Chapter 17: Dry Regions: The Geology of Deserts

How do weathering processes in deserts differ from those in temperate or humid climates?

Chemical weathering happens slowly. Soils and rocks tend to accumulate soluble minerals.

Explain the various conditions that can lead to the formation of deserts.

Formed by rain shadows. Mountains block the patch of participation. Composed of rocky or sandy landscape.

What is the process of desertification, and what causes it? How can desertification in Africa affect the Caribbean?

Land abuse may transform semiarid land into deserts. The Sahara will grow and the Caribbean could get closer to arid landscapes.

How have today's deserts always been deserts?

No, abuse of the land and land transformations have created and enlarged some deserts.

What factors determine whether a region can be classified as a desert?

Receive less than 25 cm of rain a year. Vegetation covers no more than 15% of their surface.

Describe how water modifies the landscape of a desert. Discuss both erosional and depositional landforms.

Water causes significant erosion in deserts, mostly in heavy downpours. Flash floods can occur.

Describe the process of cliff retreat and the landforms that result.

When cliffs retreat due to erosion and weathering they become mesas, buttes, and isnelbergs.

Explain the ways in which desert wind transports sediment.

Wind can also pick up silt and dust causing it to undergo saltation, can cause or shift deposition.

Desert Varnish

a dark, rusty-brown coating of iron oxide and magnesium oxide that accumulates on the surface of the rock.

Alluvial Fans

a gently sloping apron of sediment dropped by an ephemeral stream at the base of a mountain in arid or semiarid regions.

Mesa

a large, flat-topped hill (with a surface area of several square km) in an arid region.

Butte

a medium-sized, flat-topped hill in an arid region.

Dust storm/haboob

a particularly strong wind can create something that can be 100 km long and 1.5 km high and looks like a railing, opaque wave or cloud.

Sand Dune

a pile of sand deposited by wind or by flowing water.

Dune

a pile of sand generally formed by deposition from the wind.

Talus

a sloping apron of fallen rock along the base of a cliff.

Bajada

as they grow, alluvial fans emerging from adjacent valleys may merge and overlap along the front of a mountain range, producing an elongate wedge of sediment.

Cuestas

asymmetric ridges develop where strata in a region are not horizontal.

Desert

based on aridity, not temperature. Deserts may be hot or cold.

Petroglyph

drawings formed by chipping into the desert varnish of rocks to reveal the lighter rock beneath.

Natural arches

form when erosion occurs preferentially along joints, to produce wall-like fins of rock. Like when the lower part of a fin erodes away.

Cold desert characteristics

high latitudes, high elevations, near cold ocean currents, and temp usually below 20 degrees celcius.

Star Dune

if the wind shifts direction frequently, a group of crescents pointing in different directions overlap one another.

Desert Pavement forms in stages.

loose pebbles and cobbles collected at the surface. Dust settles among the stones and build up a soil layer below. The stones eventually crack into smaller pieces and settle to form a mosaic-like pavement.

Hot desert characteristics

low latitudes, low elevations, far from oceans, and high temp exceeds 35 degrees celcius.

Faces or facets

since most sand consists of quartz, a hard mineral, saltating sand not only can strip the paint off a car but it can also abrade rock surfaces in the desert and, over long periods, can carve smooth ___________ on pebbles, cobbles and boulders.

Caliche or Calcrete

since such infiltration events happen infrequently, the leached ions don't flush away entirely. Rather they precipitate underground to form new calcite cement that can bind regolith into a solid, rock-like material.

Lag deposit

the coarse sediment left behind in a desert after wind erosion removes the finer sediment.

Dry wash/arroyos/wadis

the dry channels of ephemeral streams in desert regions.

Playa

the flat, typically salty lake bed that remains when all the water evaporates in drier times; forms in desert regions.

Deflation

the process of lowing the land surface by wind abrasion.

Wind erosion

the process of removing sediment directly by the action of fast moving air.

desertification

the process of transforming non-desert areas into desert.

subtropic/subtropical desert

the world's largest deserts (such as the Sahara, Arabian, Kalanari, and Australian) form because of the global pattern of air circulation in the atmosphere.

