Chapter 13: Divergent Boundaries

What is the average thickness of the oceanic crust? A) 0 km B) 3 km C) 7 km D) 20 km

C) 7 km

Which of the following is not an example of a continental rift? A) East Africa B) Rhine Valley, Europe C) Lake Baikal, Siberia D) Appalachia, United States

D) Appalachia, United States

What rock would you expect to find associated with a mid-ocean ridge? A) Sandstone B) Granite C) Limestone D) Basalt

D) Basalt

Which part of a passive continental margin is built up by repeated deposition from turbidity currents? A) Abyssal plain B) Continental shelf C) Continental slope D) Continental rise

D) Continental rise

Match the seafloor terms to their correct definitions. *Continental Shelf*: A) Flat seafloor in the deep ocean B) Wedge of sediments deposited by turbidity currents that formed at the base of the slope C) Narrow creases in the seafloor that result from subduction D) Gently sloping, submerged extension of the continent E) A steep incline that marks the boundary between the continental crust and the oceanic crust

D) Gently sloping, submerged extension of the continent

Which of the following is associated with a passive continental margin? A) Divergent boundaries B) Convergent boundaries C) Transform boundaries D) No tectonic boundaries nearby

D) No tectonic boundaries nearby

What are the two types of continental margins? A) Transform and divergent B) Trenches and rifts C) Continental shelves and slopes D) Passive and active

D) Passive and active

Put the layers of an ophiolite complex in order from top to bottom: *Layer 2*: A) Gabbro B) Deep-sea sediments or sedimentary rocks C) Sheeted dike complex D) Pillow basalts

D) Pillow basalts

What is the definition of subduction erosion? A) Sediments from the ocean floor are scraped off a subducting tectonic plate and adhere to the overriding plate. B) Diverging tectonic plates will cause rocks to slide downward along normal faults. C) Glaciers extending off the continent will scour deep grooves in continental shelves. D) Sediment and rock are scraped off the bottom of an overriding plate and transported into the mantle by a descending plate.

D) Sediment and rock are scraped off the bottom of an overriding plate and transported into the mantle by a descending plate.

Which of the following is not a method of obtaining samples and observations of the oceanic ridge system? A) Finding segments of ocean floor that have been thrust up onto the continent B) Visual inspections using submersible equipment C) Deep sea core samples D) Side-scan sonar

D) Side-scan sonar

Which of the following will mark the furthest extent of a continent? A) The shoreline B) Tidal flats exposed during low tide C) The crest of the continental slope D) The lowest part of the continental slope

D) The lowest part of the continental slope

What is the source of flood basalts associated with oceanic plateaus? A) Decompression melting associated with seafloor spreading B) Partial melting associated with subduction C) Impact heating from a meteorite strike D) The melting of the head of a mantle plume

D) The melting of the head of a mantle plume

The measurement of ocean depth and the topography of the ocean floor are known as ________. A) geophysics B) seismic tomography C) topographic surveying D) bathymetry

D) bathymetry

A ________ forms when a rising mantle plume causes the overlying crust to dome and separate into three rifts. A) triple junction B) complex rift C) submarine canyon system D) divergent boundary

D) divergent boundary

The oceanic crust is composed almost entirely of ________ rocks that are underlain by ________ rocks, which makes up the lithospheric mantle. A) felsic; sediments B) intermediate; felsic C) sedimentary; mafic D) mafic; ultramafic

D) mafic; ultramafic

Which of the following is not a possible mechanism contributing to continental rifting? A) Changes in gravitational attraction of the moon B) Concentration of mantle plume heat beneath a continent C) Tensional stress D) Upwelling from shallow levels in the asthenosphere

A) Changes in gravitational attraction of the moon

Which of the following is a failed rift that formed in North America over one billion years ago? A) A Mid-Continent Rift B) East African Rift C) Chile Rise D) Kuril Trench

A) A Mid-Continent Rift

What is the speed of sound waves in water? A) 1,500 m/sec B) 5,000 m/sec C) 3,200 m/sec D) 10,994 m/sec

