Chapter 13 Divergent Boundaries: Origin and Evolution of the Ocean Floor
Earth: An Introduction to Physical Geology, 9e (Tarbuck/Lutgens)
13.1 Multiple-Choice Questions
1) In the 1870s, the ________ was involved in the first far-reaching, comprehensive study of
Earth's oceans.
A) H.M.S. Moonbeam
B) H.M.S. Bismarck
C) H.M.S. Challenger
D) H.M.S. Brit
Answer: C
Diff: 1
2) Water depths and seafloor topography are now routinely determined by ________.
A) ship-mounted, optical-fiber satellite uplink systems
B) computerized, satellite-mounted, parallel transponder systems
C) ship-mounted echo sounders
D) by satellite-linked, laser reflector systems
Answer: C
Diff: 1
3) Geologically, ________ are actually submerged parts of the continents.
A) coastal guyots
B) continental shelves
C) continental trenches
D) abyssal plains
Answer: B
Diff: 1
4) The ________ lies at the base of the continental slope.
A) offshore shelf
B) off-slope reef
C) continental rift
D) continental rise
Answer: D
Diff: 1
5) Submarine canyons found on the continental slope and rise 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
Answer: D
Diff: 1
1
6) 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
Answer: C
Diff: 1
7) 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.
Answer: C
Diff: 1
8) 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
Answer: B
Diff: 1
9) "Black smokers" are associated with ________.
A) oceanic ridges
B) metal-rich sulfide deposits
C) hot water
D) all of these
Answer: D
Diff: 1
10) ________ marks the edge of a continent.
A) An offshore, barrier island system
B) The lowest part of the continental slope
C) The center of an abyssal plain
D) The highest part of the continental shelf
Answer: B
Diff: 1
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
Answer: A
Diff: 1
2
12) Which one of the following is not connected in any way with submarine, hot spring vents?
A) sediment rich in metallic sulfides
B) ecological communities living without photosynthesis
C) thick turbidites on the continental rise
D) black smokers on a mid-ocean ridge
Answer: C
Diff: 1
13) Which one of the following would typically have the widest continental shelf?
A) a tectonically passive, trailing continental margin
B) a tectonically passive, mid-ocean ridge and trench system
C) a tectonically active continental margin next to a deep ocean trench
D) All of the above would roughly equal continental shelves.
Answer: A
Diff: 2
14) ________ are not associated with a mid-ocean ridge.
A) Very thin, ocean floor, sediment layer
B) Submarine basaltic lava eruptions
C) Shallower depths than abyssal plains
D) Deep ocean trenches
Answer: D
Diff: 1
15) Seafloor spreading occurs along relatively narrow areas at the crests of oceanic ridges called
________.
A) trenches
B) submarine canyons
C) rift zones
D) seamounts
Answer: C
Diff: 1
16) Oceanic ridges are elevated compared to the surrounding ocean floor because ________.
A) newly formed lithosphere is hotter and therefore less dense than the surrounding rocks
B) the older, colder lithosphere is less dense and tends to rise
C) the mantle is pushing up the lithosphere along the ridge
D) of the shield volcanoes that develop due to seafloor spreading
Answer: A
Diff: 1
17) ________ is/are not part of an ophiolite complex.
A) Basaltic lavas and pillow lavas
B) Residual, unmelted, crustal lithosphere
C) Intrusive, coarse-grained gabbro
D) Steeply inclined, basaltic dikes
Answer: B
Diff: 1
3
18) Where in the oceans are biological communities thriving without sunlight?
A) in shallow water, coral reefs
B) in muds of deep-ocean trenches
C) around seafloor, hot spring vents
D) around fissure vents for flood basalts
Answer: C
Diff: 1
19) Which one of the following statements concerning submarine canyons is not true?
A) They extend from the continental shelf to the base of the continental slope.
B) They channel turbidity currents down the continental slope to deeper waters.
C) They generally conned across the continental shelf to specific river valleys on land.
D) They were cut by streams and rivers during the Pleistocene when sea level had
dropped to the base of the continental slope.
Answer: D
Diff: 2
20) ________ 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
Answer: C
Diff: 1
21) Spreading rates of ________ are common along the Mid-Atlantic and the Mid-Indian ridges.
A) 1 to 5 meters per year
B) 1 to 5 centimeters per year
C) 1 to 5 kilometers per year
D) 1 to 5 millimeters per year
Answer: B
Diff: 1
22) All of the following are layers that comprise oceanic crust except for ________.
A) pillow basalts
B) sheeted dike complexes
C) turbidites
D) gabbro
Answer: C
Diff: 1
23) Basaltic rocks of the oceanic crust are altered by heated seawater moving through fractures
in a process known as ________.
A) contact metamorphism
B) diagenesis
C) regional metamorphism
D) hydrothermal metamorphism
Answer: D
Diff: 1
4
24) East Africa, the Rhine Valley, and the Baikal in Siberia are all examples of ________.
A) continental rifts
B) ophiolites
C) continental margins
D) passive continental margins
Answer: A
Diff: 1
25) ________ is the oceanward edge of a continental shelf.
A) The deepest portion of a deep-ocean trench
B) The top of the abyssal plain
C) The base of the continental rise
D) The top of the continental slope
Answer: D
Diff: 1
26) Plate subduction occurs because the ________, not the oceanic crust, is denser than the
underlying asthenosphere.
