P133
1. What is bathymetry?
Bathymetry is the measurement of ocean depths and the charting of ocean floor topography (shape and relief).
Determining bathymetry involves measuring the vertical distance from the ocean surface down to the mountains,
valleys, and plains on the sea floor.
2. Discuss the development of bathymetric techniques, including significant advances in technology.
Since early history, humans have determined water depth using depth soundings made by attaching a heavy weight
to a long line and lowering it over the side of a vessel to determine water depth. The first systematic bathymetry of
the oceans was made in 1872, when the HMS Challenger began its historic three-and-one-half-year voyage.
Throughout the voyage, the Challenger's crew stopped to make depth soundings every few thousand kilometers.
They also recorded various seawater properties at these locations.
In the 1920s, a new technology advanced ocean floor mapping: the invention of the echo sounder.
An echosounder sends a sound signal (a ping) into the ocean. The sound signal bounces off any object that is a
different density than seawater (such as live organisms or the ocean floor). Analogous to the use of radar on land,
the echosounder records echoes that have bounced off objects. The time that it takes for the echoes to return is used
to determine ocean depth and the shape of the ocean floor. In 1924, the German vessel Meteor was the first vessel
to use an echosounder to map the sea floor. One problem with this technique is that it lacked the resolution to show
detailed sea floor features.
The precision depth recorder (PDR) was developed in the 1950s. With a more focused high frequency sound
beam (in comparison to the echosounder), it could provide depths to a resolution of about 1 meter (3.3 feet).
Throughout the 1960s, PDRs were used extensively to provide a reasonably good representation of the ocean floor.
Data collected by thousands of research vessel tracks produced the first reliable global sea floor bathymetric maps.
These maps provided important evidence that supported plate tectonic theory and the idea of sea floor spreading.
Today, multibeam echosounders and side-scan sonar give oceanographers a far more precise picture of the
ocean floor. The first multibeam echosounder, Seabeam, made it possible for a survey ship to map the features of
the ocean floor along a strip up to 60 kilometers (37 miles) wide. The system uses sound emitters directed away
from both sides of the ship, with receivers permanently mounted on the ship's hull. With these data, a map of the
sea floor can be computer-generated. Sea MARC and GLORIA are types of side-scan sonar devices that are towed
behind a survey ship. To obtain a more detailed picture of the ocean floor, a side-scan sonar instrument must be
towed behind a ship on a cable so that it "flies" just above the ocean floor. The most recent technology in these two
deep-sea systems combines a side-scan sonar instrument with a sub-bottom imaging package. This allows a
simultaneous view of the ocean floor surface and a cross-section of the sediment below water depths of 6500 meters
(21,325 feet). The sea floor can also be mapped by using satellites that can detect slight irregularities in the height
of the ocean surface caused by gravitational attraction of different sea floor features. (see Table 3-1.)
3. Describe what is shown by a hypsographic curve and explain why its shape reflects the presence of active
tectonic processes on Earth.
A hypsographic curve is used by oceanographers to graphically represent the relationship between land elevation
and water depth (see Figure 3-4). The curve is constructed as a plot of the percentage of the Earth's surface (area of
the Earth's surface) on the horizontal axis and elevation/ocean depth on the vertical axis. The two flat areas and
three sloped areas on the Earth's hypsographic curve illustrates that there is a very uneven distribution of area at
different land elevations and ocean depths. The flat portions of the curve represent features within a plate. The
sloped portions of the curve represent the continental slopes, the deep-ocean trenches, mid-ocean ridges, and
mountains. The existence of these features suggests that some physical mechanism produces the uneven folding of
the Earth's surface such as plate tectonics. If there were no tectonic activity, the area of the Earth's surface would
be constant at every elevation/depth, producing a straight line instead of a curve.
4. Describe the differences between active and passive continental margins. Be sure to include how these
features relate to plate tectonics, and include an example of each type of margin.
A MARGIN is simply an edge of a continent: ie a coastline.
ACTIVE MARGINS are continental margins that are associated with plate boundaries, and they are marked by a
high degree of tectonic activity.
Active margins are further divided into two types, transform active margins and convergent active margins.
