Deck 17: Dry Regions: the Geology of Deserts

Question 1

The most expansive hot deserts on Earth occur in the ____________.

A) tropics, at or near the equator
B) subtropics, between 20° and 30° north or south of the equator
C) temperate zone, between 30° and 50° north or south of the equator
D) in the rainshadows of mountain ranges between 10° and 25° north or south of the equator.

Answer 1

B

Question 2

A newer theory about desert varnish suggests it does not form from chemical constituents of the rocks on which it is found.Which of the following statements about the newer theory of the formation of desert varnish is NOT true?

A) Cosmic rays (which tend to be absorbed by moisture-rich air masses over nondesert areas) reach the desert surfaces in sufficient amounts to trigger an alteration process that forms desert varnish.
B) Bacteria are involved in the formation of desert varnish.
C) The presence of both dust and dew are necessary for the formation of desert varnish.
D) Rainfall disrupts the formation of desert varnish.

Answer 2

A

Question 3

Why are there few or no deserts within 5 degrees latitude of the equator,even though this latitude averages the highest amount of sunlight of any latitiude on Earth?

A) because the equator itself crosses almost no land masses
B) because the Amazon, Nile, Congo and Mekong rivers carry sufficient water to stabilize lands at these latitudes
C) because warm air rises up from the Earth at the equator, and hence rainfall is high
D) because cool air from the upper atmosphere sinks down to the land or sea surface at these latitudes

Answer 3

C

Question 4

The Sahara is a desert primarily because it is located ____________.

A) in the subtropics
B) within the rain shadow of a mountain range
C) near the center of a large continent
D) next to a cold ocean current

Question 5

To qualify as a desert,a region must ____________.

A) be hot, with a mean annual temperature greater than 25°C (77°F)
B) be arid, with less than 15% of the ground surface vegetated
C) have a mean annual temperature greater than 25°C and less than 5 cm annual rainfall
D) have no rainfall three out of four seasons

Question 6

Coastal region deserts are most likely near ocean currents that flow ____________.

A) west
B) east
C) away from the equator toward one of the poles
D) away from one of the poles toward the equator

Question 7

Desert climate associated with a rain shadow is found ____________.

A) on the windward side of mountain ranges
B) on the leeward side of mountain ranges
C) in the middle of flat plains
D) along continental coastlines

Question 8

The Atacama of Chile is a desert primarily because it is located ____________.

A) in the subtropics
B) near the South Pole
C) near the center of a large continent
D) next to a cold ocean current

Question 9

Which statement about the differences between cold deserts and hot deserts is most accurate?

A) Cold deserts are found only in polar regions, and hot deserts elsewhere.
B) Cold deserts are found in polar regions and some high-altitude regions outside the poles, and hot deserts are found only in continental interiors or the subtropics.
C) Cold deserts can have precipitation occur as snow, whereas any/all precipitation in hot deserts is rain (or occasionally hail).
D) Cold deserts rarely have air temperatures > 20°C, whereas hot deserts often have daytime temperatures > 35° C.

Question 10

Based on the figure below,do deserts tend to form where air is moving up from or down toward Earth's surface?

A) Deserts appear to be more common where air is rising up from the Earth's surface.
B) Deserts appear to be more common where air is sinking down toward the Earth's surface.
C) Deserts appear to be more common where air is traveling parallel with the Earth's surface.
D) The location of deserts does not have any relation to the direction of air travel.

Question 11

Which of the following is NOT true regarding wind as the primary agent of erosion in most deserts?

A) Air masses move differently over deserts, resulting in higher pressure gradients and greater winds.
B) There are fewer trees, shrubs, and grasses to slow the winds, and so wind blows right over the soil/ground surface, easily eroding it.
C) There are fewer plant roots, to help bind soil in place.
D) The low soil moisture means that sand-size particles are more easily separated and blown away from one another.

Question 12

What meteorological phenomenon controls the formation of subtropical deserts?

