Chapter 3 Review Questions
Describe the origin of the three basic types of oceanic islands.
(1) Islands associated with volcanic activity along the mid-ocean ridge (such as Ascension Island along the Mid-Atlantic Ridge) (2) Islands associated with hotspots (such as the Hawaiian Islands in the Pacific Ocean) (3) Islands that are island arcs and associated with convergent plate boundaries (such as the Aleutian Islands in the Pacific Ocean). Note that all three types are volcanic in origin.
What are some differences between a submarine canyon and an ocean trench?
A submarine canyon is a deep gouge that exists on continental slopes/continental shelves, and is thought to have been created by turbidity currents. An ocean trench is a much deeper and larger feature that is the physiographic representation of a subduction zone where one tectonic plate plunges beneath another.
Describe the process by which abyssal plains are 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.
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.
Describe the major feature of a passive continental margin: deep-sea fans.
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.
Explain what graded bedding is and how it forms.
Graded bedding is a type of sediment layering that becomes progressively finer within a sequence, and it 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. The settling process may take weeks or months. This creates a graded bedding sequence of coarser material at the bottom becoming progressively finer towards the top.
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.
Describe the process by which hydrothermal vents are created.
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.
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.
Describe differences between passive and active continental margins. Include a real-world example of each type, associated features, and how these features relate to plate tectonics.
Passive margins are found on the East Coast of the Americas and in the Indian Ocean and turbidity currents travel directly down the continental margin and deposit sediment on the abyssal plains. Active margins are characterized by a deep ocean trench, earthquakes, and volcanoes. Examples of active margins are found along the Pacific Ring of Fire.
Discuss the origin of the various volcanic peaks of the abyssal plains: seamounts, tablemounts, and abyssal hills.
Seamounts are cone-shaped volcanic peaks that stand more than one kilometer above the sea floor. Tablemounts 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.
Describe the major feature of a passive continental margin: submarine canyons.
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).
Describe the major feature of a passive continental margin: continental rise.
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.
Describe the major features of a passive continental margin: continental shelf.
The continental shelf extends seaward from the shoreline as a gently sloping surface with an average slope of about one-tenth of a degree. Its topography is usually similar to that of the coastal region of the continent. It is very narrow in some areas, such as off California, and reaches widths as great as 1300 kilometers (808 miles) off the Siberian coast; its average width is about 70 kilometers (43 miles). The average depth at which the shelf break that marks its seaward limit occurs at about 135 meters (443 feet), but the break is as deep as 350 meters (1148 feet) off Antarctica.
Describe the major feature of a passive continental margin: continental slope.
The continental slope, which extends from the continental shelf to the deep-ocean basins, has an average slope of four degrees. In the Pacific Oceans where mountainous coasts are common, the continental slope averages five degrees in contrast to the Atlantic and Indian Oceans where it averages three degrees. Submarine canyons resulting from turbidity currents cut the slopes and may even extend across the continental shelf in some cases.
Describe characteristics and features of the mid-ocean ridge.
The global mid-ocean ridge is a continuous, fractured-looking submarine mountain range that extends through the middle of all the ocean basins 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: A. is entirely volcanic in origin B. is composed of lavas with a basaltic composition C. has along its crest a central down-dropped rift valley created by sea floor spreading processes where two plates are diverging D. has an abundance of earthquake activity in the form of small, shallow quakes E. has a variety of volcanic features, including seamounts and recent underwater lava flows (pillow lava) F. is associated with hydrothermal vents where hot water seeps out of the sea floor and unusual life exists G. has deposits of metal sulfides that contain economic mineral deposits
Explain how submarine canyons are created.
The mostly widely accepted phenomenon that has been used to explain the existence of submarine canyons is turbidity currents. 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.
Discuss the use of satellites to map features of the ocean floor.
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. Various satellite missions and their objectives are listed in Web Table 3.1.
List and describe the different types of hydrothermal vents.
There are three types of hydrothermal vents: a. Warm-water vents have water temperatures below 30°C (86°F). b. White smokers have water that is white in color as a result of dissolved barium sulfate particles and a water temperature between 30°C and 350°C (662°F). c. 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.