W8L2 Dester processes

Question 1

desert climate

Answer 1

extreme aridity - rainfall less than 25 cm per year
hot OR cold climate
night/day temperature contrast
evaporation > precipitation

Question 2

water in desert

Answer 2

no permanent streams originate in deserts
permanent lakes are rare & saline
sporadic precipitation (flashfloods/meltwater)
ephemeral streams & lakes = sediment dumping & evaporites
typically less than 15% vegetation cover

Question 3

desert . weathering style

Answer 3

high cliff-retreat rates + low-transport = large unweathered clasts
semi-constant wind = high-transport, highly weathered small clasts

Question 4

Wind transport

Answer 4

Wind blows from high to low pressure until equilibrium is established
Long transport + high weathering = well-sorted, well-rounded clasts

Question 5

Windblwon Sediment

Answer 5

well-rounded
high sphericity
well-sorted – grains all the same size
grain collisions remove unstable minerals & produce frosted surfaces
‘supermature’ = texturally & compositionally mature
grains often stained by Fe2O3, haematite

Question 6

Dunes

Answer 6

Large scale ripples (m to 10s of m) fed by a continuous sediment supply (ie lack of vegetation + exposed bedrock = high weathering rates)
dune laminations are steeply inclined
large-scale cross-bedding (metres to 10s of m)
‘way-up’ indicator
palaeowind direction indicator

Question 7

Dune types variables

Answer 7

Variables –
wind velocity and consistency of direction
sediment supply

Question 8

Dust

Answer 8

Sand-sized grains stay within the desert, while the finest grains are transported 1000s of km
deserts are the major global dust sources and contribute to –
fine-grained sediment in the deep ocean
atmospheric dust
loess deposits
climate change
Atmospheric dust (recovered from cores drilled through ice sheets) & loess deposits are important Quaternary climate archives
vegetation cover
wind speed

Question 9

Modern dust sources

Answer 9

amount lofted related to grain size & wind strength/persistence
long distance transport: grains lofted into troposphere
strong daytime heating OR cold fronts
raindrops nucleate around dust grains deposition
climate cooling & drying
atmospheric dust lowers albedo (surface reflectivity)
absorbs heat from the Sun climate warming
loess also sourced from volcanic ash & glacial outwash plains

Question 10

Katabatic glacial winds

Answer 10

Beyond the ice sheet edge, cold dense air flows underneath adjacent low pressure
strips fine-grained sediment (clay & silt) off the glacial outwash plain
sediment (loess) deposited downwind where pressure equilibriates

Question 11

glacial outwash plains sandurs

Answer 11

main source of quaternary loess
ex iceland
katabatic winds av. 80 km/h, but can be up to 200 km/h
can strip the paint from a car in less than a week!

Question 12

Loess

Answer 12

Loess: long-transport sediment: fine grainsize and stable mineralogy
clay + silt-sized quartz sand
dark bands are soil horizons = interstadials/interglacials

Question 13

Wind Erosion

Answer 13

vegetation-free surfaces constantly under attack
similar landforms to rocky coasts: promontories, stacks, arches
desert pavement -
fine grains constantly winnowed by wind
larger grains settle into the spaces

Question 14

abrasion

Answer 14

Desert winds carry dust and fine-grained sediment

Question 15

yardangs

Answer 15

yardangs: a softer lithology capped by a harder one
‘stalked’ landforms
elongated in the prevailing wind direction
zB “Argentinian Sphinx”
theres also yardangs on mars

Question 16

desert sediment ransport

Answer 16

Gravity: cliff retreat & rockfall (all grainsizes) – short transport + low weathering = poor sorting, angular clasts 
Flash flood (all grainsizes) - short violent transport with high clast & bedrock weathering

Question 17

dry dester weathering and erosion

Answer 17

physical weathering, accelerated by lack of vegetation
jointing & pressure-release
salt expansion
rockfalls/debris flows/alluvial fans/bajada
bajada: overlapping alluvial fans
source of sediment for dune formation
rare, violent sediment transport (flashflood) further erodes bedrock

Question 18

Flashflood

Answer 18

Flashfloods- desert transport, erosion & deposition
Talus aprons can lie undisturbed for years -
immature texture (angular, poorly sorted) & mineralogy (unstable & stable minerals)
Flashfloods - brief, high energy, destructive events
short, rapid transport & sediment dumping
clast and bedrock weathering

Question 19

Flashflood Erosion

Answer 19

Sediment-laden torrents scour steep-sided, narrow channels = arroyos
undercut cliffs
create grooves
potential for accelerated weathering and wide channels = canyons

Question 20

Peneplation

Answer 20

the result of 1000s of years of cliff retreat
Dynamic cliff retreat
pediment (foot of the cliff) covered by alluvium
rare violent storms
sediment-laden sheetwash scours the pediment = accelerated weathering & erosion
inselbergs, buttes

Question 21

Flashflood deposition

Answer 21

Water sinks/evaporates sediment load dumped
poorly sorted, compositionally and texturally immature
arkoses and polymictic breccias

Question 22

Alluvial Fan deposits

Answer 22

Debris-laden flashflood + rapid drop in stream gradient = abrupt sediment dumping
cone-shaped wedge of angular, moderately sorted sediment
coarse-grained close to source (proximal)
fine-grained furthest from source (distal)
compositionally/texturally immature arenite (arkose or lithicwacke)

Question 23

weathering in a dester

Answer 23

weathering is dominantly physical (lack of water & vegetation)
transport dominantly by wind, sporadically by water and gravity
dune formation = constant sediment supply + wind
windblown loess (dust + clay + silt) = Quaternary climate archives
‘super mature’ sediments
violent storms promote accelerated physical weathering & erosion
abrupt dumping of sediment
compositionally and texturally immature
evaporating bodies of water precipitate dissolved ions