A) downwelling of air from the upper atmosphere
B) the Coriolis Effect
C) upwelling of air
D) the cooling and drying effects of cold-water ocean currents that originate at high latitudes

Question 13

Which of the following is NOT a condition in which cold deserts occur?

A) at low latitudes
B) at high elevations
C) at latitudes > 60°
D) near oceans

Question 14

If the two counterclockwise ocean current cells shown below were to reverse their directions of flow,what would happen to the Atacama and Namib deserts?

A) Each would probably become larger and drier.
B) There would probably be no significant change to them.
C) They would probably experience more rainfall and might disappear.
D) They would probably experience tectonic subsidence.

Question 15

Which of the following is NOT a subtropical desert?

A) the Kalahari
B) the Atacama
C) the Australian
D) the Arabian

Question 16

The two deserts with the highest recorded temperatures on Earth are _________.

A) Death Valley, United States, and the Gobi Desert, Mongolia
B) the Atacama Desert, Chile, and the Sahara Desert, Libya
C) Death Valley, United States, and the Sahara Desert, Libya
D) Death Valley, United States, and the Atacama Desert, Chile

Question 17

Which of the following is NOT a component of desert varnish?

A) manganese oxide
B) iron oxide
C) clay
D) calcium oxide

Question 18

Desert areas constitute what percent of the Earth's surface?

A) 5%
B) 15%
C) 25%
D) 40%

Question 19

In deserts,what is the primary process that causes mesas and buttes to shrink with time?

A) erosion by running water
B) erosion by wind
C) chemical weathering and downslope creep
D) bedrock separation along joints, along cliff-like slopes

Question 20

The Gobi of Mongolia is a desert primarily because it is located ____________.

A) in the subtropics
B) in a polar region
C) near the center of a large continent
D) next to a cold ocean current

Question 21

"Desert pavement" results from ___________.

A) deflation
B) saltation
C) evaporation
D) salt-wedging within cracks, joints, and fractures

Question 22

How does a blowout differ from a lag deposit?

A) A blowout is formed by saltative eruption of fine sediment.
B) A lag deposit is represented by a physical depression or low point in the ground surface.
C) A lag deposit forms in a wadi or desert wash, when briefly present floodwaters remove fine sediment and sand.
D) A blowout is an area where the removal of sediment by wind has lowered the ground surface.

Question 23

Which of the following is NOT necessary for the formation of calcrete?

A) leaching of soluble mineral constituents by water
B) precipitation of soluble mineral constituents
C) the flow or wicking of water within the ground
D) warm or hot daytime temperatures

Question 24

As compared to humid climates,rates of chemical weathering in deserts are ____________.

A) much faster
B) much slower
C) very similar
D) only a bit slower

Question 25

Two other names for desert washes (dry gullies)are ____________.

A) barchans and wadis
B) wadis and arroyos
C) arroyos and inselbergs
D) inselbergs and wadis

Question 26

Native Americans produced petroglyphs by etching into ____________.

A) rocks that had been subjected to case hardening
B) rocks that had been coated with desert varnish
C) dark basalts
D) obsidian

Question 27

It appears as if more recent channels in the alluvial fan complex shown below are lighter in color than the general surface of the fan.What geologic process could have caused this effect?

A) removal of desert-varnished clasts during formation of the channels, thus exposing less varnished and lighter colored material
B) the longer and bigger growth of plants on the areas between the recent channels, which from this far away cause the intrachannel areas to appear darker
C) transport of a large amount of dark-colored material out of the canyons during recent storms
D) the formation of inselbergs

Question 28

A rock that has been significantly reshaped on multiple surfaces by saltation-related processes and sometimes has concavely shaped surfaces separated by sharp edges is a(n)__________________.

A) inselberg
B) ventifact
C) wadi
D) yardang

Question 29

Because of little rainfall,the majority of physical weathering and transport of sediments in deserts is accomplished by the wind.

Question 30

Which feature in the the photo below would a geologist study to infer that running water was the primary agent to have formed the landscape in the photo?