A) 1,500 m/sec

Which of the following groups contains all three major topographic provinces oceanographers use to define the seafloor? A) Continental margins, mid-ocean ridges, and deep-ocean basins B) Mid-ocean ridges, continental shelves, and divergent boundaries C) Submarine trenches, mid-ocean ridges, and turbidity currents D) Deep-ocean basins, active margins, and passive margins

A) Continental margins, mid-ocean ridges, and deep-ocean basins

Along which tectonic boundary are deep-ocean trenches found? A) Convergent B) Divergent C) Transform

A) Convergent

What generates the magma necessary for seafloor spreading? A) Decompression melting B) Partial melting C) Mantle plumes D) Subduction

A) Decompression melting

Which of the following locations displays a triple junction that resulted from a rising mantle plume? A) Eastern Africa/Arabian Peninsula B) East Pacific Rise C) Japan/East Asia D) Mid-Atlantic Ridge

A) Eastern Africa/Arabian Peninsula

Match the seafloor terms to their correct definitions. *Abyssal Plain*: A) Flat seafloor in the deep ocean B) Wedge of sediments deposited by turbidity currents that formed at the base of the slope C) Narrow creases in the seafloor that result from subduction D) Gently sloping, submerged extension of the continent E) A steep incline that marks the boundary between the continental crust and the oceanic crust

A) Flat seafloor in the deep ocean

What kind of subduction occurs when younger, warmer, less-dense lithosphere dips under another plate at shallower angles because it is too buoyant to sink? A) Forced subduction B) Mantle plume subduction C) Spontaneous subduction D) Overriding subduction

A) Forced subduction

Put the layers of an ophiolite complex in order from top to bottom: *Layer 4*: A) Gabbro B) Deep-sea sediments or sedimentary rocks C) Sheeted dike complex D) Pillow basalts

A) Gabbro

The continental shelf is composed of which material(s)? A) Granitic crust covered with sedimentary rocks B) Basaltic crust C) Clay sediments and sands D) Ophiolites

A) Granitic crust covered with sedimentary rocks

Which process, common near black smokers, will use hot water circulating through the crust to alter olivine and pyroxene into chlorite and serpentine? A) Hydrothermal metamorphism B) Decompression melting C) Partial melting D) Thermal contraction

A) Hydrothermal metamorphism

Which of the following locations is an example of spontaneous subduction? A) Mariana Trench B) Peru-Chile Trench C) East African Rift D) Cascadia Trench

A) Mariana Trench

A(n) ________ forms when sediments from the ocean floor and pieces of oceanic crust are scraped from a subducting oceanic plate and adhere to an overriding tectonic plate. A) accretionary wedge B) deep-sea fan C) active tectonic margin D) submarine canyon

A) accretionary wedge

Which of the following choices represents a rate of seafloor spreading commonly associated with a ridge such as the Mid-Atlantic Ridge? A) 1-5 mm/yr B) 1-5 cm/yr C) 1-5 m/yr D) 1-5 km/yr

B) 1-5 cm/yr

The Challenger Deep has previously been measured to have a depth of 10,994 meters. You have been sent to the part of the Pacific Ocean to verify this depth with an echo sounder. Assuming that Depth = 1/2 (1,500 m/sec*echo travel time), approximately how many seconds should it take for the sound wave to leave your ship, strike the bottom of the Challenger Deep, and return if the depth is correct? A) 10.5 seconds B) 14.66 seconds C) 17.39 seconds D) 22.67 seconds

B) 14.66 seconds

Approximately what percentage of the Earth's surface consists of deep-ocean basins? A) 10% B) 30% C) 47% D) 49%

B) 30%

Where do rift valleys form along the oceanic ridge system? A) Along the flanks of the continent where the slope levels out to meet the seafloor B) Along the axis of some ridge segments where the rift is down-faulted C) Transecting the ridge where transform boundaries exist D) Where the seafloor plunges back into the mantle as the plate is consumed

B) Along the axis of some ridge segments where the rift is down-faulted

Where is the deepest place on Earth? A) Peru-Chile Trench B) Challenger Deep C) Kuril Trench D) Mid-Continent Rift

