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Lesson 6: The Shape of the Land: Where in the world is that?


The Naval Meteorology and Oceanography Command has provided much of the following information about oceans.

Terms used to describe features of the underwater world are very similar to terms used to identify features on land. Oceanic ridges are similar to mountain ranges, and oceanic trenches are the Grand Canyons of the seabed. The abyssal plain (the term abyssal refers to the large, deep parts of the ocean) is the feature that constitutes the largest portion of the ocean floor. It has been compared to the great flat prairie lands of mid-America.

Figure 6.1.1, Features of the underwater world. (“Abyssal Plains,” U.S. Navy.

Abyssal plains Mid-ocean ridges

Abyssal plains are found next to the continental slopes at depths greater than 9,000 to 10,000 feet. They are areas of near-freezing water temperatures where there are no seasons or sunlight. The abyssal plains are regarded as the true ocean floor. The few marine inhabitants found in the region survive only because they have adapted to the hostile environment of bitter cold and immense pressure. Abyssal plains are among the smoothest surfaces on the planet, with less than five feet of vertical variation for every mile. These level plains are the result of a constant rain of sediments. These sediments, ranging from the remains of large marine life to microscopic particles, drift slowly downward and fill in depressions on the irregular, rocky ocean floor. Often, coastal sediments are washed down the continental slope as turbidity currents. (A turbidity current is a downward flow of suspended mud-like sediments. The descent is caused by gravity.) Sediments from large rivers reach the ocean floor primarily by way of submarine canyons, which cut through the continental slopes.

Several mid-ocean ridges are longer than the longest mountain ranges on Earth. They are tall as well, rising to 12,000 feet above the ocean floor, and their peaks penetrate the ocean's surface to form islands such as Iceland and the Azores in the Atlantic Ocean and the Galapagos Islands in the Pacific Ocean. Most of the ridges crest at a depth of about 8,000 feet and their width varies from 500 to 1,500 miles. Unlike typical continental mountain ranges that have a singular, pronounced line of peaks, oceanic ridges have two lines of peaks separated by a prominent depression known as a rift valley. This valley ranges from fifteen to thirty miles in width and cradles an active seismic belt.


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