Suspended load

tiny solid grains carried along by a stream without settling to the floor of the channel.

Parabolic Dunes

when strong winds cause a blowout gets deposited downwind, while the portions of the dune on either side of the blowout remain anchored by vegetation, whose ends point in the upwind direction.

Transverse Dunes

where enough sand accumulates to bury the ground surface completely, and moderate winds blow, sand piles into simple, wave-like shapes.

Barchan Dune

where sand is relatively scarce and the wind blows steadily in one direction, beautiful crescents with the tips of the crescents pointing downwind.

Yardangs

where wind blowing constantly in the same direction carves strata of alternating resistant and non resistant layers, it may produce ___________, streamlined caps of resistant rock resting on pillars of non resistant rock.

Longitudinal Dune

with abundant sand and a strong, steady wind, the sand streams, whose axes lie parallel to the wind direction.

What are the various types of sand dunes, and what factors determine what type of dune develops in a particular location?

(1) Barchan, star, transverse, parabolic, and longitudinal. (2) The location of other land formations, wind flow, etc.

Chimney

(1) a conduit in a magma chamber in the shape of a long vertical pipe through which magma rises and erupts at the surface; (2) an isolated column of strata in an arid region.

Polar desert

(Antarctica) so little precipitation falls in Earth's polar regions (North of the Arctic Circle and South of the Antarctic Circle) these areas are, in fact, arid. These are dry, in part, for the same reason that the subtropics are dry (the global pattern of air circulation means that the air flowing over these regions is dry), and in part, for the same reason that coastal areas along cold currents are dry (cold air holds little moisture).

Coastal deserts

(Atacama) Cold ocean water cools the overlying air by absorbing heat, thereby decreasing the capacity of the air to hold moisture.

Uluru

(Ayers Rock) in central Australia is an inselberg. The red sandstone beds comprising it are the eroded remnant of the vertical limb of a large syncline. Alluvium buried the surrounding bedrock.

Continental interiors

(Gobi) as air masses move across a continent, they progressively lose moisture by dropping rain, even in the absence of a coastal mountain range. Thus, by the time an air mass reaches the interior of a broad continent, it has become so dry that the land beneath becomes arid.

Surface load

(bed load) sediment that rolls and bounce along the ground (under the air) or along a stream bed (under water).

Rain-shadow desert

(eastern Oregon) as moist air flows from the sea toward a coastal mountain range, the air must rise. The rising air expands and cools, so the water it contains condenses and falls as rain on the seaward flank of the mountains, where it drenches a coastal rainforest. When the flowing air finally reaches the inland side of the mountains, it has lost all its moisture and can no longer provide rain.

Ventifact

(faceted rock) a desert rock whose surface has been faceted by the wind.

Desert Pavement

A mosaic-like stone surface forming the ground in a desert.

Describe the process of formation of alluvial fans, bajadas, and playas.

Alluvial - A gently sloping apron of sediment dropped by an ephemeral stream of at the base of an arid region. Bajadas - An elongate wedge of sediment formed by the overlap of several alluvial fans emerging from adjacent valleys. Playas - The flat, typically salty lake bed that remains when all the water evaporates in drier times.

Inselberg

An isolated mountain or hill in a desert landscape created by progressive cliff retreat, so that the hill is surrounded by a pediment or an alluvial fan.

Pediments

The broad, nearly horizontal bedrock surface at the base of a retreating desert cliff.

Cliff (scarp) retreat

The change in the position of a cliff face caused by erosion.

Saltation

The movement of a sediment in which grains bounce along their substrate, knocking other grains into the water column (or air) in the process.

What phenomena may lead to formation of desert varnish, desert pavement, and ventifacts?

Varnish - A dark, rust-brown coating of iron oxide and magnesium oxide that accumulates on the surface of the rock. Pavement - A mosaic-like stone surface forming the ground in a desert. Ventifacts - A desert rock whose surface has been formed by the wind.