B) Challenger Deep

Which of the following scenarios would likely result in the development of a passive continental margin? A) Two tectonic plates colliding, creating a submarine trench B) Continental blocks rifting apart and becoming separated by seafloor spreading C) Two tectonic plates sliding past each other, rotating part of the crust D) A mantle plume creating a chain of volcanic islands across the seafloor

B) Continental blocks rifting apart and becoming separated by seafloor spreading

Which of the following is associated with an active continental margin? A) Divergent boundaries B) Convergent boundaries C) Transform boundaries D) No tectonic boundaries nearby

B) Convergent boundaries

Put the layers of an ophiolite complex in order from top to bottom: *Layer 1 (Uppermost layer)*: A) Gabbro B) Deep-sea sediments or sedimentary rocks C) Sheeted dike complex D) Pillow basalts

B) Deep-sea sediments or sedimentary rocks

Which ship was involved in the first global, comprehensive study of the Earth's oceans starting in 1872? A) U.S.S. Cyclops B) H.M.S. Challenger C) R.M.S. Titanic D) The Glomar Explorer

B) H.M.S. Challenger

Which of the following locations is an example of forced subduction? A) Mariana Trench B) Peru-Chile Trench C) East African Rift D) Mid-Atlantic Rift

B) Peru-Chile Trench

What is the name of the supercontinent that preceded Pangaea? A) Amasia B) Rodinia C) Challenger D) Africana

B) Rodinia

What tool is often used to measure the topography of the seafloor today? A) High-resolution multibeam instruments B) Sonar C) Echo sounders D) Radar altimeters

B) Sonar

How do deep-ocean trenches form? A) Magma from a partially melting tectonic plate makes its way through the crust. B) Subducting lithosphere descends into the mantle. C) Two tectonic plates diverge and create a rift valley. D) Sediments from a turbidity current settle out in a deep ocean basin.

B) Subducting lithosphere descends into the mantle.

Match the seafloor terms to their correct definitions. *Continental rise*: A) Flat seafloor in the deep ocean B) Wedge of sediments deposited by turbidity currents that formed at the base of the slope C) Narrow creases in the seafloor that result from subduction D) Gently sloping, submerged extension of the continent E) A steep incline that marks the boundary between the continental crust and the oceanic crust

B) Wedge of sediments deposited by turbidity currents that formed at the base of the slope

A(n) ________ is a sequence of layers composing the oceanic crust that includes sheeted dikes, basalts, and gabbro. A) passive margin B) ophiolite complex C) lithospheric sheeting D) flood basalts

B) ophiolite complex

Why do mantle plumes tend to concentrate beneath supercontinents such as Pangaea? A) Concentrations of radioactive materials build up as continents assemble. B) Partial melting from subduction concentrates large volumes of magma. C) A large landmass acts as an insulating blanket to trap heat in the mantle. D) Friction from converging landmasses heats up the mantle.

C) A large landmass acts as an insulating blanket to trap heat in the mantle.

Which of the following materials or features are not found at mid-ocean ridges? A) Basaltic lava eruptions emitted along the ridge B) Stands higher above the surrounding seafloor C) Deep submarine trenches D) Thin layers of sediments

C) Deep submarine trenches

Volcanic island arcs are found near which features on the seafloor? A) Abyssal plains B) Coral atolls C) Deep-ocean trenches D) Rift zones

C) Deep-ocean trenches

How do researchers today routinely determine the depth of the water to the seafloor? A) Ship-mounted gravimeters B) A weighted roped or cable C) High-resolution multibeam instruments D) Laser reflector systems linked to satellites

C) High-resolution multibeam instruments

Which of the following is an example of a mid-ocean ridge? A) Puerto-Rico Trench B) Challenger Deep C) Mid-Indian Ridge D) East Pacific Rise

