EXAM 3 - chapter 19 wind and desert
Wind can only transport dry sediment because
wet sediment is too cohesive to be transported.
Sediment is deposited at alluvial fans as the result of a
decrease in stream current velocity.
Playa surfaces are typically
dry, hard and rough during the dry season, but wet and very soft in the rainy season. Bonneville Flats where race cars are tested is such a playa.
Loess soils are highly
erodible
sand blasting
erosion of a solid surface by abrasion caused by the high-speed impact of sand grains carried by wind
Transverse Dunes
form in arid regions where there is abundant sand. Long ridges are oriented at right angles to the wind directions.
Water deposited sand is not
frosted.
Convection currents occur because the Sun
heats the Earth most intensely over and near the equator
The Sahara Desert is the largest
hot desert in the world
The streams on alluvial fans consist of
intermittently flowing braided channels. These channels are susceptible to flash flooding without warning.
Rain shadow:
low-rainfall region that exists on the leeward side of a mountain range > result of mountain range causing precipitation on windward side.
Linear (Seif) Dunes
occur in areas that have a moderate sand supply, a rough pavement, and winds in the same general direction. The dunes are oriented parallel to the general wind direction.
eolian
pertaining to wind
Beach sand can be carried inland on _____ forming coastal sand dunes.
prevailing winds
Beach sand can be carried inland on
prevailing winds forming coastal sand dunes.
Windward side
rising air expands, cools, causing condensation - promoting precipitation (adiabatic cooling)
Deserts tend to be either
rocky (desert pavement) or sandy (sand dunes or ergs)
Quartz grains are common constituent of
sand dunes. Frosted quartz
In arid regions wind can be the most significant means of
sediment transport.
Leeward side
sinking air compresses, warms up, dries - promoting evaporation (adiabatic warming) "Chinook winds"
loess sometimes also refers to
soils derived from such deposits.
Loess often exposed in
steep or vertical faces.
The Coriolis effect causes
the descending air to torque and become the Trade Winds. The air is still in the "doldrums" around the equator.
The Atacama Desert is
the driest desert on Earth. Measurable rainfall has not been detected in the Valle de la Luna in history.
Transverse dune slip face is pointed in the direction of
the prevailing winds are moving.
deflation
the removal of clay, silt, and sand form dry soil by strong wind, which gradually scoop out shallow depressions in the ground
slip face
the steep leeward slope of a dune on which sand is deposited in cross-beds at the angle of repose
desertification
the transformation of semiarid lands into desert
The amount of sand and fine-grained sediment that can be moved is a function of
the wind speed. As the wind speed increases, the volume of material that can be moved increases exponentially.
Heavy precipitation occurs on
the windward side of the mountains.
What resistant rock cap the mesa?
usually sandstone, limestone or a lava flow (e.g., basalt) Less resistant rock such as shale, mudstone and siltstone make up slopes
Hadley cell:
vertical and horizontal convective air circulation in tropical/ subtropical regions creating weather patterns
loess occur as a
widespread blanket deposit that covers areas of 100s km2 and 10s meters thick.
Deflation surfaces are formed by
wind scouring.
A loess soil is composed of
wind-blown silt. The thick accumulations of loess occurred during arid and semi-arid conditions associated with glaciation during the Pleistocene.
dust
windborn material usually consisting of particles less than 0.01 mm in diameter (including silt and clay) but often including somewhat larger particles
Erosion, once initiated becomes a positive feedback cycle and is
very difficult to stop. Once a soil becomes degraded by erosion (stripped of topsoil) it becomes less productive for crops and may take centuries to repair. Soil degradation is therefore considered to be permanent!
Evaporite minerals are _____ minerals precipitated from supersaturated waters as a water body is evaporating and ultimately dries out
water soluble
Rain Shadow: The Himalayas prevent the summer monsoon rains from reaching the Tibetan Plateau.
Himalaya Mountains Form a Rain Shadow for the Tibetan Plateau
Ventrifacts:
Sandblasted pebbles or rocks. A rock sculpted by sand-blasting (abrasion) by the wind.
Wind as an Agent of Erosion
Ventrifacts Deflation surface Desert pavement
Loess
Windblown Sediment & Soil (Silt)
loess
a blanket of unstratified, wind-deposited, fine-grained sediment
pediment
a broad gently sloping platform of bedrock left behind as a mountain front erodes and retreats form its valley
desert pavement
a course, gravelly ground surface left when continued deflation removes the smaller sand and silt particles from desert soil
dry washes
a desert valley that carries water only briefly after a rain
wadi
a desert valley that carries water only briefly after a rain
desert varnish
a distinctive dark brown, sometimes shiny coasting found on may desert rock surfaces, consisting of a mixture of clay minerals with smaller amounts of manganese and iron oxides
playas
a flat bed of clay and encrusting precipitated salts, formed by the complete evaporation of a playa lake
Pediment are usually covered in
a mantle of alluvial sediment from the eroding mountains.
ventifact
a pebble with several curved or almost flat surfaces that meet at sharp ridges, formed by sandblasting of the pebbles windward side
Dunes are generally (but not always) found in
arid and semi-arid climates
Mesas and buttes are found in what climate?
arid and semi-arid. In more humid regions, weathering and erosion round the elevations forming hills and ridges.
Desert pavement produces an _____ surface.
armored -that resists further erosion.
Pediment:
A broad, gently sloping inclined surface of bedrock left behind as a mountain front erodes and retreats from its valley.
Alluvial fans
A cone- or fan-shaped accumulation of sediment deposited where a stream widens abruptly as it leaves a mountain front for an open valley.
