Chapter 19 Geology

How extensive are the desert and steppe regions of Earth?

Desert and steppe regions cover about 30 percent of Earth's land surface.

Which of the following is characteristic of a desert stream?

Desert streams lack expansive tributary systems.

How does the rate of rock weathering in dry climates compare with the rate in humid regions?

Dry climates have much lower rates of chemical weathering and mechanical weathering than humid climates.

Why is it impossible to define the boundary between humid climates and dry climates by a single precipitation amount?

Dryness is not only related to annual precipitation totals, but is also a function of evaporation.

What is interior drainage?

Interior drainage is when an arid region exhibits a discontinuous pattern of intermittent streams that do not flow out of the desert to the ocean.

What is the primary cause of subtropical deserts and steppes, and how do cold ocean currents influence some subtropical deserts?

The air over these deserts sinks, compresses, and warms. Subtropical deserts that are subjected to cold ocean currents have reduced temperatures and some of the lowest yearly rainfall totals in the world.

How does wind's suspended load differ from its bed load?

The bed load consists of sand-sized grains that move by saltation, whereas the suspended load consists of fine, silt- or clay-sized particles that can be carried far distances by the wind once they are initially ejected into the air.

Please describe the features and characteristics associated with each stage in the evolution of a mountainous desert.

The early stage includes playa lakes from intermittent rainfalls and alluvial fans at the mouths of canyons; the middle stage includes bajadas that have formed when alluvial fans coalesced together and playas that resulted from the evaporation of water from playa lakes; the late stage includes inselbergs, which are towers of bedrock that are the eroded remainders of the mountains that once existed.

How is loess related to glaciers?

When glaciers retreat, their meltwater contains loess, which can be picked up by winds and distributed elsewhere.

Describe the way in which wind transports sand. When winds are strong, how high above the surface can sand be carried?

Wind transports sand by saltation, a process where a particle of sand begins rolling and then hits another sand grain, launching one or both of them into the air; these and subsequent grains move along by bouncing along the surface. Saltating sand usually does not reach above 0.5 meter from the ground and very rarely reaches above 1 meter from the ground.

Which of the following desert features has sediment-size grading similar to that in a delta?

alluvial fan

What is an ephemeral stream?

an intermittent stream that carries water only in response to specific episodes of rainfall

Which of the following is a wind deposit?

barchan dune

Which of the following is an example of late stage desert erosion in the Basin and Range region?

inselbergs in southern Arizona

A rainshadow desert forms ______.

on the leeward side of a mountain

Of the following options, which is the most common type of chemical weathering in a desert?

oxidation

______ dunes form in areas where vegetation partially covers the sediment.

parabolic

What size sediment can be transported by wind?

sand, silt, and clay

The lower limits of a blowout are controlled by

the local water table

What factor limits the depths of blowouts?

the local water table

What is the most important erosional agent in deserts?

water

Why is wind erosion relatively more important in dry regions than in humid areas?

-Moisture in humid areas binds particles together, which inhibits wind erosion. -Vegetation in humid areas anchors the soil, which inhibits wind erosion.

When a permanent stream such as the Nile River crosses a desert, does the river's discharge increase or decrease? How does this compare to a river in a humid area?

The discharge of the Nile River decreases when it crosses a desert, in contrast with a river in a humid area, which increases in discharge.

Which choice(s) below correctly describe one or more dune types?

-Barchan dunes are crescent-shaped with their tips pointing downwind. Barchanoid dunes are somewhat of a blend between transverse dunes and barchan dunes. -Longitudinal dunes are long ridges formed by the wind blowing obliquely at the dune on either side. Transverse dunes are long ridges separated by troughs oriented at right angles to the prevailing wind. -Parabolic dunes form where vegetation partially covers the sand, with their crescent tips pointing upwind. Star dunes have complex forms caused by variable wind directions.

How do loess deposits differ from sand deposits?

-The grains that make up loess deposits have generally traveled a farther distance before deposition than a sand deposit. -Some of the loess in loess deposits has come from glacial outwash deposits, whereas sand in sand deposits does not come from glacial outwash deposits. -Sand deposits are made of larger grains than loess deposits, which are made of silt-sized grains.

Where in the United States can the main stages of desert landscape evolution be observed?

-The stage when alluvial fans merge together to form bajadas and playas form is found in Death Valley and southern Nevada. -The stage including recently uplifted mountains and an early stage of their erosion is found in southern Oregon and northern Nevada. -The late stage of erosion where only a few large knobs of bedrock project above the sediment-filled basin is found in southern Arizona.

What are two hypotheses that are used to explain the formation of desert pavement?

-Wind removes sand and silt from poorly sorted surface deposits until all of the fine particles are gone, leaving only pebbles that are too large to be moved by the wind. -Pavement develops on a surface that initially consists of coarse particles. Fine, windblown grains sift and settle downward between the larger surface stones.

What observations of desert pavements undercut the hypothesis that desert pavements form through deflation?

-Windblown silt can accumulate and infiltrate through the pebbles. -There are few to no pebbles found in the sediment beneath the desert pavement.

What are yardangs and ventifacts?

A yardang is a streamlined, wind-sculpted landform that is oriented parallel to the prevailing wind, while a ventifact is a polished, pitted, sharp-edged stone that formed as a result of wind-driven abrasion.

In which hemisphere are middle-latitude deserts more common, and why?

Middle-latitude deserts are more common in the Northern Hemisphere than the Southern Hemisphere because the Southern Hemisphere lacks extensive land areas at middle latitudes.

Why do middle-latitude dry regions exist, and what role do mountains play in their existence?

They are in the middle of landmasses and thus are distant from oceans, which are sources of moisture. Middle-latitude deserts at the leeward side of mountains receive air that has already lost moisture. If the air descends, it warms and compresses, inhibiting the formation of clouds.

How do sand dunes migrate?

Sand moves up the gentler side of the dune by saltation and accumulates just beyond the crest of the dune. As more sand collects, the slope steepens, and some of the sand slides down the slip face of the dune.