McKnight's Physical Geography

Lectures

Chapter 18

The Topography ofArid Lands

© 2014 Pearson Education, Inc.

Andrew MercerMississippi State University

A Specialized Environment

• Desert terrain stark and abrupt

• Desert special conditions– Weathering – mechanical weathering dominant; slower

weathering and angular particle formation

– Soil and regolith – soil is thin or absent, exposing bedrock

– Soil creep – minor due to lack of soil and lubricating effects of water

Top – thinner soilSlope – very thin soilFoot – thicker soil

A Specialized Environment

• Desert special conditions (cont.)– Impermeable surfaces – caprocks and hardpans; high

water runoff

– Sand – some deserts have sand abundance, allows for water input into the ground, easily moved by rain and wind

– Rainfall – is the most important shaper of landscapes in DESERT country

– Wind – also matters, but less so than even occasional rain

Even in very dry country, water is still the most important shaper of topography

A Specialized EnvironmentDesert conditions

•Utah is semi-desert

•Basins of interior drainage – most dry-land watersheds do not drain into any ocean –

•Water usually evaporates before reaching the ocean

or a large lake.

Running Water in Waterless Regions

• Running water most important external landform agent in dry country and desert

• Erosion tremendously effective with little plant cover

• Intensity of rain combined with impermeable land surfaces create intense runoff

• Unpredictable imbalance between erosion and deposition

Running Water in Waterless RegionsSurface water in the desert– Exotic streams – the Nile is

an example of a river that originates in one region and flows through an alien region to some other destination.

– Ephemeral streams – periodically flow, result in intense erosion, transportation, and deposition

– Perennial – fulltime flow, year-round

– Desert lakes – playas and Salinas (dry salt lake beds), saline lakes

C – more mild

B climate - desert

A climate

Running Water in Waterless Regions

• Fluvial erosion in arid lands

– Occurs during small portion of the year, flash floods

– Differential erosion – variations in slope and shape of landform from rock type variations

– Residual erosional surfaces – inselbergs (e.g., bornhardts), pediments

– Desert stream channels – ephemeral stream beds

Consider Mr. Allred’s yard – the rocky top of a hill was once the ‘armor-plated’ stream bed at the bottom of the hill. Eventually, everything else washed away, leaving the river bed as the new top of the hill.

Running Water in Waterless Regions• Fluvial deposition in arid lands

– Talus accumulations at the foot of steep slopes

– Piedmont – zone at the foot of a mountain range

– Piedmont angle

– Basins of interior drainage covered with fine particles since flow volume and speed are low

The Work of Wind

• Wind as a sculptor is a relatively limited effect

• Air at the surface has nearly zero wind

• Wind speed increases with distance above ground

The Work of Wind

• Aeolian erosion– Two effects, deflation and abrasion

– Deflation – shifting of loose particles via the wind, blowouts

– Abrasion – requires tools such as airborne sand and dust, sculpts landforms already in existence, ventifacts

The Work of Wind• Aeolian (wind) transportation– Only finest particles are carried in suspension as dust

– Dust storms

– Larger particles moved by saltation (curved trajectory) and traction (rolled or pushed)

– Creep by saltation

The Work of Wind• Aeolian (wind) deposition

– Fine sand laid as thin coating;

• Desert sand dunes– Some dune fields composed of unanchored sand, moved

by local winds; slip face

The Work of WindDesert sand dunes (cont.)

– Three most common dunes• Barchan – individual dunes

migrating across landscape; crescent shaped

• Transverse – supply of sand greater than for barchans; crescent shaped, but entire landscape made of these dunes

• Seifs – long, narrow dunes that are parallel; orientation seems to represent an intermediate direction between two dominant wind directions

The Work of Wind

• Common types of desert sand dunes

Loess– Wind deposited silt

– Lacks horizontal stratification

– Great vertical durability

– Formation not well understood

Millions of people have lived in silt caves across thousands of years.

However, when such soils are WET they can collapse during an earthquake – thousands of people may get killed.

The Work of Wind

Major loess (SILT) locations of the world

Two Representative Desert Landform AssemblagesNotice the effect of water even in a desert – occasional, sharp rain can result in vast erosion.

It is not a good idea to build or occupy ‘alluvial fans’ like the one below.

River deltas are similar.

Watch out.

Two Representative Desert Landform Assemblages

Death Valley– Excellent example of

basin and range terrain

– Surrounding mountain ranges

– Basin filled with alluvium

– Salt pans and mobile dunes in the basin

– Notice that salt collects in basins after washing out of mountains.

Two Representative Desert Landform Assemblages

Mesa-and-scarp terrain

•Mesa – flat topped surface

•Scarp – steep cliffs

•horizontal strata

•Some layers resist erosion well -- others erode quickly

Harder rock lasts longer

Softer rock erodes, allowing the whole formation to fail

Two Representative Desert Landform AssemblagesMonument Valley – Utah/Arizona

These features are the scattered remains of vast areas that were eroded and washed away.

Only a few “hard spots” remain – the cap rock protected softer rock nearby or underneath.

Once water got past the cap rock, the whole region washed away.

Two Representative Desert Landform Assemblages• Badlands

– Overland flows from occasional rains develop tiny rills that expand into ravines or gullies

– Characterized by maze of ravines and gullies, lifeless and nearly impassable

• Arches and natural bridges– A small amount of cap rock

is all that remains of the entire formation.

– Cap rock protects the softer rock underneath.

– All of it will be gone very quickly.