Deck 13: The Ocean Floor

Question 1

Which ocean has the greatest average depth?

A)Atlantic
B)Pacific
C)Indian
D)Arctic

Answer 1

B

Question 2

It is thought that submarine canyons on the continental slope have been generated by ________.

A)streams when sea level was much lower than it is today
B)streams when sea level was much higher than it is today
C)a submarine glaciations
D)turbidity currents

Answer 2

D

Question 3

What is the difference between an active continental margin and a passive continental margin?

A)A passive margin is the site of an ancient continental rift,left behind when seafloor spreading moved offshore,whereas an active margin is an active plate boundary.
B)A passive margin is passively carried along by plate tectonics in a transform system,like the San Andreas whereas an active margin is a trench.
C)An active margin receives large amounts of sediment whereas a passive margin is sediment starved.
D)An active margin has earthquakes whereas a passive margin has quiet erupting (aka passive)volcanos.

Answer 3

A

Question 4

Which of the following generally has the lowest surface slope?

A)continental slope
B)continental rise
C)continental shelf
D)beach shoreface

Question 5

Multibeam provides the best modern technique for obtaining seafloor bathymetry,so why hasn't it been used for the entire ocean?

A)It is too expensive,requiring many ships many years to survey the entire ocean.
B)It doesn't work in deep water.
C)It doesn't work where there is ice.
D)Oceanographers simply haven't had time to process all the data.

Question 6

Concerning the distribution of land and water,which of the following statements is true?

A)The percentage of land and water is about the same in both the Northern and Southern Hemispheres.
B)The Southern Hemisphere has much more water surface than the Northern Hemisphere.
C)The Northern Hemisphere has much more water surface than the Southern Hemisphere.
D)None of the above is true.

Question 7

An echo sounder operates by measuring the time required for a ________.

A)light beam to travel from a satellite at a known altitude to the sea bottom and back
B)radar beam to travel from a harbor patrol boat to a fuzz-buster on a speeding yacht
C)radar beam to travel from a ship to the seafloor and back
D)sound pulse travels from a ship to the seafloor and back

Question 8

The ________ Ocean is largest.

A)Atlantic
B)Pacific
C)Indian
D)Arctic

Question 9

________ develop where oceanic lithosphere bends downward and sinks into the mantle.

A)Submarine canyons
B)Abyssal seamounts
C)Deep ocean trenches
D)Rift valleys on mid-ocean ridges

Question 10

The continental rise is located ________.

A)at the top of a mid-ocean ridge
B)at the top of the continental slope
C)between an abyssal plain and continental slope
D)at the seaward edge of a deep ocean trench

Question 11

The gently sloping submerged surface extending from the shoreline toward the deep ocean is termed the ________.

A)continental shelf
B)continental slope
C)continental rise
D)submarine canyon

Question 12

Sailors in the 16th-18th century dreaded "rounding the horn" because of strong winds and heavy seas.Cape Horn is about latitude 55°S at the tip of South America.These sailors considered this a passage from the Atlantic to Pacific oceans,but this ocean is now often called ________.

A)the Southern Ocean
B)the Antarctic Ocean
C)the Australian Ocean
D)the Indian Ocean

Question 13

The best modern technique for obtaining high resolution bathymetry of the seafloor is ________.

A)sidescan sonar
B)echo sounding
C)seismic reflection profiling
D)multibeam bathymetry

Question 14

Ocean floor topography can be estimated from space using ________.

A)high resolution measurements of the earth's magnetic field
B)laser beams that penetrate to the sea floor
C)radar pulses that bounce off the ocean surface
D)powerful sound waves that echo to the satellite

Question 15

Which one of the following would most likely be covered with thick turbidite layers?

A)axial rift zone of an active mid-ocean spreading center
B)upper part of a steep,narrow,submarine canyon
C)deep-sea fan at the base of a continental slope
D)ocean floor around a conical-shaped seamount

Question 16

In the Mississippi River delta in south Louisiana shallow subsurface imaging of the sediments reveal buried channels beneath delta sediments that are less than 10,000 years,and these channels extend offshore to the edge of the continental shelf.What is a simple explanation for these buried channels?

A)They represent giant flood events before the delta was formed and before the river formed.
B)They represent channels of the Mississippi river delta when sea level was lower,during the ice age.
C)The river must have been bigger in the past,cutting a channel clear to the edge of the shelf below sea level.
D)Turbidity currents cut a submarine canyon on the shelf before the delta buried them.

