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Chapter Overview

Space travelers seeing our beautiful, blue planet would likely call it the water planet. The hydrosphere, notably the ocean, dominates the surface of Earth with over 70% of the surface covered by oceans. Clearly, the part of the geosphere covered by the oceans is important to our understanding of the Earth's systems, even though most of the seafloor is not readily accessible to direct observation.

The hydrosphere is, of course, vital to the biosphere. Primitive life began in the sea and evolved over billions of years into the rich and diverse plant and animal life that we see today. Life on land is a relative newcomer. It has been only a few hundred million years since the first creatures ventured out of the sea and land-dwelling life evolved and flourished.

Most of what we know about the seafloor has been discovered since the second half of the twentieth century. Because of the difficulty in accessing the deep ocean floor, our maps of its surface are not as complete as those of some of our neighboring planets.

We do know that the rocks and topography of the sea floor are different from those on land. To understand the evidence for plate tectonics in the next chapter, you need to understand the nature of major sea-floor features such as mid-oceanic ridges, oceanic trenches, and fracture zones, as well as the surprisingly young age of the sea-floor rocks.

The material discussed in this chapter and the next are an excellent example of how the scientific method works. This chapter is concerned with the physical description of most sea-floor features - the data gathering part of the scientific method. The next chapter shows how the theory of plate tectonics explains the origin of many of these features. Geologists generally agree upon the descriptions of features but often disagree on their interpretations. As you read, keep a clear distinction in your mind between data and the hypotheses used to explain the data.

Learning Objectives

1. The ocean formed from the condensation of gasses released by degassing of the earth as it cooled. Salt content was added from the gasses and chemical weathering (Box 18.1).

2. Continental shelves are shallow, submarine platforms of variable widths inclined very gently seaward covered by young sediments that become finer offshore. Wide shelves may have gravel at their outer edges that was deposited during lower Pleistocene sea levels. Continental slopes are steeply inclined toward ocean depths.

3. Submarine canyons are erosional features that cross continental shelves and slopes caused by a combination of down-canyon sand movement, bottom currents, river erosion during low sea level, and turbidity currents. Abyssal fans are formed of land-derived sediment moved down the submarine canyon by turbidity currents. Turbidity currents were responsible for the 1929 Grand Banks cable breaks, and they produce graded bedding.

4. A passive continental margin includes a continental shelf, slope and rise, and lacks earthquakes, volcanoes, and young mountain belts. The continental rise is a wedge of sediments deposited by turbidity currents (deposits exhibit graded bedding) moving down the continental slope and contour currents (not graded and wedge shaped) moving along the slope. Abyssal plains are exceedingly flat, covered by graded sediments deposited by turbidity currents that cover irregularities on the sea floor.

5. Active continental margins lack continental rises and abyssal plains, but possess oceanic trenches associated with Benioff zones, volcanoes, low heat flow, and negative gravity anomalies.

6. Mid-oceanic ridges have rift valleys along their crests, except for the Pacific Ocean ridge. Other geologic activity includes shallow-focus earthquakes, high heat flow, basaltic eruptions, hydrothermal activity, and exotic organisms.

7. Fracture zones cross and may offset the mid-oceanic ridges. They produce shallow-focus earthquakes and along segments called transform faults.

8. Conical seamounts are extinct volcanoes, sometimes forming islands. Guyots are flat-topped seamounts, cut by wave activity, that have subsided to their present depths. Aseismic ridges link guyots and seamounts on the sea floor.

9. Reefs are wave-resistant ridges formed by coral and other calcareous organisms. Three types include: fringing - attached directly to shore, barrier - separated from land by a lagoon, and atoll - circular reefs that rim lagoons that form around subsiding volcanoes.

10. Terrigenous sediments on the sea floor were derived from land and deposited by turbidity and/or contour currents. Pelagic sediments are clays and fine-grained skeletons of microorganisms that settle through the ocean water. Pelagic sediments are absent from mid-oceanic ridges.

11. Oceanic crust is divided into three layers: 1 - marine sediment, 2 - pillow basalt overlying sheeted dikes, 3- thought to be sill-like gabbro, but unsampled by deep drilling at the moment. Ophiolites found in mountain chains exhibit a similar three-layered sequence. They are interpreted as sea floor from marginal ocean basins, but are not typical ocean floor.

12. Rocks of the sea floor are younger than 200 million years, in contrast to continental crust that is 3-4 billion years old.

Related Readings

Anderson, R. N. 1986. Marine Geology - A Planet Earth Perspective. New York: John Wiley and Sons.

Dietz, R. S. 1964. Origin of Continental Slopes. American Scientist 52:50-69.

Heezen, B. C., and C. D. Hollister. 1971. The Face of the Deep. New York: Oxford University Press.

Lutz, R. A., and R. M. Haymon. 1994. Rebirth of a Deep-sea Vent. National Geographic 186(5): 114-26.

Macdonald, J. B., and P. J. Fox. 1990. The Mid-ocean Ridge. Scientific American 262(6): 72-79.

Menon, S. 1997. Ocean Watch: Deep Sea Rebirth. Discover no. 7:34.

Nicolas, A. 1995. The Mid-ocean Ridges: Mountains below Sea Level. New York: Springer-Verlag.

Pinet, P. 1996. Invitation to Oceanography. St. Paul, MN: West Publishing.

Rona, P. A. 1986. Mineral Deposits from Sea-floor Hot Springs. Scientific American 254(1): 84-93.

-------. 1992. Deep-sea Geysers of the Atlantic. National Geographic 182(4): 105-109.

Scientific American Editors. 1983. The Ocean. New York: W. H. Freeman.

Scrutton, R. A., and M. Talwani, eds. 1982. The Ocean Floor. New York: Wiley-Interscience.

Seibold, E., and W. H. Berger. 1982. The Sea Floor. New York: Springer-Verlag.

Waters, T. 1995. The Other Grand Canyon. Earth 5(3): 42-51.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 18:

14.A, 15.D, 16.C, 17.C, 18.D, 19.D, 20.B, 21.D, 22.B, 23.B

Boxed Readings

This chapter contains the following boxed readings:

Earth Systems
Box 18.1: Does the Earth Breathe?

Environmental Geology
Box 18.2: Geologic Riches in the Sea





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