MULTIPLE CHOICE
1. An oceanic-continental plate boundary is:
a. the same type as oceanic-oceanic
b. a convergent type
c. a divergent type
d. a and b
e. a and c
feedback
The amount of crust on the surface of the earth
remains relatively constant. Therefore, when a divergence (or separation)
occurs in one area, a convergence (or collision) must occur in another
area. Three types of converging plate boundaries exist: oceanic-continental,
oceanic-oceanic and continental-continental.
2. A geoid is:
a. a sphere
b. an ellipsoid
c. the true shape of the earth
d. a mathematical approximation of the shape of the earth
e. an estimate of the shape of the earth used
for mapping
feedback
students will have to use additional sources
to answer this question
3. There are three layers of the earth that you want to map. They are 300, 550, and 1200 km deep. How would these layers be represented at a 1:1000 scale?
a. 0.03, 0.055, 0.12
b. 30, 55, 120
c. 3, 5.5, 12
d. 0.3, 0.55, 1.2
e. none of the above
feedback
students will have to use additional sources
to answer this question
4. Two of the most recent areas of sea-floor topography discovered are the:
a. Pacific-Antarctic rise
b. Juan de Fuca ridge
c. Louisville seamount
d. a and b
e. a and c
feedback
For example, the Pacific-Antarctic rise, which
has an area about equal to South America, is a broad rise of the ocean
floor caused by sea floor spreading between two major tectonic plates (see
Poster southeast of New Zealand). To the west of the ridge lies the Louisville
seamount chain which is a chain of large undersea volcanoes having a length
equal to the distance between New York and Los Angeles. These features
are unfamiliar because they were discovered less than 20 years ago.
5. Corrections to radar pulses from satellites are made for:
a. ionoshperic delays
b. atmospheric delays
c. tidal corrections
d. a and b
e. a, b, and c
feedback
Corrections to the travel time of the pulse are
made for ionospheric and atmospheric delays and known tidal corrections
are applied as well. The difference between the height above the ellipsoid
and the altitude above the ocean surface is approximately equal to the
geoid height N = h* - h.
6. A ____ repitition rate of ______ is used to improve the signal to noise ratio, especially when the ocean surface is _________.
a. high, 1000 pulses per minute, calm
b. low, 1000 pulses per minute, calm
c. high, 1000 pulses per second, rough
d. low, 1000 pulses per second, rough
e. high, 1000 pulses per minute, rough
feedback
A high repetition rate (1000 pulses per second)
is used to improve the signal to noise ratio, especially when the ocean
surface is rough.
7. What are the units of gravity?
a. 9.8 ft/sec
b. 8.9 m/sec2
c. 9.8 m/sec
d. 9.8 ft/sec2
e. 9.8 m/sec2
feedback
One mgal is about one millionth the normal pull
of gravity (9.8 m/s2 ). Typical variations in the pull
of gravity are 20 milligal although over the deep ocean trenches they exceed
300 mgal.
8. As the sea floor ages it:
a. reflects cooling of the plates
b. becomes covered in sediments
c. reflects subsidence of the plates
d. a and b
e. a, b, and c
feedback
On a broad scale the topography of the ocean
floor reflects the cooling and subsidence of the plates as they move away
from the spreading center. While this process is fairly well understood,
there are interruptions in this normal subsidence caused by mantle plumes
and other types of solid-state convection in the mantle of the Earth that
are current topics of research. As the seafloor ages it also becomes covered
by a slow rain of sediments. The analysis of the gravity data along with
measured can be used to map the thickness of the sedimentary layers.
9. Spreading ridges on the ocean floor are characterized by:
a. transform patterns of ridges
b. orthogonal patterns of ridges
c. transform faults
d. a and b
e. b and c
feedback
The Indian Ocean Triple junction (27 deg S latitude,
70 deg E longitude) is a textbook example of seafloor spreading. Spreading
ridges are characterized by an orthogonal pattern of ridges and transform
faults. The scar produced in the active transform valley is carried by
seafloor spreading out onto older seafloor leaving evidence of the past
plate motions. At this Indian Ocean site, three spreading ridges intersect
forming a triple junction as described by plate tectonic theory. The theory
predicts that the ridges would intersect at 120û angles if the three
ridges were spreading at exactly the same rate
10. Undersea volcanoes:
a. can be found by gravity maps if the volcanoes are 1000 m tall or
more
b. only about ½ have been charted previously
c. can occur alone
d. can occur in chains
e. all of the above
feedback
The global gravity grids reveal all volcanoes
on the seafloor greater than about 1000 m tall. Approximately 1/2 of these
volcanoes were not charted previously. One of the more important
aspects of these new data will be to locate all of these volcanoes and
identify spatial patterns that may help determine how they formed.
Many volcanoes appear in chains, perhaps associated with mantle plumes,
there are many more that do not fit this simple model.
11. Measuring the amplitude and width of a gravity low and relating this to echo soundings can be used to measure:
a. petroleum deposits
b. length of volcanic chains
c. height of volcanoes
d. thickness and elasticity of tectonic plates
e. all of the above
feedback
By measuring the amplitude and width of the gravity
low and relating this to the size of the volcano as measured my a ship
with an echo sounder, one can establish the thickness and strength of the
elastic plate.
TRUE/FALSE
1. Oceanic-continental collisions can result in volcanic activity.
True. When an oceanic-continental convergence
occurs the denser oceanic plate will most commonly subduct beneath the
less dense continental plate creating a trench. Once the subducting
oceanic crust reaches a depth of about 100 kilometers, melting of the crust
and the overlying mantle begins forming a magma.
2. Hawaii is an example of an oceanic-continental convergence.
False. An oceanic-oceanic convergence often
results in the formation of an island arc system. As the subducting oceanic
crust melts within the asthenosphere the newly-created magma rises to the
surface and forms volcanoes. If the activity continues, the volcano may
grow tall enough to breech the surface of the ocean creating an island.
3. The ocean surface has broad bumps and dips that mimic the topography of the ocean floor.
True. Fortunately, such a major mapping
program is largely unnecessary because the ocean surface has broad bumps
and dips which mimic the topography of the ocean floor. These bumps and
dips can be mapped using a very accurate radar altimeter mounted on a satellite.
In this brief report we attempt to answer some basic questions related
to satellite measurements of the ocean basins.
4. Oceanic flow and sea floor depth are proportional to s1/ square root of age, but inversely proportional to square root of age according to cooling halfspace models.
False. The thermal state of the oceanic lithosphere
is largely controlled by its age. Physical models invoking a cooling halfspace
predict that oceanic heat flow and sea floor depth are proportional to
s1/(sqrt(age) and sqrt(age), respectively (Davis and Lister, 1974); whereas
plate cooling models assume a constant thickness after a certain age
5. The actual surface of the ocean deviates from the shape of an ideal ellipsoid by 100 miles.
False. However because the earth is rotating, the equipotential ocean surface is more nearly matched by an ellipsoid of revolution where the polar diameter is 43 km less than the equatorial diameter. While this ellipsoidal shape fits the earth remarkably well, the actual ocean surface deviates by up to 100 meters from this ideal ellipsoid.