# Earth Science Chapter 13

## 20 terms

### How does the area of Earth's surface coveres by the oceans compare with that of continents? Describe the distrution of land and water on Earth.

The oceans cover almost 71 percent of the globe, and the continents account for the remaining 29 percent of Earth's surface. Although both the Northern and Southern Hemispheres have more water than land, the oceans dominate the Southern Hemisphere. A much greater percentage of Earth's land area is in the Northern Hemisphere.

### Excluding of Southern Ocean, name the four main oceans basin. Of the four: A) Which one is the largest in area? Which one is smallest? B) WQhich one is the deepest? Which one is the shallowest? C) Whihc one is almost entirely within the Southern Hemisphere? D) Which one is exclusively within the Northern Hemisphere?

The four main ocean basins are the Pacific Ocean, Atlantic Ocean, Indian Ocean, and Arctic Ocean.
(a) The Pacific Ocean is largest in area; the Indian Ocean is smallest
(b) The Pacific Ocean is the deepest; the Arctic Ocean is the shallowest.
(c) The Indian Ocean is almost entirely in the Southern Hemisphere.
(d) The Arctic Ocean is exclusively in the Northern Hemisphere.

### How doew the average depth of the ocen compare to the average elevation of continents

The average depth of the ocean basins is 3800 meters below sea level, and the average elevation of the continents is a mere 840 meters.

### Describe differences between sidescan sonar and multibeam sonar.

The difference between sidescan sonar and multibeam sonar is that multibeam produces water depth data.

### Assuming that the average speed of sound waves in water is 1500 meters per second, determine the water depth in meters if a signal sent out by an echo sounder requires 6 seconds to strike bottom and return to the recorder.

The echo sounder pulse travels a distance equal to twice the water depth. In 6 seconds the pulse travels 9000 meters (1500 m/sec × 6 sec = 9000 m); thus the depth (1/2 of 9000 meters) is 4500 meters.

### Describe how satellites orbiting Earth candetermine features on the seafloor without being able to directly observe them beneath kilometers of seawater.

Satellites equipped with radar altimeters are able to determine features on the seafloor by measuring subtle differences in elevation on the ocean surface. The satellites bounce microwaves off the ocean surface, which is affected by the gravitational pull of seafloor features. Therefore, mountains and ridges produce elevated areas on the ocean surface, whereas canyons and trenches cause slight depressions.

### List the three major subdivision of a passive continental margin. Which subdivision is considered a flooded extension of the continent? Which has the steepest slope?

The three major features listed in their order from the coastline seaward are the continental shelf, continental slope, and continental rise. The shelf is a relatively flat, flooded extension of the continent. Water depths increase very gradually seaward to the edge of the shelf. The continental slope extends seaward and downward from the shelf edge and merges downward into the continental rise. It has the steepest slopes of the three listed features. On trailing passive continental margins, the shelf is part of the continent. The slope marks the eroded scarp left from an original continental rift zone, and the rise marks a fan-shaped accumulation of clastic sediments carried down from the shelf and slope by turbidity currents.

### Describe the difference between active and passive continental margins. Be sure to include how various features relate to plate teconics and give a geographic example of each type of margin.

An active continental margin exhibits a very narrow continental shelf and a relatively steep, narrow, continental slope that merges into a deep-ocean trench along most of the coastline. Along a passive continental margin, the continental shelf is wide, and the shelf is terminated by a well-defined continental slope with a wide, gently sloping continental rise at its base. There are no deep-ocean trenches, and the rise merges oceanward into an abyssal plain. In terms of plate tectonics, the east coast of North America is a good example of a passive continental margin and therefore is not very active tectonically. In contrast, the west coast of South America represents an active continental margin that is characterized by subduction, volcanism, and earthquakes—all evidence of a tectonically active region.

### Defend or rebut the statement " most submarine canyons found on the continental slope and rise extened theirvalleys seaward."

Submarine canyons do, in general, lie on the projected courses of major rivers across the continental shelves and slopes. It is also true that sea level was much lower during the maximum stages of the
Pleistocene glaciations. However, sea level was not lowered enough to expose the continental slopes to
subaerial erosion. It is known that turbidity currents scour and erode submarine canyons, and once
established, the canyons can be maintained and deepened by underwater erosion. Thus these channels are
most likely very old features formed long before the Pleistocene glaciations at a time when the continental
shelf areas were higher than they are today. Perhaps those on the eastern North American continental
margin date from as far back as the time of the breakup of Pangaea, when streams incised short, steep
canyons into the scarp bordering the newly opened rift.

### What are turbidity currents? How do they differ from turbidites? What is meant by the term graded bedding?

Turbidity currents are episodic downslope movements of dense, sediment-laden water that in part are
responsible for the formation of submarine canyons. Turbidites are the sediments that are deposited by
turbidity currents and lithified into sedimentary rocks. They are typically deposited in deep water below
the influence of surface waves and water currents. Consequently, turbidites are not reworked and retain
most of their primary depositional characteristics, one of which is graded bedding. This term refers to an
upward decrease in grain size (from small pebbles, granules, or sand at the base to mud at the top) within
a single depositional unit. As the sediment grains sink in still water the larger, heavier grains are
deposited first and the finer-sized silt and clay settle out last, forming the top of the deposit. The
depositional unit (bed) rarely exhibits much internal stratification, but the upper and lower boundary
contacts are easy to recognize. The vertical trend to finer-grain sizes always points toward the original top
of the deposit; thus graded bedding can be used to establish the original stratigraphic-top direction in
deformed turbidite strata.

