Answers to STUDY BREAK Questions
Essentials 5th
Chapter 4
OCEAN BASINS
1. How was bathymetry accomplished in years past? How do scientists do it now?
The simplest methods involved lowering a weight on a line. The length of line is measured, and the depth determined. Sometimes the weight was tipped with wax to retrieve a sample of bottom sediment. Scientists now use beams of sound to measure depth (see next answer).
2. Echo sounders bounce sound off the seabed to measure depth. How does that work?
Echo sounders sense the contour of the seafloor by beaming sound waves to the bottom and measuring the time required for the sound waves to bounce back to the ship. Unlike a simple echo sounder, a multibeam system may have as many as 121 beams radiating from a ship’s hull.
3. Satellites orbit in space. How can a satellite conduct oceanographic research? Why does the surface of the ocean “bunch up” over submerged mountains and ridges?
Satellites cannot measure ocean depths directly, but they can measure small variations in the elevation of surface water using radar beams. This is useful because the pull of gravity varies across Earth’s surface depending on the nearness (or distance away) of massive parts of Earth. An undersea mountain or ridge “pulls” water toward it from the sides, forming a mount of water over itself, and that mount is detected by the orbiting satellite.
4. How would you characterize the general shape of an ocean basin?
Ocean basins are not bathtub-shaped. The submerged edges of continents form shelves at basin margins, and the center of a basin is often raised by a ridge.
5. If you could walk down into the seabed, the transition from granite to basalt would mark the true edge of the continent and would divide ocean floors into two major provinces. What are they?
Try drawing a diagram of a typical ocean basin’s cross-section. Now look at Figure 4.9. Did your drawing include the continental margin (continental shelf + continental rise) and the deep ocean floor? Did you remember to raise the center of the deep floor to indicate a mid-ocean ridge (as in Figure 4.7)? The true edge of a continent is marked by the transition from granite to basalt (seen in Figure 4.7).
6. How does a continental margin differ from a deep-ocean basin?
The transition to basalt marks the true edge of the continent and divides ocean floors into two major provinces. The submerged outer edge of a continent is called the continental margin. The deep seafloor beyond the continental margin is properly called the ocean basin.
7. What are the features of the continental margins?
The continental margin is characterized by thick (and less dense) granitic rock of the continents. Near shore the features of the ocean floor are similar to those of the adjacent continents because they share the same granitic basement. Relatively thin (and denser) basalt forms the adjacent deep seafloor.
8. How is an active tectonic margin different from a passive tectonic margin?
Continental margins facing the edges of diverging plates are called passive margins because relatively little earthquake or volcanic activity is now associated with them. Continental margins near the edges of converging plates (or near places where plates are slipping past one another) are called active margins because of their earthquake and volcanic activity.
9. How do the widths of continental shelves differ between active margins and passive margins?
The width of a shelf is usually determined by its proximity to a plate boundary. The shelf at a passive margin is usually broad, but the shelf at the active margin is often very narrow.
10. How has sea level varied with time? Is sea level unusually high or low at present?
Figures 4.15 provides a graph of sea level through time. Sea level is high at the moment, and is rising as the ocean warms.
11. What are submarine canyons? Where are they found, and how are they thought to have been formed?
Submarine canyons cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment. Most geologists believe that the canyons have been formed by abrasive turbidity currents plunging down the canyons.
12. Where would you look for a continental rise? What forms continental rises?
Along passive margins, the oceanic crust at the base of the continental slope is covered by an apron of accumulated sediment called the continental rise. Sediments from the shelf slowly descend to the ocean floor along the whole continental slope, but most of the sediments that form the continental rise are transported to the area by turbidity currents.
13. What are typical features of deep-ocean basins?
Deep-ocean basins consist mainly of the oceanic ridge systems and the adjacent sediment-covered (abyssal) plains. Basin edges may be rimmed by trenches or aprons of sediments that have fallen along the continental shelves. The abyssal plains are sometimes punctuated by islands, hills, and volcanoes.
14. What is the extent of the mid-ocean ridge system? Are mid-ocean ridges always literally in mid-ocean?
Oceanic ridges are Earth’s most remarkable and obvious feature. They stretch 65,000 kilometers. Although these features are often called mid-ocean ridges, less than 60% of their length actually exists along the centers of ocean basins.
15. Draw a cross section through an active mid-ocean ridge. Where are the hydrothermal vents located? Where is new seabed being formed?
Check Figure 4.20 after you make your drawing.
16. What are fracture zones? What causes these lateral breaks?
Fracture zones extend outward from the ridge axis. They are seismically inactive areas that show evidence of past transform fault activity. While segments of a lithospheric plate on either side of a transform fault move in opposite directions from each other, the plate segments adjacent to the outward segments of a fracture zone move in the same direction.
17. What are abyssal plains? What is unique about them?
Abyssal plains are flat, featureless expanses of sediment-covered ocean floor found on the periphery of all oceans. Abyssal plains are extraordinarily flat.
18. Why are abyssal plains relatively rare in the Pacific?
Because the extensive system of trenches along the active margins of the Pacific trap much of the sediments flowing off the continents, preventing them from building the broad, flat abyssal plains typical of the Atlantic. There are a few abyssal plains in the Pacific (notably adjacent to China and Southeast Asia), but none approaches the extent of, say, the Canary Abyssal Plain west of the Canary Islands in the North Atlantic, with an area of 900,000 square kilometers (350,000 square miles).
19. How do guyots form? How were lines of guyots and sea-mounts important in deciphering plate tectonics?
Guyots are flat-topped seamounts that once were tall enough to approach or penetrate the sea surface. The flat top suggests that they were eroded by wave action when they were near sea level. Movement of the lithosphere away from spreading centers has carried them outward and downward to their present positions.
20. How are the ocean’s trenches formed? How are earthquakes related to their formation?
A trench is an arc-shaped depression in the deep-ocean floor that forms where a converging oceanic plate is subducted. Earthquakes are understandably associated with subduction zones.
21. Why are trenches and island arcs curved? Is the descent to the bottom steeper on the convex side of the arc or on the concave side? Why do you think most trenches are in the western Pacific?
Trenches are curved because of the geometry of plate interactions on a sphere. The convex sides of these curves generally face the open ocean. The trench walls on the island side of the depressions are steeper than those on the seaward side, indicating the direction of plate subduction. Trenches are prevalent in the western Pacific because that is an area of vigorous plate subduction.
22. How do you think graphics like The Grand Tour have assisted our understanding of geological processes?
These large-scale remote sensing techniques have allowed researchers to (quite literally) see the big picture. For the first time, global processes may been in context in relation to one another. For example, in Figure 4.32, look at the area numbered “9” just south of the tip of South America. Notice how the tip of the continent – along with the adjacent northern tip of Antarctica’s Palmer Peninsula -- has been nudged to the east. The South Sandwich Trench marks the easternmost terminus of the large feature. The relationship of all these was unclear until images like this Figure provided the “aha” moment.
Page 4 of 4