Question 17

The best definition of the outer edge of the continental shelf is that point where ________.

A)the gradient becomes very gentle
B)the water depth reaches 100 fathoms
C)it meets an oceanic ridge
D)a rapid steepening of the gradient occurs

Question 18

The oceans cover approximately ________ percent of Earth's surface.

A)40
B)50
C)60
D)70

Question 19

Submarine canyons found on the continental slope are believed to have been created ________.

A)by rivers during the ice age
B)by faulting
C)because of a plate plunging into the mantle
D)none of these

Question 20

Which one of the following is not part of the continental margin?

A)continental shelf
B)continental slope
C)continental rise
D)continental trench

Question 21

The crests of mid-oceanic ridges ________.

A)are heavily mantled with sediment
B)lie at depths exceeding 6 kilometers
C)contain active rift zones
D)are geologically old features

Question 22

Seamounts ________.

A)are a special type of oceanic trench
B)are volcanoes that form on the ocean floor
C)form only in the Pacific Ocean basin
D)are submarine canyons found near Australia

Question 23

Sediments derived primarily from the products of weathering on the continents are called ________.

A)terrigenous sediment
B)biogenous sediment
C)hydrogenous sediment
D)both terrigenous and biogenous sediment

Question 24

"Black smokers" are associated with ________.

A)oceanic ridges
B)mineral-rich waters
C)hot water
D)all of these

Question 25

Which one of the following concerning mid-ocean ridges is false?

A)They are sites for submarine eruptions of basaltic lava.
B)They are where young lithosphere is added to the edges of spreading,oceanic plates.
C)Terrigenous sediment coverings are very thin or absent.
D)Sediments include thick siliceous ooze deposits and sandy turbidite beds.

Question 26

Abyssal plains are very flat because ________.

A)basalt volcanos on the seafloor produce little terrain
B)sediments accumulate to cover the seafloor topography
C)they have flat faults,the form flat,planar topography
D)they are not really flat; it is just that there is no data so they are shown as flat

Question 27

Why are there virtually no sediments at ocean ridges,even when they are close to land?

A)The sediments get buried by volcanic rocks,so are not visible.
B)The sediments get deflected from the topographic high of the ridge.
C)The crust is too young; there hasn't been enough time for sediments to accumulate.
D)No one knows,it is one of the mysteries of geology.

Question 28

Which of the following is not true of deep ocean trenches?

A)They are long and narrow depressions.
B)They are sites where plates plunge back into the mantle.
C)They are geologically very stable.
D)They may act as sediment traps.

Question 29

Ocean ridges form because ________.

A)rocks bulge up in ridges along transform faults associated with the ridge
B)they are a broad seafloor highland formed by sea floor spreading
C)they are submarine collisional mountain belts,forming submarine mountains that haven't yet risen from the sea floor
D)they are chains of volcanos,like the Hawaiian Islands,that form along hot spots

Question 30

The ________ Ocean has more extensive abyssal plains than the Pacific Ocean because it has fewer trenches to trap sediments moving down the continental slope.

A)Indian
B)Atlantic
C)Arctic
D)none of these

Question 31

Earlier we discussed the concept of isostasy,where lower density rocks rise higher than higher density rocks.How is the variation of water depth at spreading centers (ridges)controlled by isostasy?

A)Volcanic rocks are lighter than other rocks,and so the abundant volcanic rocks at the ridges are lower density features that produce the seafloor topography.
B)It has nothing to do with isostasy; the areas are simply high because there are chains of active volcanos along the ridge crest,producing the topography.
C)Oceanic ridges are sites where the lithosphere is carried into the mantle on one side (aka subduction zones),and the volcanos along these margins produce the ridge by buildup of lower density crust.
D)The lithosphere cools as it moves away from the ridge axis by sea floor spreading,and cooler rocks are lower density,so the sea floor gets deeper as the lithosphere gets more dense.

Question 32

If Yellowstone were on the ocean floor,it would produce ________.

A)a chain of line islands,like Hawaii
B)an island arc
C)a trench
D)a mid-ocean ridge

Question 33

Deep sea trenches are important features for plate tectonic studies because ________.

A)they are sites where oceanic lithosphere is formed
B)they are sites where two plate move past each other horizontally
C)they are sites where the oceanic lithosphere is consumed into the mantle
D)none of the above

Question 34

Where are the deepest parts of the oceans?