### What are differences between a submarine canyon and a deep-ocean tranch?

Submarine canyons are deep, steep-sided valleys that are cut into the continental slope and may extend
across the entire continental rise to the deep-ocean basin. Deep-ocean trenches are long, relatively narrow
canyons in the seafloor that form the deepest parts of the ocean.

### Decribe the process by which abyssal plains are created. Why are ayssal plain more extensive on the floor of the Atlantic than on the floor of the Pacific?

Abyssal plains represent thick accumulations of sediment that bury and mask an otherwise rugged ocean
floor. The fine sediments found on the abyssal plains are carried out to sea by turbidity currents and also
result from suspension settling. To a great extent, the Pacific Ocean is rimmed by deep-ocean trenches or
deep nearshore basins that trap sediment and prevent turbidity currents from moving farther offshore. The
Atlantic Ocean is rimmed mainly by passive continental margins, and turbidity currents with their
sediment load can move far out into the deeper parts of the ocean basin. The abyssal plain marks the
smooth upper surface of turbidity current sediments deposited on the original basaltic bedrock of the
ocean floor. For the most part, the Atlantic Ocean floor is not being subducted, so there has been plenty of
geologic time to allow turbidite sediments to accumulate.

### Discuss how seamonts and gutots are created and why they have different shape.

Seamounts are isolated volcanic peaks that rise hundreds of meters above the ocean floor. Over time, the
volcano may grow large enough to emerge as an island. During the time they exist as islands, some of the
volcanoes are eroded to near sea level by running water and wave action. Over millions of years, the
islands gradually sink below the surface of the water as a moving plate carries them away from their
place of origin. The submerged flat-topped seamounts are called guyots.

### Describe the environmental conditions required for the development of coral reefs.

The environmental conditions necessary for coral reef development are warm ocean temperatures with an
average annual temperature of 24˚C, a suitable attachment site for the reef-building organisms, and clear
sunlit water (shallow water depths of 45 meters or less). Coral reefs begin as fringe reefs along the
margins of volcanoes. Later, they develop into a barrier reef with a volcano in the middle and finally
evolve into a continuous or broken ring of coral reef surrounding a central lagoon called an atoll.

### What paradox was there concerning atolls? Describe Darwin's porposal on the orgin of atolls. How was his proposal confirmed or disproved.

The paradox of atolls was, How do corals—which require warm, shallow, sunlit water to live—create such thick structures that extend into deep water? Darwin proposed an origin for the development of atolls that is explained in question 13. His proposal has been confirmed by drilling through atolls, which has revealed that volcanic rock underlies the oldest (and deepest) coral reef structures.

### Describe features associated with the oceanic ridge.

Mid-ocean ridges are topographically elevated features located near the center of most ocean basins. The ridges are broken into segments that are offset by large transform faults. Also, along the axis of some of the segments are deep down-faulted structures called rift valleys. The rift valleys are characterized by active volcanoes, recent underwater lava flows, and black smokers, which are a type of hydrothermal vent that spews dark, mineral-rich water.

### How are oceanic ridges and deep-ocean trenches related to late tectonics?

Mid-ocean ridges are spreading centers. That is, they are divergent plate boundaries where magma wells up from below, creating new oceanic crust. Ocean trenches form where crustal plates plunge back into the mantle. Thus, these long, narrow depressions are associated with convergent plate boundaries.

### Distinguish among the three basic types of seafloor sediment, and give an example of each.

Terrigenous—These sediments are composed of rock and mineral grains derived from subaerial and submarine weathering, erosion, and mass wasting of preexisting rocks and minerals. They are detrital sediments with clastic textures. Lithogenous, a term describing the same sediments, is also used in introductory oceanography and earth science texts.
Biogenous—The sediment particles were precipitated by once-living organisms. Most biogenous or biogenic sediments are oozes, consisting mainly of minute coccolithophores and foraminifera (calcareous; calcium carbonate) or diatoms and radiolarians (siliceous; silica).
Hydrogenous—These are sediments chemically precipitated from seawater without biological intervention. Examples are the manganese-rich nodules on parts of the ocean bottom and metal-rich sediments around seafloor hot-spring vents.

### Describe how seafloor sediments are useful in studying past climates. What kind of information can sediments reveal?s

Seafloor sediments are useful in studying past climates because the numbers and types of the organisms that once lived at the atmosphere-ocean interface reflect the temperatures of that time. Therefore, by studying the types and numbers of organisms incorporated within the sediment, scientists can construct a record of past climatic changes. For example, seafloor sediments have been useful in unraveling the fluctuating atmospheric conditions of the Ice Age.

### Discuss the present impotance and future propects for the production of offshore petroleum, gas hydrates, sand and gravel, evaporation salts, and mananese nodules.

Currently, offshore petroleum accounts for about 30 percent of world oil production. Because the likelihood of finding new oil reserves on land is slim, offshore exploration and production will be very important in the future.
Gas hydrates represent a huge potential source of energy in the form of methane trapped in sediments. Although gas hydrates rapidly decompose at surface temperatures and pressures, they will probably help power our society in the future.
Sand and gravel deposits are second in economic value only to the petroleum industry. Sand and gravel is currently mined using barges. These valuable offshore resources will undoubtedly continue to be very important in the future.
Evaporative salts, most notably the mineral halite, have been an important source for human consumption since ancient times. Currently, about 30 percent of the world's salt is produced by evaporating seawater.
Manganese nodules contain significant concentrations of manganese and iron along with smaller concentrations of copper, nickel, and cobalt. From a technical standpoint, mining nodules from the deep-ocean floor is possible but not economically profitable in today's society. In the future, such resources may become an important source of cobalt and other strategic elements. However, in addition to economic issues, there are also political problems (establishing mining rights) and environmental concerns associated with mining on the deep-ocean floor.