A) the sand dunes
B) the dendritic patterns
C) the alluvial fans
D) the volcanoes

Question 31

Why would the Antarctic ventifact shown below be black?

A) It is made of dolomite.
B) It has been carbonized.
C) It is coated with desert varnish.
D) It formed from coal.

Question 32

Which two terms go together?

A) lag deposit and playa
B) playa and talus
C) talus and evaporate
D) deflation and talus
E) lag deposit and deflation

Question 33

Two means of sediment transport by wind are ____________.

A) sand load and silt load
B) wind load and storm load
C) dune load and storm load
D) bed load and suspended load

Question 34

Saltation of sand involves ____________.

A) spherical grains rolling along the surface of dunes
B) grains traveling short distances (approximately 0.5 m) in the air
C) grains traveling long distances (approximately 1-100 km) in the air by strong wind
D) the mixing of mineral sand and salt to form a weakly cemented soil

Question 35

Drawing upon information in this chapter and previous chapters,which of the following would NOT be a reason why flowing water "dries up" so quickly in deserts?

A) The stream-bottom sediments are often coarse particles with correspondingly large pore spaces between them, allowing water to quickly infiltrate downward.
B) The water table in the desert is usually deep and is not close to the ground surface.
C) The air is usually dry, and so evaporation rates are usually high.
D) The desert-adapted vegetation common in dry washes, arroyos, wadis, and the like, has a very high initial water uptake in order to store water for dryer periods, and the roots literally "soak up water."

Question 36

Some of the physiographic features that cause extreme aridity may be found together within a single desert,but the deserts found in the western United States are primarily due to being located ____________.

A) in the subtropics
B) within the rain shadow of one or more mountain ranges
C) near the center of a large continent
D) next to a cold ocean current

Question 37

Which of these factors allows the bouncing of sand grains to exceed 0.5 m in height (off the ground)during saltation?

A) Particle size. Very fine and fine-grained sand may bounce as high as 2 m, after hitting other sand particles in saltation.
B) Lower density. Weathered platy minerals (clays and micas, primarily) are lofted further off the ground.
C) Bouncing off of bedrock instead of off of other sand or silt grains.
D) Air pressure. Extremely low air pressure (as occurs near or within tropical storms and depressions) can allow sand clasts to bounce as much as 2 m in height before being pulled back downward.

Question 38

During heavy rainstorms,rates of physical weathering and erosion are ____________.

A) greater in humid climates than in deserts because dry desert soils can soak up all of the available moisture
B) greater in deserts than in humid climates because soil tends to be more easily eroded in deserts than in humid regions
C) greater in humid climates than in deserts because vegetation tends to break up the soil
D) about equally fast in deserts and in humid climates

Question 39

Desert varnish is caused by ____________.

A) pesticide and other pollutant residues
B) dusts of iron and magnesium oxides
C) dust deposits altered by microorganisms
D) carbonate cements with iron impurities, for example, siderite

Question 40

Why is the land surface higher underneath the clumps of vegetation?

A) evaporation, causing compaction of soils
B) compaction of the ground from the hoofs of cattle or other grazing animals
C) deflation of the ground surface by wind erosion
D) excretion of soil particles by the bushes during their lifetimes

Question 41

Why do the sand dunes in the west vary from those in the east in the figure below?

A) The average size of the sand clasts differs in the two areas.
B) There is slightly more rain to the east.
C) The material to the east is closer to an igneous-rock source area, from which the dune sediment is significantly derived.
D) The winds are predominantly in one direction in the west but blow more complexly in the east.

Question 42

A vast region of desert sand dunes is called a(n)_________.

A) wadi
B) dune field
C) erg
D) loess region

Question 43

What statement describing shared features of alluvial fans and pediment is FALSE?

A) Both form from clastic sediment.
B) Both form along flanks of mountains.
C) Both form where canyons exit from the side of a mountain range.
D) Both have radial or "fan shapes."

Question 44

The downwind side of a dune is called the _____________.

