Earth: An Introduction to Physical Geology, 11e (Tarbuck)
Chapter 13 Divergent Boundaries
13.1 Multiple-Choice Questions
1) 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
2) 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
3) Which of the following is not one of the topographical provinces oceanographers use to
define the seafloor?
A) Continental margins
B) Mid-ocean ridges
C) Submarine trenches
D) Deep-ocean basins
4) How do researchers today routinely determine water depths and seafloor topography?
A) Ship-mounted gravimeters
B) Computerized parallel transponder systems mounted to satellites
C) Ship-mounted echo sounders
D) Laser reflector systems linked to satellites
5) Which of the following tectonic boundaries is associated with an active continental margin?
A) Divergent
B) Convergent
C) Transform
6) 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
7) Which of the following scenarios cannot trigger a turbidity current?
A) Earthquakes
B) Large surface storms
C) Overload of sediment on slope
D) Salinity differences
8) Submarine canyons found cutting into the continental shelf and slope are believed to have
been created ________.
A) By subduction
B) By scouring by coastal glaciers
C) By rivers during the ice age
D) By tectonic rifting
9) What percentage of the Earth’s surface consists of deep-ocean basins?
A) 10%
B) 30%
C) 37%
D) 49%
10) Volcanic islands such as the Hawaiian islands form as a result of ________.
A) Subduction
B) Rifting
C) Mantle plumes
D) Batholith emplacement
11) Which of the following is not a method of obtaining evidence of the oceanic ridge system?
A) Finding segments of ocean floor that have been thrust up onto the continent
B) Visual inspects using submersible equipment
C) Deep sea core samples
D) Satellite imagery
12) Where do rift valleys form along the oceanic ridge system?
A) Along the flanks of the ridge 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) Acting as a valley trending from the ridge peak to the seafloor
13) ________ 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) Thermal expansion
14) What is the source of magma necessary for seafloor spreading?
A) Decompression melting
B) Partial melting
C) Mantle plumes
D) Subduction
15) 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
16) Which of the following features are not found at mid-ocean ridges?
A) Basaltic lava eruptions emitted along the ridge
B) Shallow depths than deep ocean plains
C) Deep submarine trenches
D) Thin layers of sediments
17) 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
18) 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
19) 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
20) 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
21) 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
22) What kind of subduction occurs when old, dense lithosphere sinks into the mantle by its own
weight?
A) Forced subduction
B) Mantle plume subduction
C) Spontaneous subduction
D) Overriding subduction
23) 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
13.2 True/False Questions
1) The Challenger Deep of the Mariana Trench is named for the H.M.S Challenger, the ship that
first measured it.
2) Because massive submarine structures such as seamounts and ridges exert stronger than
average gravitational attraction, they produce depressed areas on the ocean’s surface.
3) A passive continental boundary is located hundreds of miles away from the nearest tectonic
boundary.
4) Continental shelves on active margins are larger and broader than those on passive margins.
5) Continental shelves are valuable locations that support important fishing grounds.
6) Continental shelves can be exposed during glaciations because of the large volumes of water
taken from the ocean to precipitate as ice and snow.
7) Abyssal plains are flat areas of the ocean floor that are likely the most level places on Earth.
8) Oceanic plateaus form from turbidity currents that flow down the continental slope.
9) The East Pacific Rise is a mid-ocean ridge.
10) Oceanic ridges form as a result of compressional stress.
11) Oceanic ridge are the longest topographic feature on Earth.
12) The greatest volume of magma produced annually on Earth is associated with seafloor
spreading and oceanic ridges.
13) Based on its slow rate of spreading, the East Pacific Rise has a gradual profile with gently
sloping sides.
14) Black smokers can be found along continental rift valleys.
15) The Mid-Continent Rift in North American is a failed rift that formed over one billion years
ago.
13.3 Short Answer Questions
1) What tool is often used to measure water depths?
2) What modern methods using satellites are used to determine the topography of the seafloor
today?
3) What are the two types of continental margins?
4) What three major features comprise a passive continental margin?
5) Where is the deepest place on Earth?
6) Along which tectonic boundary are deep-ocean trenches found?
7) Why does the Atlantic Ocean have the most extensive abyssal plains?
8) What rock would you expect to find associated with a mid-ocean ridge?
9) How do rift valleys form?
10) Why are oceanic ridges elevated above the rest of the seafloor?
11) Why does the lithosphere thicken as it moves away from a mid-ocean ridge?
12) What is the average thickness of the oceanic crust?
13) What is the name of the supercontinent that preceded Pangaea?