Arid Lands Flashcards
Special Desert Conditions
1) Weathering = only mechanical since no water for chemical
2) Soil/Regolith = thin + absent in most places –> lack of soil
3) hardpans widespread –> soil impermeable to water
4) sand = water infiltrates ground + easily moved by heavy winds
5) limited rainfall creates short-distance temporary streams + lack of vegetation
6) underdeveloped drainage systems
Weather type in arid regions
1) B climates
Main agent of erosion
WATER even in arid landscapes
Exotic Streams
1) exotic - water in stream comes from outside desert, usually from higher mountain area
2) ex = Nile River = water comes from mountains of Africa/Ethiopia
3) able to sustain itself through arid environments
Ephemeral Streams
1) carry water only seasonally or after rainstorm
2) when carries water = intense erosion, transportation, deposition
3) dry beds of ephemeral streams –> flat floors, sandy bottoms, steep sides
Playas
1) Dry lake beds (occasionally fill with water)
2) high salinity bc of evaporation of water
Saline Lakes
1) Permanent desert lakes BUT extremely high saline concentrations –> much more evaporation of water compared to inflow
How does fluvial erosion begin in desert
1) Begins with brief but intense thunderstorm
2) lots of water on ground BUT bc most of desert soil is impermeable –> water runs off into nearby ephemeral stream
3) fills up with water –> creating flash flood + debris flow
Flash Flood Hazard in desert
1) Risk to humans in arid lands
2) Desert cities –> flood-control channels + basins
Differential weathering effect in arid landscape
1) striking difference in erosion + weathering bc of sparse cover of soil/vegetation
2) Rocks resistant to weathering –> form cliffs, pinnacles, spires (pointy stuff) while less resistant rocks form gentle slopes
Fluvial Deposition
more common than fluvial erosion, most prominent along piedmonts (base area around mountains)
alluvial fan
1) Deposition of sediment in piedmont zone
2) occurs because river goes from flowing through mountain gorge into an open piedmont zone (water spreads out –> reduced CC –> drops sediment)
Aeolian processes
1) Related to wind action
2) most pronounced + effective in regions with fine grain sand + little vegetation and moisture (deserts_
2 divisions of Aoelian erosion
1) deflation
2) abrasian
Deflation
1) loose particles blown through air or along ground –> moved by deflation
2) GENERALLY wind not strong enough to move anything more than dust/sand grains
Blowout (deflation hollow)
1) shallow depression where abundance of fine material has been deflated
2) generally 1 mile in diameter
Abrasion
1) similar to fluvial abrasion BUT much less effective
2) the sand/dust hitting rocks results in pitting, etching, faceting, polishing of exposed rock surfaces (ventifacts)
3) found in mesa-and-scarp regions
Aeolian Transportation
1) finest particles carried in suspension as dust
2) larger particles moved through saltation + traction (like streamflow)
Unanchored dunes
1) moved by local winds
2) force sand up + over crest (on windward side)
3) goes onto leeward side (slip face) which has a 32-34 degree angle of repose
Anchored dunes
1) held in place by vegetation BUT they do not provide much nourishment/moisture for plant growth
Types of dunes
1) Barchan
2) transverse
3) seif (longitudinal)
4) star
Sheltered Pocket
On opposite side of dune (opposes wind direction)
Barchan dunes
1) Crescent shaped + formed bc of strong winds blowing in one direction
2) fastest moving dunes
Transverse Dunes
1) less crescent shaped + occur where entire landscape is sand-covered
2) much more sand than barchans
3) looks like ripples of water
Seif (longitudinal)
1) Long, narrow + generally parallel
2) created by two dominant wind directions blowing in one direction half of the year and another direction the other half
Star Dunes
1) pyramid shaped dunes that are star shaped
2) formed in areas where wind constantly changes directions
Basin-and-range terrain features
1) ranges
2) piedmont zones
3) basins
basin-and-range place without external drainage + some exotic rivers
How did basin + range topography form
1) repeated process caused by tension forces (inactive rifts)
Ranges
1) rugged mountain ranges in all directions
2) tend to be long, narrow, parallel to one another
3) have alluvial fans (V-shaped cross sections)
Piedmont Zone
1) Base of ranges –> zone of transition between steep range slopes to flatness of basin
2) full of alluvial fans + bajadas
Bajada
1) interconnected alluvial fans
2) as deposition occurs, fans grow larger + interconnect
Basins
1) Beyond flattish floor –> gently slopes from all sides to low point (usually playa)
2) poor drainage channels
3) salt accumulation common around the playa –> becomes salina if there is enuf salt
Mesa-and-Scarp Terrain
1) Tall, steep vertical cliffs in the Four Corners Country (Colorado, Utah, Arizona, New Mexico)
2) Grand Canyon
Weathering of mesa-scarp formation
1) horizontal + sedimentary strata
2) each level has different degrees of erosion which causes abrupt changes in slope angle
3) most resistant layer = at bottom = scarp
4) alternates between scarp + inclined + scarp + inclined
5) top scarp = mesa
Plateaus
1) Extensive elevated flatlands of uniform horizontal layers
Progression from plateau
1) Plateau –> mesa –> butte –> pinnacle
2) each consecutive is smaller + more eroded
Badlands
1) rugged, heavily eroded landscapes
2) lots of ravines + gullies formed by overland flow from rain (effective erosive agent)
