche face, slipface
is the leeward or windward slope usually steeper
leeward slope (30-43 degrees)
up to how high can dune crests be
15m
give the 5 steps of dune initiation and development
- sand patch initiation
- wind-ripple protodune
- streamlined protodune
- protodune
- dune with slipface
what is flow separation
the wind closest to the ground starts to lose energy and saltating sand falls to the ground
which stage of dune development enhances flow separation to begin
when it is a wind-ripple protodune
at which stage of dune development does flow separation occur
when it becomes a protodune
describe how the sand dune form is maintained
aerodynamic control: wind increases velocity as it is pushed upwards, saltation causes sand to shoot off crest
gravity control: sand falls on lee face and sis deposited
how do dunes move
sand is eroded from the windward face, accumulates at the crest and avalanches down the slipface
do larger or smaller dunes move faster
smaller
what can cause a dune to become stabilised
vegetation
what is sand drift
the process of sand movement across the desert as a result of surface winds
what is drift potential
capacity of surface wind to drag sand across dry deserts
what is resultant drift direction
the vector sum of drift potentials
what is resultant drift potential
the amount of sand moving in the net direction
what ratio can be used to interpret forms of dunes in a sandy desert
ratio of resultant drift potential to drift potential
what is the main control on dune type
the wind directional variability versus the amount of sand in the dunes
where do barchan dunes form
relatively sediment starved locations where there is a unimodal wind
where do transverse/barchanoid dunes form
where there is a higher supply of sediment (eg from the sea) and a fairly uniform wind direction
where do linear dunes form
where there are 2 wind directions (eg summer and winter) that push the sand into a ridge
where do sand dunes form
where there have been multiple wind directions, arms are more mobile than core
what are compound dunes
larger dunes with smaller dunes on the windward slope
what are complex dunes
dunes with smaller dunes on top of a different type