A)along the base of the continental slope
B)in rift valleys at sea floor spreading center
C)in the abyssal plain
D)in trenches

Question 35

How do calcareous oozes form?

A)The particles are precipitated in warm surface waters and sink to the bottom.
B)The particles are precipitated by bottom-dwelling organisms.
C)The particles are precipitated in the water column below the depth of sunlight penetration and then sink to the bottom.
D)The particles settle out from calcite-rich turbidity currents at depths greater than 15,000 feet.

Question 36

Manganese nodules are an example of ________.

A)terrigenous sediment
B)biogenous sediment
C)hydrogenous sediment
D)both terrigenous and biogenous sediment

Question 37

Subduction of oceanic lithosphere at the trenches can act like a bulldozer to produce ________.

A)subduction erosion
B)an accretionary wedge
C)a subduction slope
D)a strike slip fault like the San Andreas

Question 38

Which of the following is associated with ocean ridges?

A)rift zones
B)mountainous topography
C)volcanic structures
D)all of these

Question 39

Convective cooling cools rocks much more rapidly than heat conduction.Hydrothermal circulation represents convective cooling at ocean ridges and is well known from things like black smokes,but only occurs close to the spreading ridge axis.When geophysicists measure the geothermal gradient in areas along ridges where there is no hydrothermal activity,the thermal gradient is far below what you would predict theoretically,but near hydrothermal vents it is far more than you would predict.Why would this be?

A)The measurements are incorrect because it is too hot to measure thermal gradient in molten rock.
B)Most of the heat is carried away by convection as hydrothermal systems,so the average geothermal gradient away from the hydrothermal circulation is depressed.
C)Organisms growing around hydrothermal vents disturb the thermal properties,insulating the surface,to make an apparent high thermal gradient.
D)The thermal gradient in water represented by the hydrothermal system must be different than the rock,so it is measurement artifact.

Question 40

The seafloor spreading process at ridges produces what kind of faults?

A)normal faults
B)thrust fault
C)strike-slip faults
D)oblique-slip faults

Question 41

manganese nodules calcareous ooze metal sulfides evaporites

Question 42

Minerals that crystallize directly from seawater are examples of ________.

A)terrigenous sediment
B)biogenous sediment
C)hydrogenous sediment
D)both terrigenous and biogenous sediment

Question 43

-calcareous ooze

Question 44

-abyssal plain clay

Question 45

echo sounder Doppler radar sidescan sonar radar altimeters

Question 46

In the present day,what is the most valuable non-living commodity obtained from sea floor?

A)sand and gravel
B)gold
C)oil and gas
D)gas hydrate

Question 47

continental shelf continental rise continental slope continental volcanic arc

Question 48

rift valley subduction volcanic island arc trench

Question 49

A geologist is studying sediments on land that were originally deep sea sediments.She finds the minerals zircon and garnet in the sediments,which could only come from a continental region.These sediments must be ________.

A)terrigenous sediment
B)biogenous sediment
C)hydrogenous sediment
D)both terrigenous and biogenous sediment

Question 50

The deepest point in the ocean is in the Indonesian trench,off Sumatra,where the 2004 Earthquake occurred.

Question 51

Much of the Gulf of Mexico is underlain by large thicknesses of salt that formed in a small ocean basin like the red sea,as the north Atlantic spreading began in Mesozoic time.This salt is a good example of ________.

A)terrigenous sediment
B)biogenous sediment
C)hydrogenous sediment
D)both terrigenous and biogenous sediment

Question 52

When organisms living in seawater grow shells,the ratio between the isotopes of oxygen (18O/16O)trapped in their shell is dependent on the sea temperature.Why might geologists collect these sea shells from deep-sea sediments that are Pleistocene in age and measure their oxygen isotope ratios?

A)They might want to know how cold the water was at the seafloor in the past.
B)They want to know how metabolism of the organism affects the oxygen isotope ratios.
C)The organisms live in the photic zone,and so photosynthesis must be the main factor in the oxygen content.
D)The organisms live near the surface,so their oxygen isotope ratios record surface temperature during Earth's great climate fluctuations of the ice age.

Question 53

Micro-organisms called foraminifera have tiny coiled shells with chambers,not unlike the chambering nautilus but much smaller.In modern oceans the coiling direction of foraminifera shells is correlated to temperature.Using coiling direction from foraminifera fossils is an example of ________.

A)using the fossil as a paleo-depth indicator because water temperature varies with depth
B)using the fossil's coiling direction is a proxy for climate because the surface water temperature would be highly correlated to surface temperature
C)using isotopes to fingerprint temperature
D)geomagic; the whole idea is preposterous

Question 54

Most of our table salt comes from ________.

