SCIENCE AND THE SEA 



The tectonic features of the Mid-Atlantic' Ridge seem to be a part o( 

 a 40,000 mile long world-encircling belt which passes through the 

 South Atlantic, around the Cape of Good Hope into the Indian 

 Ocean, around Australia, up the Eastern Pacific, through the Arctic 

 Basin and the Norwegian Sea. South of the Azores, this ridge is 

 very complex with many mountains and valleys. In the South 

 Atlantic, the ridge has a layer of sediments from about 300 to 650 

 feet thick, but in other zones sediments are sparse or entirely 

 lacking. 



Some elongated valleys extend down the slopes of the Atlantic 

 basins with steep sides and flat floors that are 10 to 100 fathoms 

 lower than the surrounding area. These depressions are from about 

 three to five miles wide. Terraces like plateaus are found on both 

 sides of the Mid-Atlantic Ridge. 



Oceanic rises are elongated elevations with gentle smooth slopes 

 having continuous layers of oceanic sediments. The lack of terri- 

 genous sediments and the sharp change in slope at the base of such 

 rises indicate that the turbidity currents have not affected these 

 areas. 



In 1947 an expedition of the Woods Hole Oceanographic Institu- 

 tion made the important discovery that large areas of the deep- 

 ocean floor in the North Atlantic are flat, almost level plains with 

 a bottom gradient of less than 1:1000. These plains are called abyssal 

 plains and have been found on the deep-sea floor throughout the 

 world mostly at the base of the continental rises. 



Current opinion suggests that these plains were produced by the 

 deposition of coarse sand and shallow-water fossils carried by tur- 

 bidity currents fanning out on the sea floor. These plains only exist 

 in areas where the turbidity currents are not blocked by the bottom 

 relief. Since turbidity currents cannot pass island arcs and trenches, 

 the Pacific has fewer abyssal plains than the Atlantic. Abyssal 

 plains have been observed also in marginal seas, such as the Western 

 Mediterranean, the Gulf of Mexico, and the Caribbean. 



Although many soundings have been made over the deep-sea 

 floor with standard echo-sounding apparatus, large errors in these 

 instruments made a study of the abyssal plains next to impossible. 

 To make a more effective study of these plains feasible, a Precision 

 Depth Recorder (P.D.R.) was developed in 1954. A more accurate 

 curve of the bottom relief was obtained on an 18-inch-wide fatho- 

 gram by recording depths in a 400-fathom range. For example, a 

 depth of 3,745 fathoms will be shown when the fathogram records 

 depths between 3,600 and 4,000 fathoms. Even at such great depths 

 a slight variation can be easily observed on the fathogram. An ac- 

 curacy of the P.D.R. and Precision Graphic Recorder of 1 fathom in 

 3,000 has been achieved. 



An additional improvement in sounding occurred with the devel- 

 opment of a high-resolution narrow-beam echo-sounder, electron- 

 ically stabilized, so that the return echo represents the accurate 

 depth under the ship. The reception of echoes from surrounding 

 slopes has been greatly reduced. 



Deep-sea channels at the seaward end of submarine canyons, 

 cut out in the continental slope, often extend across the boundary in 

 the abyssal plains. On the seaward side of most abyssal plains, 

 adjoining areas are covered by abyssal hills. These hills vary in 

 height between 50 and 200 fathoms and are between two and six 

 miles wide. They are most numerous in the Pacific, where they 

 cover most of the floor. If an area is almost completely covered by 

 hills, it is called an abyssal-hills-province. Abyssal hills are found in 

 abundance in basins separated from land areas by trenches, ridges, 

 or rises. Two strips of abvssal hills run parallel to the Mid-Atlantic 

 Ridge for nearly its entire length. The Western Atlantic Abyssal- 

 Hills Province, southeast of Bermuda, is over 500 miles wide. 



A seamount, mostly of volcanic nature, is an isolated circular or 

 elliptical elevation on the ocean floor, rising at least 500 fathoms 

 above its surroundings and having comparatively steep slopes. 

 Soundings, taken with a Precision Depth Recorder over seamounts 

 often show craters on their tops. 



Resembling a huge underwater apear, 

 this cover, used to extract samples of the 

 sea bottom, is checked by a scuba diver. 



Whi-n the topography of the ocean floor is similar to the abyssal- 

 hills-province but with higher peaks, the area is designated as a 

 "seamount province". A typical seamount province can be found in 

 Baja California, where 1,000 seamounts are located in a region 

 covering 410,000 square miles. Considering the number of seamounts 

 present in a densely-surveyed area, it has been estimated that the 

 number of seamounts in the whole Pacific Basin may reach roughly 

 10,000. 



Seamounts having flat tops are called "guyots" or "tablemounts". 

 To find an explanation for the flat-top appearance of the guyot, a 

 topographic survey was made with echo sounders of five tablemounts, 

 followed by dredging and coring for rocks and sediments. The 

 results suggested that the tops of the guyots have once been at or 

 near sea level, when erosion took place, but have sunk on account of 

 subsiding of the ocean floor and the guyots, or the rising of the 

 sea level. 



Although seamounts and guyots are present in the Atlantic, 

 greater numbers are known to exist in the Pacific. Guyots, fewer in 

 number than the seamounts, appear in three linear groups— the 

 Emperor Seamount Chain, south of Kamchatka; the Marcus-Necker 

 Ridge, west of Hawaii; and the region stretching out from the 

 Marianas to the Marshall Islands. Furthermore, 10 guyots, close 

 together, in the Gulf of Alaska, seem to be forming parallel lines. 

 Other guyots exist as isolated mountains. 



Trenches are long and narrow depressions with steep sides in 

 the ocean floor. These trenches attain the greatest depths of the 

 ocean, and are often adjacent to island chains with active volcanoes 

 and earthquake activity, while others border the coast of Mexico, 

 Central and South America. 



For many years the Mindanao Trench off the Philippines was 

 thought to be the deepest, but soundings have disproved this belief; 

 greater depths have been found in the Tonga Trench of the South 

 Pacific Ocean, the Mariana Trench and the Kuril Trench. 



The Middle-America Trench, extending from the southern end of 

 the Gulf of California almost to Panama, has been explored most 

 thoroughly. It was found, after producing a contour map of the area, 

 that the floor was flat in part for several miles across and was V- 

 shaped elsewhere. A thick layer of sediment was found underneath 

 the flat floor, while the V-shaped floor was free of sediment. Several 

 submarine canyons cut the landward wall of the trench, and a group 

 of basins with varied depths up to 3,700 fathoms and some hills, 

 presumably submarine volcanoes, are spread out on the floor. 



Many scientists indicated the connection between the trenches, 

 and earthquakes and volcanoes. It is generally believed that the 

 trenches are surface expressions (faulting of the crust) of large- 

 scale processes acting deep within the earth. Along the trenches 

 earthquakes are more frequent than in any other region in the 

 world. Near the trenches tremors are of shallow origin, but farther 

 landward the earthquakes originate at greater depths and their 

 origins reach depths exceeding 200 miles when on a line 200 miles 

 removed from the trench. This phenomenon has been observed for 

 all trenches of the world. If the origins of the earthquakes farther 

 landward do not increase in depths, the trenches are lacking. 



C 



EQUATOR 