Washes, arroyos, wadis:
A desert valley that carries water only briefly after a rain. These valleys are prone to flash flooding, without warning. Names, such as gulch, wash, arroyo or wadi vary by region.
Playa:
A dry or ephemeral lakebed or remnant of a permanent lake.
Loess:
A homogeneous, typically nonstratified, porous, friable, slightly coherent, often calcareous, fine-grained, silty, pale yellow or buff, windblown (eolian) sediment consisting mainly of silt.
Desert pavement:
A remnant ground surface of gravel too large for the wind to transport, left when continued deflation removes the finer-grained particles from a mixture of gravel, sand and silt in the sediments and soils.
Sand Sea - Erg
A sand sea occupying a large, relatively flat area of desert covered with windblown sand with little or no vegetative cover. The formation of ergs requires a long period of time for sand accumulation, believed to require at least a million years.
Mesas & Buttes:
A small, flat-topped elevation with steep slopes on all sides created by differential erosion of bedrock of varying hardness.
Evaporite (Salt) Deposits
An accumulation of minerals precipitated out of sea water and from lakes in arid regions where there is no river outlet.
Dune
An elongated ridge of sand formed by wind or water.
Types of Dunes
Barchan Dunes Transverse Dunes Blowout (Parabolic) Dunes Linear (Seif) Dunes Star Dunes
Areas Without Vegetation are Prone to
Blowout When vegetation is "blown out" by strong winds parabolic dunes are formed.
Structural Cross-Section of Death Valley
Death Valley occupies a graben or downthrown structural blocks in an extensional terrain.
Wind as an Agent of Deposition
Dunes
_____ may become stabilized with vegetation as climate becomes more humid and become active again as the climate becomes more arid.
Dunes
Dune formation:
Dunes form in relatively few environments. Require a large supply of dry sand in an area with no or limited vegetation and steady winds.
Trade winds:
Earth's rotation deflects descending air (Coriolis effect) to create steady winds flowing from northeast/west to the N & S of equatorial region.
Gypsum sand:
Evaporite minerals (e.g., gypsum) can form sand such as White Sands in New Mexico.
The wind is forced to rise along the mountain front leaving the sand at the base of the range.
Great Sand Dunes National Park San Luis Valley, Colorado
_____ air expands, rises, and cools
Heated
Convection currents creating extensive deserts
Heated air expands, rises, and cools Condensation and precipitation occur The equator's constant heat causes this process to repeat Supporting tropical rain forests The now dry air "spills over" north and south of the equator Descending over subtropical areas (30o N & S) and creating extensive deserts The descending air is also rotated by the Earth's rotation (Coriolus effect)
Barchan Dunes Occur in Areas With
Limited Sand Supply
Orographic Effect of the Cascades Palouse Falls
Low Rainfall (Rainfall 15-8 Inches/Year): Palouse Falls, Eastern Washington
The Sand Hills of Nebraska is the largest dune field in
North America. They were formed during more arid times during the Pleistocene Ice Age.
Pediments should not to be confused with merged groups of alluvial fans (bajadas), which also may appear to gently slope from an escarpment, but are composed of material eroded from canyons, not bedrock.
Pediment can be thought of as a ramp of sediment rising up to an eroded mountain front or remnant.
Orographic Effect Of The Cascades in Seattle
Seattle Lies in the Rain Shadow of the Olympic Range & Receives 96 cm/year (38 inches/years) Seattle and Portland, while moist most of the year generally receive less than 100 cm per year (40 inches per year) of precipitation as they both lie in valleys in the rain shadow of the coastal ranges. Mt. Rainier receives over 15 meters (50 feet) of snow during the winter.
_____ form where the prevailing wind comes from several different directions heaping the sand into dune clusters.
Star dunes
Dunes are Initiated by
Surface Obstacles A Rock or Object Forms an Eddy That Accumulates Sand A small rock forms a wind shadow that causes sand to accumulate in its lee. A small incipient dune forms that can then start to grow a more sand accumulates by positive feedback.
Desertification - Soil Degradation
The formation and expansion of degraded soil and vegetation covering arid, semiarid and seasonally dry areas, caused by climatic variations and human activities.
Cross-Bedding in Sand From Wind
The lower dune was truncated by erosion and deposition of a new dune.
Cross-bedded sediments:
The presence of cross-beds in sand indicates the creation of ripples and larger beds form as the result of unidirectional current action (in this case wind).
Deflation surface:
The removal of dust, silt and sand from dry soil by strong winds that gradually scoop out shallow depressions in the soil. Sand dunes in the distance is where sediment removed from the deflation surface was deposited.
Carbonate sand: is formed where there are
abundant shells and coral. Carbonate sand dunes and beaches in the Bahamas and Bermuda.
Playas are also known as
alkali flats, sabkhas, dry lakes or mud flats. If the surface is primarily salt then they are called salt pans, salt lakes or salt flats.
Blowout (Parabolic) Dunes
are almost the reverse of barchans. In barchan dunes the slip face is concave downwind, with blowout dunes the slip face is convex downwind.
Barchans Dunes
are crescent-shaped dunes, always the products of limited sand supply and unidirectional winds. Barchans point downwind.
Star Dunes
are formed where the wind blows from several dominant directions. Essentially, the sand is heaped up where several limbs of the dune intersect.
Playas consist of fine-grained sediment, usually
clay (mud), alkali minerals and salts.
Wet sand is too _____ to be transported by wind.
cohesive
Bajada:
consists of a series of coalescing alluvial fans along a mountain front.
