Arid Landscapes
Which are the middle-latitude deserts?
Great Basin, Gobi and Patagonian
What do barchanoid dunes look like?
a combination of barchan and transverse dunes, like a series of barchan dunes that form ridges at right angle to the wind
What is the leeward side of a dune characterised by?
a decrease in energy and sand avalanches which occur due to over steepening
What is the difference between desert and steppe?
a matter of degree, steppe is just a more humid variant that surrounds the desert and separates it from the adjacent humid climates
What do longitudinal dunes look like?
a series of long parallel very narrow ridges that are formed by slightly variable wind direction
How are barchan dunes are formed?
are crescent shaped, the movement is like a pacman and the open part of the crescent is where the downwind is, found in areas where there is moderate supply of sand but often they move along bare bedrock, exhibits concave slipface,
How are transverse dunes formed?
are linear dunes that form perpendicular to the wind direction and form in areas where there is more sediment, consistent wind direction forms linear ridges
What are the two hypotheses of desert pavement?
as coarse material develops on the surface it blocks the fine-grained sediment below from the wind which prevents the possibility of further deflation, OR wind energy diminishes between large grains of the surficial layer leading to the deposition of fine-grained sediments between large grains
How do cross-beds form?
by the movement of sediment dunes and ripples, currents of flowing water or blowing wind erode grains form the up-current side of the dune and carry them up to the dune crest, where they avalanche down the steep down-current face of the dune
How does deflation work?
causes small sediments such as silt to be lifted into the air, while larger sediments, like sand are pushed along the ground, larger sediments such as pebbles cobbles and boulders are too heavy to be transproted
What do star dunes look like?
complex shape because they form where wind direction varies, shaped like mountains, typically having several ridges stemming from a high centre point
What do parabolic dunes look like?
crescent shaped, much like barchan dunes, except the tips of the crescent shape move slower then the centre of the dune, these tips point into the wind because the sand is anchored by vegetation
What is the relationship between water currents and cross beds?
cross beds sit at an angle to current direction
What are dunes and ripples?
curving ridges of loose sediment that move along with water or wind currents, or move back and forth beneath oscillating water waves
In deserts wind erosion can cause?
deflation
What is the erosion by wind, in arid and semiarid areas such as desert and steppe, known as?
deflation
What are the two types of dry climates?
desert - arid and steppe - semiarid
Very dry or arid regions are known as what?
deserts
What are one of the main depositional features created by wind?
dunes
Describe the distribution of air pressure and winds on Earth's surface in the equatorial low pressure belt
heated air rises to great heights at the equator and spreads out, when the upper level flow reaches 20-30 degrees latitude it sinks toward the surface, rising air expands and cools, this process produces clouds and precipitation, therefore the region influenced by the equatorial low is rainy
What is the best explanation for the abundance of desert areas in the low latitudes?
high-pressure systems in these regions prevent the formation of clouds that could carry water, as air sinks in these regions it is both warmed and compressed thus inhibiting cloud formation and precipitation
How do climatologists classify a dry region?
if yearly precipitation is less than the potential yearly evaporation
Where do subtropical deserts form?
in areas of high pressure that stretch worldwide as a part of global atmospheric circulation, dry air descends from high in the atmosphere, often having the appearance of a desert with clear skies, sunshine and veery dry conditions
How are parabolic dunes formed?
in areas where the sand is some what anchored by vegetation, and often times the wind will be coming off a body of water and will slam into the semi-stabilised dunes creating an area of blowout, crescent shaped but the slip face is on the convex side of the crescent so the open mouth is facing in the up wind direction, opposite to the barchan dunes, convex slip face
What is a rain shadow effect?
in middle latitude deserts that form where a mountain chain interrupts the prevailing winds, the wind goes up and over the mountains, the water vapour it carries condenses, forming clouds and rain on the windward (facing the wind) side of the mountains and a desert on the leeward (sheltered from the wind) side where the air (which is now very dry) descends the mountains
Where do the middle-latitude deserts and steppes occur?
in the deep interiors of landmasses far from the ocean and/or in places where moisture-laden winds are blocked by mountains
Where do middle latitude deserts form?
in the interiors of continents, far from the ocean or other sources of moisture and anywhere land is cut off from sources of moisture
Where does the heart of the low-latitude subtropical deserts lie?
in the vicinity of the Tropic of Cancer and the Tropic of Capricorn, the deserts here are the result of prevailing distribution of air pressure and winds
What are cross-beds?
inclined layers in sediment or sedimentary rock that reveal current or wave transport of sediment, they mark sequential positions of the migrating, down-current side of the dune
How are barchanoid dunes formed?
intermediate in their form between barchan and transverse dunes, look like barchan dunes that have been lined up side by side,
What happens to air as it sinks?
it is compressed and warmed
What is cross-bedding?
layers deposited on a dune slip face that are inclined in the direction of wind transport
On which side a mountain is dry?
