30 



OCEANOGRAPHY: 



land causes this area to accumulate muds that may 

 be white, red, green, blue, or black; these muds later 

 may compress to sedimentary rock. The organic re- 

 mains are also different. In contrast to pressure in 

 deep areas, water pressure here is sufficiently low to 

 permit more kinds of skeletons and shells to ac- 

 cumulate rather than to dissolve; hence, fossils are 

 more likely to be preserved. 



Ocean mountains possess many truly remarkable 

 features. Nothing on land really compares with the 

 great volcanic ranges like the Mid-Atlantic Ridge, a 

 range that extends roughly from Iceland to Antarctica 

 and is some 10,000 miles long and 500 miles wide. 

 Most of the range is 3 miles high but also 1 mile 

 below the surface of the sea. However, in some places 

 the ridge is closer to the surface, and in certain areas 

 its summits rise above the sea. Such islands as As- 

 cension Island, the Rocks of St. Paul, and the Azores 

 are peaks of the Mid-Atlantic Ridge. The highest 

 peak in the range, Pico Island, is in the Azores. Pico 

 is 7613 feet above sea level, but its base on the ocean 

 floor is about 20,000 feet below the surface. 



Other remarkable topographic features are the 

 volcanic, oceanic islands. These islands are termed 

 "oceanic" because they originate from vulcanism 

 that is not directly involved in continent formation. 

 Also, such islands are usually outgrowths of deep- 

 sea ridges. This means that the highest peaks of the 

 Mid-Atlantic Ridge are true oceanic islands. How- 

 ever, the Hawaiian Islands are an even more conspic- 

 uous group of peaks. Among these peaks of a single 

 1600-mile ridge is Mauna Kea, the world's tallest 

 mountain. Although unheralded because it reaches 

 only 13,823 feet above sea level, Mauna Kea rises 

 about 31,000 feet above the surrounding ocean floor. 



Perhaps strangest of all ocean mountains are 

 guyols. These look like once cone-shaped mountains 

 that have lost their pointed tops, a form suggesting 

 volcanoes whose tops were sheared off, probably by 

 wave action. The main difficulty with this wave 

 premise is that the mountain tops are from a half to 

 one mile below the surface and known water currents 

 at those depths are insufficient to explain the shear- 

 ing. Therefore, it is hypothesized that guyots formed 

 at the surface and then sank, either from their own 

 weight, or from a collapse of the ocean floor. There 

 are more than five hundred guyots known in the Pa- 

 cific Ocean and a few in the Atlantic Ocean. 



The greatest mystery of the deep-sea floor is the 

 "deeps," or trenches. These take the form of long 



incisions in the ocean bottom near continents and 

 islands. As their names imply, the deepest areas 

 known in the ocean are in such places. The greatest 

 measured depth, 35,640 feet, is in the Mariana 

 Trench, but bottoms of the Mariana, Philippine, and 

 Japanese trenches all appear to be about 7 miles be- 

 low the surface and 3 miles below the surrounding sea 

 floor. 



There is much speculation but little actual knowl- 

 edge about how these trenches formed. Because 

 they are near continents, there is a hypothesis that 

 deeps are a reciprocal reaction to mountain forma- 

 tion on land next to the sea. A second hypothesis sug- 

 gests that they form when sediments produce suf- 

 ficient weight to cause a collapse in the ocean floor. 

 Another hypothesis results from there being many 

 earthquakes in trenches. Earthquakes are move- 

 ments of the earth's crust along lines of weakness 

 (faults) in the crust itself. Although many trenches 

 are more curving than fault lines on land, there are 

 theories relating deeps to fault lines. 



CONTINENTAL SHELF 



A continental shelf is an area of transition be- 

 tween a continent and an ocean floor. It also is a 

 deposition basin for sediments from the land. Most 

 of these transition zones are from 10 to 200 miles 

 wide and extend from land to depths of 200 to 600 

 feet. They are narrowest where there are young, 

 rugged mountains along the coast and broadest where 

 coastal relief is low or where great rivers enter the 

 ocean. Owing to a particular combination of physical 

 features, the widest continental shelf extends some 

 800 miles from the USSR into the Arctic Sea. 



CONTINENTAL SLOPE 



Continental slopes are areas of remarkably uniform 

 but steep descent from the continental shelf to the 

 ocean floor. However, the rate and amount of descent 

 vary in relation to adjacent coastal plains. For broad 

 coastal plains, the slope is relatively insignificant, 

 with a descent rate of about 2 miles in 100; off 

 narrow coastal plains, the descent is about 4 miles 

 per 100. In some cases, the continental slope appears 

 to continue into the 7-mile depths of certain trenches. 

 The land has no feature that rises or falls at so con- 

 tinuous a rate or for so great an amount. 



Various other features characterize continental 



