Lesson 29- Arid Processes and Landforms

C. Hamada (Barren Bedrock)

- Exposed bedrock. - Any eroded material is usually quickly carried away. > Ergs, regs, Hamada are restricted to plain areas. > Regs and Hamadas are flat, whereas ergs are steep. > Distinct boundaries between each three due to differences in friction layer.

> Aeolian Deposition

- Finer material is usually deposited in one thin layer of silt. - Prominent feature of deposition are sand dunes:

1. Barchan

- Individual dunes migrating across a non-sandy surface. - Crescent shaped with horns. - Result when wind blows in one strong direction. - Move quickly. - Found in Australia/Eurasia.

> Aeolian Erosion

1. Deflation = loos particles are blown through the air, deflation hallows occur in places where deflation has occurred. 2. Abrasion = winds carries particles which blast into other rocks, causing rock to break down into sometimes vertical structures.

> Exotic Streams

- Exotic streams originate at wetter places away from the desert. Ex: The Nile river flows from the mountains of Southern Africa. - As exotic stream increases, the narrower it gets (in humid regions the river increases due to the addition of water from tributaries).

> Sand

- Absorbs some moisture, prevents large excess of runoff. - Sand is easily moved by rains. - Sand can be transported.

> Rainfall

- Brief but effective. - Streams are ephemeral (immediately flow after rains).

> Impermeable Surfaces

- Caprocks (resistant bedrock surfaces) - Hardpans (water impermeable layers) - Does not readily absorb water.

> Weathering

- Chemical weathering usually absent. - Salt wedging is more common in drier locations.

Erosion of Escarpment Edge

- Cliffs retreat due to the rapid removal of less resistant strata beneath the caprock. - Sapping = process in which groundwater seeps and trickles out of the scarp face and erodes fine particles. - Over time larger features erode into smaller ones (plateau > mesa > butte > pinnacle)

B. Reg (Stony Deserts)

- Coarse gravel, finer particles were removed. - Usually found in places that have been undisturbed. - Pebbles fit closely together, creating a type of "armor".

> Basins of Interior Drainage

- Contain watersheds that do not easily drain to large bodies of water because drainage networks are underdeveloped.

> Playas

- Dry lake bed occupies lowest place of elevation. - Claypan = lake bed surface filled with clay. - Flat layer of dry mud.

> Residual Surfaces and Features

- Inselbergs = steep sided ridges/mountains. - Bornhardt = a type of inselberg, where the land around highly resistant rock erodes. - Pediment = gently inclined bedrock form that can be a result of exhumed weathered bedrock or detachment erosion and subsequent weathering.

> Saline Lakes

- Lakes that permanently exist in deserts have high concentrations of salt.

A. Erg (A Sea of Sand)

- Large accumulation of sand, usually in the form of dunes. - Accumulated during period of moisture, then dryness.

4. Star Dunes

- Large pyramid shaped dunes with arms radiating out in three or more directions. - Develop in areas where the wind frequently varies direction.

> Vegetation

- Less vegetation, easier to loosen and remove sediment.

3. Seifs

- Long narrow dunes. - Result from strong forces of two different winds. - Not common in North America.

Sand Dunes

- Loose windblown sand is piled into a hill. - Unanchored dunes can be moved by the sand. Process = wind erodes on windward side of dune and carries sand to the slip face (opposite, steeper side of the dune). Dunes can move without changing shape - Dune has an angle of repose between 32 and 34 degrees. - Vegetation can hold dunes in place.

B. Piedmont Zone

- Marks change in slope between the range and the basin. - Causes speed and load capacity to drop and deposition results. - Creates alluvial fans, if alluvial fans overlap they become bajadas.

2. Mesa and Scarp Terrain

- Mesa refers to table. - Scarp refers to steep side.

2. Transverse Dunes

- Migrate down wind. - Parallel waves, with crests perpendicular to wind. - Occur where there is more sand, if sand decreases transverse dunes will become barchans dunes.

C. Basin

- Mountain gently slopes towards one point of lowest elevation. - If salt accumulates it becomes a salina. - If large accumulation of salt takes place long enough, a salt pan will occur. - Playa lakes = occur when basins temporarily fill with water. Ex: Death Valley, CA

1. Coastal Dunes

- Occur along flat coastlines. - Parabolic Dunes = dunes where vegetation begins to grow.

> Ephemeral Streams

- Occur briefly after rainfall. - Leads to intense erosion and deposition.

Arches and Natural Bridges

- Occur when rock underneath is eroded.

C. Fluvial Deposition in Arid Lands

- Piedmont zone is where there is an abrupt change of slope, accumulation of talus usually occurs in this area. - Alluvial fan = streams run down slope and break up into smaller streams, streams carry and then deposit rocks, building up the alluvial fan (large boulders are dropped near the apex)

> Flash Flood Hazards

- Rains are not common but intense. - Water flows rapidly across flat bedrock into ephemeral streams. - Campers advised not to camp in dry riverbeds. - Cities build flood controls.

D. Climate Change and Deserts

- Rains more irregular. - Deseritfication = expansion of deserts.

Structure of Mesa and Scarp Landforms

- Resistance layers (limestone, sandstone) form extensive caprocks that become mesa. - At edge of the eroded caprocl, , hard layer protect underlying strata and produces escarpment.

A. The Ranges

- Ridges, crests, sharp peaks. - Long, narrow, and parallel to one another. - Develop dry v-shaped drainage channels, where sediment can fill.

> Differential Weathering and Erosion in Deserts

- Rocks resistant to weathering form cliffs, pinnacles, spirals, crests. - Significant differences in resistant between strata. - Escarpments = vertical structures, characterized by abrupt change in slope.

> Winds

- Sand particles can be shifted.

> Soil and Regolith

- Soil creep does not occur due to lack of soil and moisture. - This causes land forms to look extremely angular.

1. Basin and Range Landforms

- Southwest of the US. - No external drainage, few exotic rivers. - Aridity in basin-range area due to rain shadows- that occur in the Sierra Nevada. - Three principle features:

3. EXPLAIN HOW AEOLIAN (WIND) PROCESSES CAN AFFECT LANDORM DEVELOPMENT IN ARID REGIONS IN TERMS OF EROSION, TRANSPORTATION AND DEPOSITION.

- Strong winds, wind speed increases with distance above the ground. - Different wind speeds causes turbulence. - Winds most effective where sand is fine-grained.

> Aeolian Transportation

- Suspension = sand particles carried by wind. - Traction = larger particles are rolled and pushed along by the wind. - Saltation = larger particles picked up by wind then dropped.

2. Loess

- Wind deposited silt. - Lacks horizontal stratification. - Tiny grains have great molecular attraction which make particles cohesive. - Absorbs large amounts of water which makes land fertile for agriculture. - Occurs in the midlatitudes.