Water landforms in arid environments

Case study: USA

1) Canyons 

2) Mesas, buttes, spires

3) Alluvial fans

4) Tafoni

5) Salt flats

1. Canyons

Ephemeral river= one that flows occasionally

Exogeneous river= permanent, originates in uplands outside of the arid environments.

Slot Canyons:

Location: Antelope Canyon 

• Canyons from ephemeral rivers 
• Made from one rock type
• Prone to flash floods as there is no vegetation

Key features:

• Narrower than it is tall
• Smooth sides 
• Sinuous 
• Uniform geology (Navajo sandstone)

Overland flow from where the local LP system breaks through the global HP system causing a flash flood. Due to limited interception and baked ground means there is a high amount of surface run-off. The overland flow picks up weathered material from wetting and drying, salt crystallisation, insolation/exfoliation, hydrolysis and freeze-thaw weathering. The weathered material abrades the sides of the canyon meaning there is a wider bottom than the sides.

Grand Canyon:

• Canyon formed with glaciation (relic landform)
• Stepped appearance
• Key processes: abrasion, hydraulic action

FACTS: 

1. Colorado river has had the same course for the last 6 million years.
2. River at deepest= 1600m

2. Mesas, Buttes, Spires

Location: Monument Valley

Shinarump cap rock (most resistant)

DeChelly sandstone (least resistant)

Organ shale (average resistant)

Parallel retreat= Occurs because rock debris is removed as fast as it accumulates.

3. Alluvial Fan

Location: Panamint Range, Death Valley, California

Increased friction means a decrease in energy leading to the deposition of sediment dropping the largest particles first.

4. Tafoni

Location: Snake Valley, Crystal Peak, Utah

Facts:

• 300m high
• Inselberg=crudely stratified
• rock type= porous & permeable (petrified volcanic ash)
• contains CaCO3 (calcite)

Definition= large hollows found in rock faces in semi-arid environments commonly <4m wide. They are a product of weathering (salt crystallisation).

Process:

1. Weakly acidic due to CO2 in the atmosphere is dissolved in the rainwater forming carbonic acidic.
2. Carbonic acid dissolves the calcite (Carbonation)
3. The evaporation of this leaves salt crystals of calcite in the pores of the rock. 
4. This causes the process of spalling leaving holes in the rock.

5. Salt Flats

Location: Bonneville Salt flats, Utah

Process:

1. High rates of evaporation bring salts to the surface
2. Surface run-off dissolves the salts running into the nearest lake or river
3. High temperatures lead to evaporation of the lake

Facts:

• 1.8m deep
• 4 levels are sodium chloride, sodium sulphate, sodium carbonate and calcium sulphate.

Wind/Aeolian landforms in arid environments

Case study: Namibia

Context:

• hyper-arid (<30mm ppt)
• very dry
• one of the oldest deserts 55Ma
• aridity maintained by the dry descending air of the Hadley Cell
• main source of ppt= Advection fog (contact of warm land air with cold sea air)

Landforms:

1) Ventifacts & Desert Pavements 

2) Yardangs

3) Sand dunes

1. Ventifacts & Desert Pavements (wind erosion)

Location: Walvis Bay

• Occurs in every desert
• Takes a while to form 
• Size and energy of the wind relationship

Desert Pavements= The wind removes the fine materials and deflation continues until an accumulation of stones virtually blankets the surface. 

Ventifacts= Are small stones that have been shaped by attrition and abrasion due to wind blown sediment (Saltation). Flat faces develop in the prevailing wind direction. They are separated from the lee side by a sharp edge. (If there are different flat surfaces (facets) developed, there is more than one wind direction prevailing. Ventifacts with 3 facets are called dreikanters.

2. Yardangs

Location: Mouth of the orange river

The trough is formed either from deflation of the softer rock (geology) or from the erosion of weaknesses and joints (Lithology). The abrasion of the sides goes up to the height of 0.5m as that is the highest that rocks are transported via saltation. Are formed where there is a uni-directional prevailing wind.

3. Sand dunes 

Location: Namib Sand Sea/Cunene Sand Sea

NEED:

1) Large amount of loose sand

2) A wind or breeze

3) An obstacle

Formation:

1) Friction stops sediment leading to deposition

2) Positive feedback loop

3) Migration

This is how a sand dune may actually move over time 100m/yr, rolls along maintaining its shape as it goes.

5 types:

1) Transverse

2) Barchans

3) Linear (longitudinal dune)

4) Star

5) Parabolic

Transverse and Barchans are the most common.