C) Mid-Indian Ridge

Match the seafloor terms to their correct definitions. *Submarine Trench*: A) Flat seafloor in the deep ocean B) Wedge of sediments deposited by turbidity currents that formed at the base of the slope C) Narrow creases in the seafloor that result from subduction D) Gently sloping, submerged extension of the continent E) A steep incline that marks the boundary between the continental crust and the oceanic crust

C) Narrow creases in the seafloor that result from subduction

Which region is surrounded by an active continental margin? A) Atlantic Ocean B) Arctic Ocean C) Pacific Ocean D) Indian Ocean

C) Pacific Ocean

Put the layers of an ophiolite complex in order from top to bottom: *Layer 3*: A) Gabbro B) Deep-sea sediments or sedimentary rocks C) Sheeted dike complex D) Pillow basalts

C) Sheeted dike complex

What kind of subduction occurs when old, dense lithosphere sinks into the mantle by its own weight rather than by compressional force? A) Forced subduction B) Mantle plume subduction C) Spontaneous subduction D) Overriding subduction

C) Spontaneous subduction

What is the definition of a turbidity current? A) Subsidence of the seafloor B) Fast-moving water created by density differences C) Submarine landslide D) Focused jet of water blasting across the shelf from a surface stream

C) Submarine landslide

Although many areas of the continental shelves are relatively featureless, there are some locations on the shelves that have received extensive glacial deposits and significant dissection by streams. How would this have been possible? A) Glaciers and streams were powerful enough to shape the shelf underwater. B) The shelves were once above sea level but have since dropped below sea level due to tectonic activity. C) The sea level dropped during the last glacial episode because water was stored in large ice sheets, exposing the shelves. D) As the continents collided in the past, the shelves were lifted above sea level and became exposed to the elements.

C) The sea level dropped during the last glacial episode because water was stored in large ice sheets, exposing the shelves.

________ accounts for greater ocean depths moving away from the oceanic ridge toward the deep ocean basin. A) Mantle plume B) Convection C) Thermal contraction D) Gravity

C) Thermal contraction

Thermal springs known as ________ are often associated with oceanic ridges, where hot water containing dissolved minerals gushes from the seafloor. A) hot springs B) geysers C) black smokers D) ophiolites

C) black smokers

Submarine canyons found cutting into the continental shelf and slope are believed to have been created ________. A) by subduction B) by icebergs C) by rivers during the ice age D) by tectonic rifting

C) by rivers during the ice age

Volcanic islands such as the Hawaiian islands form as a result of ________. A) subduction B) rifting C) mantle plumes D) batholith emplacement

C) mantle plumes

What happened when the spreading center that generated the Farallon plate collided with the North American plate? A) A mantle plume formed. B) A divergent plate boundary was created. C) The spreading center subducted, resulting in the Yellowstone Hot Spot. D) A transform boundary was created.

D) A transform boundary was created.

Match the seafloor terms to their correct definitions. *Continental slope*: A) Flat seafloor in the deep ocean B) Wedge of sediments deposited by turbidity currents that formed at the base of the slope C) Narrow creases in the seafloor that result from subduction D) Gently sloping, submerged extension of the continent E) A steep incline that marks the boundary between the continental crust and the oceanic crust

E) A steep incline that marks the boundary between the continental crust and the oceanic crust

True or False? Based on its slow rate of spreading, the East Pacific Rise has a gradual profile with gently sloping sides.

False

True or False? Because massive submarine structures such as seamounts and ridges exert stronger than average gravitational attraction, they produce depressed areas on the ocean's surface.

False

True or False? Black smokers can be found along subduction zones.

False

True or False? Continental shelves on active margins are larger and broader than those on passive margins.

False

True or False? Oceanic plateaus form from turbidity currents that flow down the continental slope.

False

True or False? Oceanic ridges form as a result of compressional stress.

False

True or False? A passive continental margin is located hundreds of miles away from the nearest tectonic boundary.

True

True or False? Abyssal plains are flat areas of the ocean floor that are likely the most level places on Earth.

True

True or False? Continental shelves are valuable locations that support important fishing grounds.

True

True or False? The Challenger Deep of the Mariana Trench is named for the H.M.S Challenger, the ship that first measured it.

True