A)salt taken from mines
B)salt obtained by evaporating seawater in dry climates
C)combining the metal sodium and the nonmetal chlorine gas in a chemical reaction vessel to make pure NaCl
D)China

Question 55

Radiolarian chert is a common sedimentary rock accreted along active margins in the circum-pacific.It is produced in the deep sea by the accumulation of siliceous radiolarian shells.What kind of sediment is radiolarian chert?

A)terrigenous sediment
B)biogenous sediment
C)hydrogenous sediment
D)both terrigenous and biogenous sediment

Question 56

Why would a large concrete producing company be interested in the sea bed?

A)as a source of lime from sea shells
B)as a source of mud to mix with limestone to make Portland cement
C)They can rape the sea bed without being prosecuted like they would in operating a quarry or gravel pit improperly.
D)as a source of sand and gravel

Question 57

Why isn't manganese mined on the seabed when there are abundant manganese nodules in some parts of the ocean floor?

A)The manganese is too difficult to extract from the nodules; a new technology is needed.
B)It is not economical to mine manganese nodules at present-day world prices for manganese and associated metals in nodules.
C)The manganese market is held by a global cartel who prevent the material from being mined.
D)It is impossible to extract the nodules from the seafloor with present-day technology.

Question 58

The 2010 deep water horizon oil spill in the Gulf of Mexico was a tragedy brought on in part by new technology allowing development of oil and gas reserves in deep water.Given the scope of the spill,why would companies take a risk of such a disaster given the scope of the litigation following the disaster?

A)They didn't care about the risk,they just drove forward.
B)The oil and gas industry is poor at assessing risk,and didn't understand the risk.
C)The potential profit is so large from a major oil find that the companies take the risk,try to use all cautions to minimize risk,but sometimes fail.
D)They just assume their lawyers can win any lawsuit,regardless of the disaster.

Question 59

African dust from the Sahara desert is observed across the Atlantic as far west as the Caribbean region where it produces a hazy sky condition during the annual dry season.You are a climate scientist and make a prediction that the Sahara was not a desert during glacial periods in the northern hemisphere.How could you test this hypothesis by looking at deep sea sediments from the Atlantic off west Africa?

A)You would look at fossils to see if there were organisms that preferred dusty conditions vs.clear water conditions.
B)You could look at the sediments to see if there were biogenic sediments interbedded with sediments with terrigeneous input corresponding to the dry,interglacial intervals.
C)You could measure the isotopes in the sediment to estimate the sea surface temperatures.
D)The hypothesis is untestable with deep sea sediment.

Question 60

-manganese nodule

Question 61

Valleys that lead from the continental shelf into deeper waters are known as ________.

Question 62

Oil and gas resources are rare on the continental slope and continental rise.

Question 63

Seafloor hot springs occur mainly in oceanic,abyssal plains.

Question 64

________ continental margins typically exhibit wide,extensive,continental shelves.

Question 65

The lowest point on earth is in the ________ Ocean.

Question 66

________ continental margins occur where oceanic lithosphere is being subducted beneath the edge of a continent.

Question 67

The Atlantic and Pacific basins have oceanic ridges; the Indian Ocean has no oceanic ridge.

Question 68

Submerged,flat-topped seamounts are known as guyots.

Question 69

The gently sloping submerged surface extending from the shoreline toward the deep ocean is termed the ________.

Question 70

Submarine canyons form the deepest parts of the ocean basins.

Question 71

Most of the world's landmass is in the northern hemisphere.

Question 72

Flat-topped volcanic structures located on the floor of the deep ocean basin are termed ________.

Question 73

Manganese nodules do not accumulate below 4500 meters depth because the manganese minerals are highly soluble in seawater below that depth.

Question 74

The principal technique for finding ocean depth today is ________.

Question 75

Abyssal plains with sediments covering the seafloor igneous rocks are more extensive in the central Pacific basin than in the North Atlantic.

Question 76

Sand,silt,and clays deposited on the ocean floor are described as terrigenous sediments.

Question 77

The second most important economic mineral resource extracted from the seafloor today is manganese from manganese nodules.

Question 78

The west coast of South America and the east coast of North America have very different continental margins.

Question 79

Turbidites and siliceous oozes are both biogenous sediments.

Question 80

The continental rise lies at the bottom of the continental slope.