Transform active margins - side-to-side fault. San Andreas Fault.
Convergent active margins - ocean-continent convergent plate boundaries. Island arcs and offshore
trenches delineate the plate boundary. Aleutian Island chain.
PASSIVE MARGINS are continental margins that are embedded within the interior of plates and are not near
any plate boundary. Rifting of continental landmasses produces passive margins. An example of a passive margin
exists along the east coast of the United States where there is no plate boundary. Passive margins lack tectonic
activity (earthquakes, volcanoes, mountain building). A passive continental margin includes the continental shelf
and shelf break, the continental slope and the continental rise that extends toward the deep-ocean basins.
Unlike the Pacific Ocean, the Atlantic Ocean has an abundance of passive plate margins.
5. Describe the major features of a passive continental margin: continental shelf, continental slope,
continental rise, submarine canyon, and deep-sea fans.
The continental shelf extends seaward from the shoreline as a gently sloping surface with an average slope of about
one-tenth of a degree.
- topography is usually similar to that of the coastal region of the continent.
- very narrow in some areas, such as off California, or as wide as 1300 kilometers off the Siberian coast.
The average depth at which the shelf break that marks its seaward limit occurs is about 135 meters (the break is as
deep as 350 meters off Antarctica.)
-The continental slope, which extends from the continental shelf to the deep-ocean basins, has an average slope
of four degrees. In the Pacific Ocean where mountainous coasts are common, the continental slope averages five
degrees in contrast to the Atlantic and Indian Oceans where it averages three degrees. [NOTE: When airliners
land, their final approach path is a glide slope of about 3.5˚]
- Submarine canyons resulting from turbidity currents cut into the slopes and may even extend across the
continental shelf in some cases.
-The continental rise is a gently sloping surface that has about the same slope as the continental shelf. It
extends from the base of the continental slope into the deep ocean basins as a thinning wedge of sediment, most
of which is probably deposited by turbidity currents when they reach the base of the continental slope and lose their
energy. This sediment is greatly modified by deep boundary currents that erode it from some locations and deposit
it as drifts as the currents slow while turning at bends in the continental slope.
-Submarine canyons are similar in size and design to river-cut canyons on land. They are found mostly on
the continental slope, but some canyons extend landward across the continental shelf (such as the Hudson
Canyon that extends toward the mouth of the Hudson River in New York).
-Deep-sea fans are formed when the turbidity currents reach the base of the continental slope and lose speed
moving across the flatter deep ocean floor. The sediment settles out of the slow moving water to create deep-sea
fans at the base of the slope. As the fans grow from deposits of distributary channels flowing across their surface,
they merge to produce the gently sloping continental rise at the base of the continental slope. They are basically
underwater river deltas
(left: Nile; right: Mississippi)
6. Explain how submarine canyons are created.
The most widely accepted phenomenon that has been used to explain the existence of submarine canyons is
turbidity currents [aka seafloor avalanche]. Although no one has directly witnessed turbidity currents in action,
there is good evidence to support their existence and erosive power. It is believed that the sediment on the
continental slope may be dislodged by an event such as oversteepening of sediment, an earthquake, a hurricane, or
another disturbance. The dislodged sediment mixes with water as it moves down the slope under the influence of
gravity to produce a turbidity current. This flow of dense fluid is erosive and cuts a gully in the slope. Subsequent
turbidity currents deepen it to canyon proportions.
7. What are the differences between a submarine canyon and an ocean trench?
A submarine canyon is a deep , eroded gouge into a continental slope, created by turbidity currents. An ocean
trench is much deeper and larger and is “pulled down” by as a subduction zone, where one plate slips beneath
another.
8. Explain what graded bedding is and how it forms.
Graded bedding is a type of sediment layering that is the result of deposition by turbidity currents. Material that was
suspended in a turbidity current settles out in a distinct pattern: the larger pieces settle first, then progressively
smaller pieces settle, and eventually even very fine pieces settle out. This creates a graded bedding sequence of
coarser material at the bottom becoming progressively finer towards the top. The settling process may take weeks or
months.
