Geo Chap 19: Wind and Arid Regions

rainshadow

When air rises, expands and cools, a process that can produce clouds and precip. The windward sides of mtns, have high precip--leaving the leeward sides of mtns a lot drier!

barchanoid dunes

a common dune and intermediate between isolated barchans and transverse dunes. Form scalloped rows of sand oriented at right angles to the wing. The rows resemble a series of barchans that have been positioned side by side

abrasion

a form of wind erosion. *means to scrape away. occurs in dry regions as well as along some beaches, where windblown sand cuts and polishes exposed rock surfaces

deflation

a form of wind erosion. The lifting and removal of loose material. Sometime difficult to notice because the entire surface is being lowered at the same time

steppe (semi arid)

a marginal and more humid variant of the desert and separates it from bordering humid climates. These, and deserts, and concentrated in subtropics and middle latitudes

sub tropical highs

air is subsiding. air sinks and compresses and warms. Opposite needed to produce clouds/precip. *clear skies, sunshine, & drought (((tropics of Cancer and Capricorn))

Middle Latitude deserts and steppes

are not controlled by the subsiding air masses of high pressure. far removed from the ocean. Usually have high mountains, which force the air to lose its water (Gobi Desert, C.Asia)

interior drainage

arid regions that exhibit a discontinuous pattern of intermittent streams that do NOT flow out of the desert to the ocean

bed load

carried by wind that consists of sand grains

ephemeral streams

carry water ONLY in response to specific episodes of rainfall. MIGHT flow a few days, hours, or not at all. Can often create flash floods ((AKA: wash and arroyo, wadi, donga, nulla))

West coast subtropical deserts

cold, foggy, very dry. No clouds ((Atacama, SA; Namib, Africa))

ventifacts

created by abrasion where interestingly shaped strobes are left on the side of the stone that is exposed to the prevailing wind. It will polish, pit, abrade, and create sharp edges. IFF the wind was not consistently from one direction, to may have several surfaces

covers 30% of earths surface and <25 cm of precipitation. A region thats yearly precipitation is not as great as the potential loss of water by evaporation (depends on temp)

dry regions

Star dunes

dunes confined largely to parts of the Sahara and Arabian deserts. Isolated hills of sand that inhibit a complex form. Their bases resemble pointed stars

transverse dunes

dunes in regions where the prevailing winds are steady, sand is plentiful, and vegetation is sparse or absent, the dunes form a series of long ridges that are separated by troughs and oriented at right angles to the prevailing wind. *Many coastal dunes are this type. Also common in arid regions where the extensive surface of wavy sand is called a SAND SEA (sahara/arabian desert)

parabolic dunes

dunes that form where vegetation partially covers the sand. The shape of these dunes resembles the shape of barchans exempt that their tips point into the wind rather than downwind. Often form along coasts where there are strong onshore winds and abundant sand

longitudinal dunes

dunes with long ridges of sand that form more or less parallel to the prevailing wind where sand supplies are moderate. Are relatively small, but can get quite long in large deserts

Causes of Deserts and steppes.....

global distribution of air pressure and winds-heated air in belt (equatorial low) increase to gray heights and the spreads out. Air that rises to atmos. expands & cools **development of clouds and precipitation; regions under the influence of the equatorial low are the rainiest on the earth!!!

how can you know in a desert if MECHANICAL weathering occurred..

much of the weathered debris in deserts consists of unaltered rock mineral fragments. Dry lands lack moisture, therefore rock weathering is reduced **wind is primary weathering agent!

playa lake

occurs on rare occasions! when an abundant rainfall where streams may flow across the bajada to the center of the basin, converting the basin floor into a shallow _______. lasts only a few days or MAYBE weeks before evaporation and infiltration remove the water

suspended load

particles of dust swept high into the atmosphere by the wind, usually airborne for hours or days. Usually both silt and clay can be carried, but sold commonly makes up the majority because of the reduced level of chemical weathering in deserts proves only small amounts of clay

dunes

sand mounds or ridges from wind deposited sand

how can you know in a desert if CHEMICAL weathering occurred...

slopes of rock edges are rounded in humid climate, clay and thin soils oxidize (rust colored deserts)

cross beds

sloping layers as sand is deposited on the slip face. Layers from that are inclined in the direction the wind is blowing. When the dunes are eventual buried under other layers of sediment and become part of the sedimentary rock record, their asymmetrical shape is destroyed. **no where are these more prominent than in the sandstone walls of Zion Canyon in Utah

barchan dunes

solitary dunes shaped like crescents and with their tips pointing downwind. These dunes form where supplies of sand are limited and the surface is relatively flat, hard, and lacking vegetation

desert pavement

surface of the desert, consisting on closely packed pebbles and cobbles is usually 1 or 2 stones thick. Beneath is a layer containing a significant proportion of silt and sand. When this feature is present, it is a important control on wind erosion because the stones are too large for deflation to remove

slip face

the angle of repose for loose dry sand (34 degrees). The steepest angle at which loose material remains stable. Continued sand accumulation, coupled with these___, shows the migration of the dune in the direction of air movement

playa

the dry, flat lake bed left behind from a playa lake. Typically composed of fine silts and clays and occasionally with salts precipitated during evaporation

inselbergs

the late stage of the erosion of a mtn mass. The few large bedrock knobs projecting about the surrounding sediment filled basin that are left when an entire mtn mass is almost gone (ongoing erosion)

blowouts

the most noticeable results of deflation. Shallow depressions, ranging from small dimples (1m deep/3m wide) to depressions 50 meters deep and sever km wide. The depth of these basins are controlled by the WATER TABLE. **when these are lowered to the water table, damp ground and vegetation prevent further delation

saltation

the primary movement of sand being carried by the wind. A bouncing or skipping process along a surface. *This movement begins when wind reaches a velocity sufficient to overcome the inertia of the resting particles. At first it rolls, then a moving grain will jump and hit another, resulting in the other, or both, particles to jump. *chain reaction ((don't travel very far lower than 1m))

yarding

usually small wind sculpted landforms that are aligned parallel with the wind (>5m). Because sand blasting effects are greatest near the ground, these landforms will be the smallest near its base

What is does most of the erosional work in deserts?

water. Most desert landforms are carved by running water

bajada

when a fan enlarges, and eventual coalescing with fans from adjacent canyons to produce an apron of sediment called _____ along the mtn front

alluvial fan

when the occasional torrents of water produced by sporadic rains move down the mountain canyons, they are heavily loaded wit sediment. This runoff spreads over the slopes at the base of the mountains and loses its velocity, leaving most of its load dumped within a short distance and creating a CONE OF DEBRIS at the mouth of the canyon referred to as ________

loess

windblown silt over periods of 1,000's of years, where dust storms deposited this material. the distribution of this worldwide indicates that there are 2 primary sources of this sediment: deserts and glacial outwash