leeward side
What do transverse dunes look like?
long parallel ridges of sand that are found where sand is plentiful and winds impacting the ridges at a right angle are steady
Deserts generally have _______ humidities and _______ of sunshine.
low, lots
Which desert type may have rain shadows forming where mountains cross prevailing winds?
middle latitude desert
How are rainshadow deserts formed?
middle-latitude deserts are an example of how tectonic processes affect climate, where mountains are produced when plates collide
What are rainshadow deserts?
moisture-rich winds meet mountain barriers, air is forced to rise, expanding, cooling and produces clouds and precipitation, therefore the windward side of the mountains often receive lots of precipitation, air reaching the leeward sides of the mountains has lost much of its moisture making these regions much drier
Desert regions occur ___________ and always have a __________ climate adjacent to them
on every continent, steppe
What is the primary difference between the two desert pavements hypotheses?
one is based on a sediment volume decrease through time (erosion) while the other is based on an increase in sediment volume through time (deposition)
Approximately how much of Earth's land surface has a dry climate?
one third
How are longitudinal dunes formed?
parallel to the prevailing wind direction, wind direction has to vary, the average wind direction will be parallel to the dunes
What is dust?
particles carried in suspension by wind
Do plants and animals live within deserts?
plants and animals also thrive in pockets within desert landscapes where water is available
What is the term of the rolling and bouncing of sand grains during transport?
saltation
How are dunes originally formed?
sand saltating along the ground is prone to being trapped where an obstruction to its movement (shrub or boulder) exists, sand is then deposited because wind bends around objects, as more sand accumulates the areas that are protected from the wind expand which will eventually cause a dune
What are regions called that are dry but have slightly more water called?
semiarid regions or steppe
What property of a piece of material will most directly determine how it is carried by wind?
size
What latitudes are deserts found in?
subtropical and middle latitudes
Where are dry lands concentrated?
subtropics and middle latitudes
What happens to the dune over time?
the erosion on the up-current side and deposition on the down-current side cause the dune to migrate in the direction of current flow
Deserts that lie outside of the subtropics and mid-latitude belts are most likely the result of what?
the presence of a rainshadow
Why are barchan dunes crescent shaped?
the wind blows the edges of each mound of sand faster than the center
What is coincidental about all of Earth's low-latitude subtropical deserts and steppes?
they all coincide with the subtropical high pressure belts
How are star dunes formed?
they have lots of little ridges that emanate out from a central point, form in areas where the wind direction is variable
How do dunes move?
they move over time with the wind as sand grains climb the windward slope of the dune until they are pushed it the leeward side where they slip down the lee slope (slip face)
How else can desert pavement form?
when sediments transported by wind and deposited atop gravels, fall down below the gravel, this would result in a layer of fine-grained sediment below the gravelled surface
How is desert pavement formed?
when silt and sand on the ground are picked up by deflation and the larger rocks are left behind, resulting in a pebbled surface
How are depositional features created?
when the velocity of the wind decreases, sediment that is moved by the wind drops
What is saltation?
where the wind pushes larger sediment like sand along the ground, rolling and bouncing
What is the definition of a dry climate?
where water deficiency exists year after year, annual precipitation is less than potential water loss by evaporation
Why do desert dunes migrate?
wind erodes material on the back side of the dune and deposits it on the front side of the dune
On which side of a mountain does precipitation occur?
windward side
What are the six major types of sand dunes?
barchan, transverse, barchanoid, longitudinal, parabolic, star
Why are soils uncommon in deserts?
because plants are less abundant, in more humid climates, plants add organic matter to the soil and their roots break rocks into fragments adding inorganic matter to the soil
At what latitudes do the large-scale convection cells bring dry air downward toward the Earth's surface?
between 20 and 30 degrees north and south
Wind erosion can create shallow depressions called what?
blowouts
How does deflation lead to the formation of desert pavement?
by carrying fine sediment away and leaving a layer of larger particles at the surface
How do pieces of rock move under the influence of wind?
rolling, sliding, bouncing, suspension
What are the features that dunes can be and where are they found?
waves, mounds or ridges of sand and are often found in deserts or coastal environments where sand is abundant
What is abrasion?
when sands saltate across the ground, they are able to erode the larger rocks, in a process similar to sandblasting
What are the subtropical highs?
30 degrees north and south latitude are zones of high air pressure, here the air is sinking, when air sinks it is compressed and warmed, opposite of what is needed to cloud formation and rain
How much land area does dry regions cover?
42 million square kilometres of Earth's land area, 30 percent, no other climate covers so large an area
What are examples of rainshadow deserts?
North America - Great Basin desert, Patagonian Desert in the rainshadow of South America's Andes Mountains and the Gobi Desert in the rainshadow of Asia's Himalayan chain
Where are major deserts found?
North America, South America, Africa, Eurasia and Australia
Which are the low-latitude or subtropical deserts?
Sahara, Atacama, Arabian and Great Australian deserts