9. Describe the process by which the abyssal plain was created.
Abyssal plains are formed by fine particles of sediment slowly drifting down onto the crust of the deep ocean floor
and, over millions of years, producing thick sedimentary deposits analogous to marine dust. This process is called
suspension settling because fine sediment is constantly settling out of suspension in the water column. With enough
time, most irregularities of the deep ocean are covered by these deposits.
10. Discuss the origin of the various volcanic peaks of the abyssal plains: seamounts, tablemounts (guyots),
and abyssal hills.
-Seamounts are cone-shaped volcanic peaks that stand more than one kilometer above the sea floor.
-Tablemounts/Guyots are seamounts that have had their tops flattened by wave energy. Most are now submerged
beneath 1800 to 3000 meters (5900 to 9800 feet) of ocean water.
-Abyssal hills (seaknolls) are small gently rounded volcanic hills that stand less than one kilometer above the ocean
floor.
11. In which ocean basin are most ocean trenches found? Use plate tectonic processes to help explain why.
Most ocean trenches are located in the Pacific Ocean because of the prevalence of convergent plate boundaries
(with subduction zones) that ring the Pacific Ocean basin.
12. Describe characteristics and features of the mid-ocean ridge, including the difference between oceanic
ridges and oceanic rises.
The global mid-ocean ridge is a continuous, fractured-looking submarine mountain range that extends through the
middle of all the ocean basins and extends for some 65,000 kilometers (40,400 miles). The width of the mid-ocean
ridge varies along its length, but averages about 1000 kilometers (620 miles). The mid-ocean ridge is a
topographically high feature, extending an average of 2.5 kilometers (1.5 miles) above the surrounding sea floor.
The mid-ocean ridge:
• Is entirely volcanic in origin.
• Is composed of lavas with a basaltic composition.
• Has along its crest a central down-dropped rift valley created by sea floor spreading processes where two plates
are diverging.
• Has an abundance of earthquake activity in the form of small, shallow quakes.
• Has a variety of volcanic features, including seamounts and recent underwater lava flows (pillow lava).
• Is associated with hydrothermal vents where hot water seeps out of the sea floor and unusual life exists.
• Has deposits of metal sulfides that contain economic mineral deposits.
Segments of the mid-ocean ridge are called oceanic ridges where they are mountainous with steep irregular slopes
and oceanic rises where the slopes are gentler. As explained in Chapter 2, the differences in overall shape between
ridges and rises are due to spreading rate differences: oceanic ridges have slower spreading rates than oceanic rises.
13. List and describe the different types of hydrothermal vents
Hydrothermal vents are hot springs associated with mid-ocean ridge rift valleys that are caused by seawater
seeping along fractures in the ocean crust. The water is heated when it comes in contact with underlying magma
and then rises upward and spews out of the sea floor. There are three types of hydrothermal vents:
• Warm-water vents have water temperatures below 30°C (86°F).
• White smokers have water that is white in color as a result of dissolved barium sulfate particles and a water
temperature between 30 and 350°C (662°F).
1.
• Black smokers have water that is black in color as a result of dissolved dark-colored metal sulfides and a water
temperature above 350°C (662°F). Many of these black smokers spew water out of chimney-like structures
composed of metal sulfide deposits up to 20 meters (66 feet) high.
14. What kinds of unusual life can be found associated with hydrothermal vents? How do these organisms
survive?
Hydrothermal vent communities include large clams, mussels, vestiminiferan tubeworms, and other animals. The
vents discharge hydrogen sulfide gas that is oxidized by bacteria and archaeons to provide the starting products to
fix organic carbon in the absence of sunlight.
15. Use pictures and words to describe the differences between a fracture zone and a transform fault.
- fracture zone – fault line perpendicular to a diverging plate boundaries where the area on either side of the fault moves
in the same direction
- transform fault - fault line perpendicular to a diverging plate boundary between laterally-offset regions of the boundary.
The area on either side of the fault moves in the opposite directions.
ADDITIONALLY
1.
back arc spreading center – area of seafloor spreading that parallels a subduction zone. (The surface of upper
plate is stretched as the subducting plate drags down the front edge of the upper plate)