Chapter 13
The oceans cover approximately __________ percent of Earth's surface
70
What is the difference between an active continental margin and a passive continental margin?
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
The ___________ Ocean has more extensive abyssal plains than the Pacific Ocean because it has fewer trenches to trap sediments moving down the continental slope.
Atlantic
The gently sloping submerged surface extending from the shoreline toward the deep ocean is termed the _____________.
Continental rise
Which of the following generally has the lowest surface slope?
Continental shelf
Which one of the following is NOT part of the continental margin?
Continental trench
_____________ develop where oceanic lithosphere bends downward and sinks into the mantle.
Deep ocean trenches
Why isn't manganese mined on the seabed when there are abundant manganese nodules in some parts of the ocean floor?
It is not economical to mine manganese nodules at present-day world prices for manganese and associated metals in nodules.
Multibeam provides the best modern technique for obtaining seafloor bathymetry, so why hasn't it been used for the entire ocean?
It is too expensive , requiring many ships many years to survey the entire ocean
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?
Most of the heat is carried away by convection as hydrothermal systems, so the average geothermal gradient away from the hydrothermal circulation is depressed.
The best modern technique for obtaining high resolution bathymetry of the seafloor is ____________.
Multibeam bathymetry
The seafloor spreading process at ridges produces what kind of faults?
Normal faults
"Black smokers" are associated with _____________.
Oceanic ridges mineral-rich waters hot water ALL OF THESE
The __________ Ocean is largest
Pacific
Which ocean has the greatest average depth?
Pacific
Which one of the following concerning mid-ocean ridges is false?
Sediments include thick siliceous ooze deposits and sandy turbidite beds.
Cape Horn the tip of South America called "rounding the horn" because of strong winds and heavy seas. These sailors considered this passage from Atlantic to Pacific oceans, what ocean is it called now?
The Southern Ocean
Why are there virtually no sediments at ocean ridges, even when they are close to land?
The crust is too young; there hasn't been enough time for sediments to accumulate
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?
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
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?
The organisms live in the photic zone, and so photosynthesis must be the main factor in the oxygen content.
How do calcareous oozes from?
The particles are precipitated in warm surface waters and sink to the bottom
Concerning the distribution of land and water, which of the following statements is true?
The southern Hemisphere has much more water surface than the Northern Hemisphere
Which of the following is NOT true of deep ocean trenches?
They are geologically very stable
It is thought that submarine canyons on the continental slope have been generated by ___________.
Turbidity currents
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?
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.
If Yellowstone were on the ocean floor, it would produce _____________.
a chain of line islands, like Hawaii
The best definition of the outer edge of the continental shelf is that point where __________.
a rapid steepening of the gradient occurs
Which of the following is associated with ocean ridges?
all of these
Subduction of oceanic lithosphere at the trenches can act like a bulldozer to produce ____________
an accretionary wedge
The crests of mid-oceanic ridges _____________.
are geologically old features
Seamounts ________________
are volcanoes that form on the ocean floor
The continental rise is located ______________.
between an abyssal plain and continental slope
Radiolarian chert is a common sedimentary rock accreted along active margins in the circum-pacifc. It is produced in the deep sea by the accumulation of siliceous radiolarian shells. What kind of sediment is radiolarian chert?
biogenous sediment
Which one of the following would most likely be covered with thick turbidite layers?
deep-sea fan at the base of continental slope
Manganese nodules are an example of __________
hydrogenous sediment
Minerals that crystallize directly from seawater are examples of ____________.
hydrogenous sediment
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 ________.
hydrogenous sediment
Where are the deepest parts of the ocean?
in the abyssal plain
Submarine canyons found on the continental slope are believed to have been created _______________.
none of these
In the present day, what is the most valuable non-living commodity obtained from sea floor?
oil and gas
An echo sounder operates by measuring the time required for a ___________.
radar beam to travel from a ship to the seafloor and back
Ocean floor topography can be estimated from space using ________.
radar pulses that bounce off the ocean surface
Most of our table salt comes from ________.
salt obtained by evaporating seawater in dry climates
Abyssal plains are very flat because __________.
sediments accumulate to cover the seafloor topography
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 ________.
terrigenous sediment
Sediments derived primarily from the products of weathering on the the continents are called ___________
terrigenous sediment
Deep sea trenches are important features for plate tectonic studies because ___________.
they are sites where oceanic lithosphere is formed
Ocean ridges form because __________.
they are submarine collisional mountain belts, forming submarine mountains that haven't yet risen form the sea floor
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 ________.
using the fossil's coiling direction is a proxy for climate because the surface water temperature would be highly correlated to surface temperature
