1. Assuming that the average speed of sound waves in water is 1500 meters per second, determine the water depth if the signal sent out by an echo sounder requires 6 seconds to strike bottom and return to the recorder (Fig. 18.2).
The echo sounder pulse travels a distance equal to twice the water depth. In 6 seconds the pulse travels 9000 meters (1500 m/sec X 6 sec = 9000 m); thus the depth (1/2 of 9000 meters) is 4500 meters.
2. List the three major features that comprise the continental margin. Which of these features 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 (Figs. 18.3 & 18.4). 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.
3. How does the continental margin along the west coast of South America differ from the continental margin along the east coast of North America?
The west coast of South America has 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 the east coast of North America, the continental shelf is wide; 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 trailing continental margin that is moving away from the Mid-Atlantic Ridge spreading center; such margins are not very active tectonically. The west coast of South America is above a subduction zone; such margins are very active tectonically.
4. Defend or rebut the following statement: "Most of the submarine canyons found on the continental slope and rise were formed during the Ice Age when rivers extended their valleys seaward."
Submarine canyons do, in general, lie on the projected courses of major rivers across the continental shelves and slopes, and it is 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; 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 today. Perhaps those on the eastern North American continental margin date from as far back as the time of the Pangaean breakup, when streams incised short, steep canyons into the scarp bordering the newly opened rift.
5. What are turbidites? What is meant by the term graded bedding?
Turbidites are sediments and lithified, sedimentary rocks deposited from turbidity currents (Fig. 18.6). 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; 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.
6. Discuss the evidence that helped confirm the existence of turbidity currents in the ocean. (Box 18.1)
Following the 1929 Newfoundland earthquake, the North Atlantic telegraph cables linking North America and Europe were broken (Box 18.1). Subsequent studies demonstrated that a turbidity current, triggered by the earthquake, was responsible; these studies confirmed the existence of turbidity currents in the deeper parts of the oceans. The current began as a submarine sediment slide, shaken from the continental shelf and slope by the earthquake. As the slide moved downslope, it entrained water and evolved into a turbidity current with enough speed to break the seafloor cables. From the known times and positions of the cable breaks, the track and speed of the current could be reconstructed. Subsequently, coring and other studies related to laying new cables showed that sediments around the broken cables had graded bedding and contained remains of land plants and shallow water organisms that had been transported from the continental shelf into much deeper waters. In recent years, turbidity currents have been observed and filmed in submarine canyons off California.
7. Why are abyssal plains more extensive on the floor of the Atlantic than on the floor of the Pacific?
To a great extent, the Pacific Ocean is rimmed by deep ocean trenches or deep, near shore basins that trap sediment and prevent turbidity currents from moving farther offshore. The Atlantic Ocean is rimmed mainly by passive, continental margins; 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 for turbidite sediments to accumulate.
8. What is an atoll? Describe Darwin's theory on the origin of atolls. Was the theory ever confirmed?
Atolls are small, relatively low, flat, coral limestone islands (Fig. 18.8) dotting the deeper parts of an ocean basin (most are in the Pacific). They commonly have interior lagoons and are ringed by living coral reefs which thrive in waters with strong wave action. An atoll is the limestone-capped, eroded top of an old volcano built upward from the ocean floor. After the volcano rose above the sea surface, eruptive activity ceased and it began to slowly subside. The part above sea level was cut away by wave erosion; but upward growth of fringing coral reefs kept pace with subsidence, forming an atoll when the old volcano eventually sank well below the surface (Fig. 18.9).
Darwin proposed that atolls form by upward growth of fringing reefs that once encircled a slowly subsiding, volcanic island. Subsequent studies and deep drilling have established beyond question that atolls are thin, coral limestone cappings on the eroded tops of old volcanoes, confirming his original hypothesis.
9. Distinguish among the three basic types of seafloor sediment.
Terrigenous - These sediments are composed of rock and mineral grains derived by 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) (Fig. 18.10).
hydrogenous - These are sediments chemically precipitated from seawater without biological intervention (Box 18.2). Examples would be the manganese-rich nodules on parts of the ocean bottom (Fig. 18.B.) and metal-rich sediments around seafloor, hot spring vents (Fig. 18.D).
10. If you were to examine recently deposited biogenous sediment taken from a depth in excess of 4500 meters (15,000 feet), would it more likely be rich in calcareous materials or siliceous materials? Explain.
The stated depth (4500 meters) is the calcium carbonate compensation depth, below which crystalline calcium carbonate is unsaturated in seawater. The higher pressures and lower temperatures combine to make calcium carbonate soluble below this depth, whereas it is only sparingly soluble in the warmer waters nearer the surface. Thus, below the 4500 meter depth, the sediment would be depleted in or devoid of calcium carbonate because of dissolution. Opaline silica and quartz are very weakly soluble at all temperatures and depths in the oceans; thus the biogenic sediment would be siliceous and evidence for large diatom/radiolarian populations living in the water column above the site.
11. How are mid-ocean ridges related to seafloor spreading?
Seafloor spreading initiates at the mid-ocean ridges, where basalt magma rises from the mantle and crystallizes as seafloor basalt and intrusive gabbro, forming new, oceanic crust at the edges of the diverging plates. Most geologists agree that Earth's diameter has more-or-less remained constant since the Hadean; thus as new seafloor is formed at spreading centers, an equivalent area of old seafloor sinks into the mantle at subduction zones.
12. What is the primary reason for the elevated position of the oceanic ridge system?
Oceanic ridges are thought to form over rising plumes of hot, mantle rock. Partial melting occurs as the plume approaches the top of the mantle; basaltic magma rises and solidifies as lava flows and intrusive gabbro, forming new, oceanic crust. Temperatures are higher at any given depth under the ridge than under parts of the seafloor distant from the ridge. Magmas and high-temperature rocks occupy more space than equivalent masses of cool, solid rock; thus the seafloor is elevated along the mid-ocean ridge. Also, the rising-mantle, convection currents may dynamically sustain some of the "excess" elevation of the ridges.
Slow-spreading ridges, such as the Mid-Atlantic Ridge, stand higher and have more pronounced, deeper, crestal rifts than fast-spreading ridges such as the East Pacific Rise.
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