A) lee face
B) depositional slope
C) slip face
D) lag face

Question 45

In this high-altitude photo of a desert,what are the dark grey features in the bottom right of the photo? Hint: The area that looks like water flowing from top right to bottom left of the photo is not water.

A) volcanoes
B) chimneys
C) inselbergs
D) cuestas

Question 46

Cuestas can form when ____________.

A) winds remove certain sizes of sediment from the land surface
B) water flows intermittently
C) beds or layers are dipping
D) the wind blows strongly from two different directions and there is an abundant supply of sand

Question 47

If you excavated an exploratory trench from left to right through either of the sand dunes in the figure below,which way would you expect to find the cross-beds dipping?

A) vertically
B) to the right
C) to the left
D) horizontally

Question 48

A(n)____________ consists of sand and gravel deposited by ephemeral distributary channels that occur where a gully or canyon discharges from a canyon mouth.

A) alluvial fan
B) ventifact
C) wadi
D) yardang

Question 49

Which would be a better place to build a house-at point A or at point B?

A) at A
B) at B
C) A and B are equally good locations.
D) The picture does not provide any useful information for answering the question..

Question 50

What is the most important difference between a talus deposit and an alluvial fan?

A) size of the clasts
B) climate of formation. Alluvial fans form in desert regions; talus forms in all climates.
C) the agent that moves clasts: water, for alluvial fans; gravity, for talus
D) degree of weathering of clasts. In alluvium, materials have undergone extensive chemical weathering; in talus, clasts have undergone little chemical weathering.

Question 51

The factors that control the shapes of sand dunes include all of the following EXCEPT for _________.

A) the average speed of wind when it blows
B) the constancy/variability in the direction that moderate and strong winds blow
C) the supply of sand
D) the size fraction of sand that composes the dunes

Question 52

Mesas,buttes,and chimneys have which of these in common?

A) They form only in hot deserts.
B) They form from basalt flow caps on softer rocks.
C) They form in flat-lying rocks.
D) They form only in subtropical deserts.

Question 53

Which of these was NOT a contributing cause of the desertification of the Sahel?

A) higher-than-normal temperatures
B) increased grazing
C) increased farming
D) disease-related death of the predominant grass types

Question 54

The dominant process of clast transport in active sand dunes is ____________.

A) saltation
B) sliding
C) deflation
D) suspension

Question 55

A geologist discovers several lithified horizons that contain ventifacts within matrices material that can be age-dated by carbon-14.Which of the following should the geologist NOT infer,even with good field and laboratory methods of research?

A) average rates of sediment deposition
B) the predominant type(s) of geologic materials being transported through the area at the time/age of each soil horizon
C) the predominant wind directions and age of each horizon
D) the age of the rock from which the ventifacts formed

Question 56

Which of the following was NOT a factor contributing to the Dust Bowl event in 1933 in the United States and Canada?

A) lower-than-normal rainfall
B) high winds
C) withering and crumbling of vegetation due to lack of water
D) higher-than-normal temperatures

Question 57

Which of these is NOT necessary for the formation of a bajada?

A) clastic sediment
B) an arid environment
C) multiple canyon, stream, or other outlets along a mountain front
D) shallow groundwater rich in calcium and other ions

Question 58

The geometry of sand dunes is strongly influenced by the ____________.

A) strength of the wind
B) consistency of wind direction
C) abundance of sand
D) All of the above are correct.

Question 59

What is a playa?

A) an outcropping of bedrock within a large area of alluvium, soil, or sediment
B) a sand dune formed when winds blow predominantly from two different directions oriented at approximately 90 degrees to one another
C) a narrow gully or small canyon that intermittently contains flowing surface water
D) the lowest point of a basin that contains fine sediments, evaporate minerals, and occasionally standing water

Question 60

Which of these factors does NOT contribute to the salinity of the Great Salt Lake and similar water bodies?

A) evaporation
B) the inflow of water from streams
C) the outflow of water from the Great Salt Lake
D) chemical weathering