Arid and Semi-Arid Geomorphology
Plateau Erosion
-A plateau is a relatively flat but elevated landscape -In arid regions, scarp retreat can produce many landforms by dissecting a plateau
Arid and hyperarid deserts
-Affected by water and wind -Most are ROCKY NOT SANDY!
Aridity Index
-Aridity is related to the amount of rainfall and temperature -Expressed as Climatic Aridity Index = p/ETP -P = precipitation -ETP = potential evapotranspiration
Hoodoo
-Column of rock with caprock on top -Typically variable width along column -Formed by frost wedging and dissolution of fin -Can evolve from separation of adjacent windows (arches)
Drylands
-DRY!!! -Mean Annual Precipitation<2/3rds of potential evapotranspiration -Ecosystem Type: -Desert Biome -Covers 1/3rd of the Earth's surface
Desertification
-Deserts are formed by natural changes in climate over long time frames -Shrink & expand over time -Desert margins are transitional environments -Deserts may also develop due to human activity
Deserts
-Dry, relatively little to no rainfall -Arid or Hyperarid climate -Covers 1/5th of the Earth's surface -Can be cold or hot! -Antarctica = driest continent on Earth -Australia = most arid continent on Earth -Affects vegetation
Pinnacle
-Eventually, further erosion will leave a pointed spire -No appreciable summit surface area -ex: buttes and pinnacles in monument valley, utah
Dryland Streams: Exotic vs. Local
-Exotic streams- begin in areas that are not arid and flow into the desert. -Ex. Nile River, Colorado River, Okavango River -Local streams (Arroyos) - begin in the arid region, and often end there as well.
Butte
-Further scarp retreat and erosion of a mesa -Flat top, but narrower than butte height
Natural Arches
-Generally, arch refers to an opening formed without running water -Continued weathering will result in arch collapse -Formed by multiple processes Fin weathering or breached alcove Ex: Landscape Arch, Utah
Alcove
-Horizontal depression in the side of a cliff -Usually sandstone -Formed when groundwater flowing above non-permeable rock layer weakens cement holding sand grains together -Erosion removes loose grains
Aridity Categories
-Hyper-arid = arid index of 0.03, 4.2% of land, <100 mm rain/year -Arid = arid index of 0.03-0.2, 14.6% of land, 100-300 mm of rain/year -Semi-arid = arid index of 0.2-0.5, 12.2% of land, 300-600 mm/year -Dry subhumid = arid index of 0.5-0.75, 9% of land, 700-800 mm of rain per year
Relationships with vegetation
-Less Vegetation -Types Ephemeral Annuals Succulent Perennials Nonsucculent Perennials
Climate Types
-Mediterranean - winter rain -Tropical- summer rain -Continental- even distribution, high evapotranspiration
Causes of desertification
-Overgrazing of arid or semi-arid lands -Cultivation of marginal lands in semiarid regions -Destruction of vegetation in arid regions -Poor irrigation practices that add salt to the soil, eventually killing plants
Dryland Stream types
-Perennial -Year round water -Spring -Exotic -Ephemeral -Only floods when rains -Gullies, Arroyos, Canyons
Playas
-Playas occupy the flat central basins of desert plains. -interior drainage -eEvaporation greatly exceeds inflow. -When flooded, a playa lake -fine-grained sediment -salts concentrate.
Types of drylands
-Polar -Subtropical -Cold winter -Cool coastal
Biggest Deserts
-Polar Antarctic Polar Artic Polar -Non-Polar Sahara
Factors used to classify climate regions:
-Precipitation -Timing of Precipitation -Intensity -Evapotranspiration -Temperature -Wind -Vegetation
Results of desertification
-Reduced ability to support plant and animal life -Increased soil erosion -Increased run-off of rain water, and reduction of groundwater resources
Mesa
-Section of plateau that has been detached on all sides from the main plateau -Spanish for table -Still has substantial summit area -Top is wider than height
Causes of aridity
-rain shadow -latitude and wind -west coasts near tropics
What causes erosion and deposition Drylands?
-water -weathering (moisture) -streams -gravity -wind
Fins
Differential erosion may favor the formation of slender walls of rock
What if there are vertical joints?
Vertical fractures allow water to penetrate and weather rock at more than scarp face
Mass movement
movement of regolith and masses or rock downhill -gravity: -shear stress - force of gravity acting on an object on a slope -cohesive strength - resistance of an object to move
Scarp or cliff retreat
over time, location or a scarp face will move form weathering and erosion (see pics)
The role of water and examples
water may act to increase or decrease cohesive strength examples: -rockfalls -avalanches -rockslides -landslides -lahars -pyroclastic flows -land subsidence