A) outer core
B) seafloor sediments
C) lithospheric mantle
D) trailing continent
Answer: C
Diff: 1
27) The idea that rifting and dispersal of one large landmass is followed by a long period during
which the fragments are gradually reassembled into a new single landmass is called
________.
A) plate tectonics
B) plate assemblage
C) uniformitarianism
D) the supercontinent cycle
Answer: D
Diff: 1
28) 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
Answer: C
Diff: 1
5
29) 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 to travel from a ship to the seafloor and back
Answer: D
Diff: 1
30) 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.
Answer: D
Diff: 1
31) Oceans cover approximately ________ of Earth's surface area?
A) 10%
B) 90%
C) 50%
D) 70%
Answer: D
Diff: 1
13.2 Word Analysis Questions
Examine the words and/or phrases for each question below and determine the relationship among the
majority of words/phrases. Choose the option which does not fit the pattern.
1) A) continental slope
B) continental rise
C) active continental margin D) continental
shelf
Answer: active continental margin
Diff: 1
2) A) trench
B) submarine canyon C) abyssal plain
Answer: submarine canyon
D) seamount
Diff: 1
3) A) manganese nodules B) pillow basalts
Answer: manganese nodules
C) sheeted dikes
D) gabbro
C) Red Sea
D) Baikal Rift
Diff: 1
4) A) East African Rift
Answer: Red Sea
B) Basin & Range
Diff: 2
6
13.3 True/False Questions
1) The west coast of South America and the east coast of North America have very different
continental margins.
Answer: TRUE
Diff: 1
2) Abyssal plains with sediments covering the seafloor igneous rocks are more extensive in the
central Pacific basin than in the North Atlantic.
Answer: FALSE
Diff: 1
3) Submerged, flat-topped seamounts are known as guyots.
Answer: TRUE
Diff: 1
4) The Atlantic and Pacific basins have oceanic ridges; the Indian Ocean has no oceanic ridge.
Answer: FALSE
Diff: 1
5) Oceanic ridges are lower then the surrounding ocean floor due to the warm, less dense
oceanic crust found along the ridges.
Answer: FALSE
Diff: 1
6) Deep-ocean trenches mark the sites where lithospheric plates are subducting back into the
mantle.
Answer: TRUE
Diff: 1
7) Rift zones are narrow areas along the oceanic trenches where hot solid mantle rocks rise
upward to replace the material that has shifted horizontally.
Answer: FALSE
Diff: 1
8) Submarine canyons form the deepest parts of the ocean basins.
Answer: FALSE
Diff: 1
9) Spreading rates along the East Pacific Rise are relatively fast with many areas spreading
more than 9 centimeters per year.
Answer: TRUE
Diff: 1
10) The continental rise lies at the top of the continental slope.
Answer: FALSE
Diff: 1
7
11) Seafloor hot springs occur mainly in oceanic, abyssal plains.
Answer: FALSE
Diff: 1
12) The sequence of rocks that comprise an accretionary wedge is called an ophiolite.
Answer: FALSE
Diff: 1
13.4 Short Answer Questions
1) The ________ is a gently sloping submerged surface that extends from the shoreline to the
deep-ocean basin.
Answer: continental shelf
Diff: 1
2) The chaotic accumulation of deformed sediment and scraps of oceanic crust along an active
continental margin is called an ________.
Answer: accretionary wedge
Diff: 1
3) The sequence layers that comprise the oceanic crust is call an ________.
Answer: ophiolite complex
Diff: 1
4) Submerged, flat-topped volcanoes that rise from the deep ocean floor are called ________.
Answer: guyots
Diff: 1
5) Valleys that lead from the continental shelf into deeper waters are known as ________.
Answer: submarine canyons
Diff: 1
6) ________ continental margins typically exhibit wide, extensive, continental shelves.
Answer: Passive
Diff: 1
7) A(n) ________ is a volcanic mountain built up from the seafloor that never reached the sea
surface.
Answer: seamount
Diff: 1
8) A(n) ________ is the vast, relatively deep, flat, sediment-covered portion of the deep-ocean
basin.
Answer: abyssal plain
Diff: 1
9) A(n) ________ marks the site where old, oceanic lithosphere begins its descent into a
subduction zone.
Answer: deep-ocean trench
Diff: 1
8
10) Submarine, basaltic lava flows chill quickly to form tube shaped protuberances called
________.
Answer: pillow basalts
Diff: 1
13.5 Critical Thinking Questions
Use complete sentences, correct spelling, and the information presented in Chapter 13 to answer the
questions below
1) Where are most of the current active continental margins located on Earth today? Why?
What is the relationship of the age of rocks on the ocean floor to the location of these active
margins?
Diff: 2
2) Briefly discuss the formation and structure of oceanic crust. What rock compositions are
involved and how do these compositions change as a result of interaction with seawater?
Diff: 3
3) List several areas on Earth where continental rifting is occurring today. Eventually, what
will happen to these areas as rifting continues? Finally, how is continental rifting similar to
seafloor spreading and how is it different?
Diff: 3
13.6 Visualization Questions
1) The illustration below is a topographic profile of a passive continental margin. Fill in the
blanks with the correct name of the feature labeled.
Answer: a) continental shelf b) continental slope c) continental rise
Diff: 1
9
2) What is the name of the valley in the central portion of the diagram below?
Answer: rift valley
Diff: 1
3) Label the layers that make up the oceanic crust on the diagram below.
Answer: See figure 13.17 from chapter 13 in Earth, 9e.
Diff: 2
10
4) On the diagram below, match the features to the correct answer for each one.
a) continental volcanic arc b) trench c) accretionary wedge
Answer: A = b B = c C= a
Diff: 2
11

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