GEOLOGICAL OCEANOGRAPHY 



Features prevalent beneath the sea. 



Many trenches located in regions with earthquake activity are 

 paralleled by linear groups of volcanoes. In the same manner, the 

 Aleutians, the Kurils, and the Marianas are arc-shaped island chains 

 located on the concave side of the arcuate trenches. 



In the sea floor of the northeastern Pacific, H. W. Menard found 

 four great parallel fractured zones with an average width of sixty 

 miles between 1950 and 1953. The Mendocino Fracture Zone, the 

 northernmost, extending westward from Cape Mendocino for a dis- 

 tance of more than 1,400 miles, stands out for its steep slope, called 

 "sea scarp", which in places is over 10,000 feet high. Menard also 

 discovered many smoothly sloping areas extending to the deep-sea 

 floor around some island masses, which he called archipelagic aprons, 

 presumably caused by lava flows originating from the islands. 



Depressions, resembling canyons, have been found in the abyssal 

 floor. They often have steep walls and flat floors. East of 

 Newfoundland four of these depressions have been discovered, which 

 are part of a continuous canyon extending for a length of 1,500 miles 

 from Greenland to the Southeast Newfoundland Ridge. Other ocean 

 canyons have been found in the equatorial Atlantic and in the 

 Hikurangi Trench. 



Oceanic islands, mostly limited in area and volcanic in character, 

 rise from the ocean floor far from the continents. Large continental 

 islands, such as Iceland, Greenland, and New Zealand are not in this 

 category. Japan and the Aleutians are in a borderline class. Oceanic 

 islands are either a part of a chain or are isolated. Older oceanic 

 islands in the tropics are often surrounded by reefs, and the volcanic 

 base may be covered by limestone. Oceanic islands, which appear to 

 be isolated, are sometimes the highest peaks of oceanic ridges such 

 as in the Atlantic, South Pacific and Indian Oceans. 



Volcanic oceanic islands in the Pacific Ocean are often located 

 in linear groups, such as the Hawaiian, Society and Marquesan 

 Islands. These islands and their ridges are built of lava, and rise 

 from submarine ridges, which may be caused by faulting. When 

 volcanoes are spaced far apart as those in Tubuai (Austral) Islands 



Group, their lava output has not been large enough to build a ridge 

 between them. 



A ring-shaped reef lying at or near sea level enclosing a lagoon 

 is called an atoll. Most of them rise from the ocean floor, but others 

 are found on the continental shelves. Kwajalein in the Marshall 

 Islands, the largest existing atoll, covers about 840 square miles. 

 Most atolls on the ocean floor have probably been produced by coral 

 growth on top of truncated volcanoes. Basalt, a volcanic rock, has 

 been obtained from the slopes of some atolls in the Marshall Islands. 



SEDIMENTS ON THE OCEAN FLOOR 

 Since the Challenger Expedition (1872-1876), cores of bottom 

 samples have been extracted from the ocean floor. In the beginning 

 cores were only one foot long, but after the development of the 

 piston corer, cores up to 70 feet have been raised. 



According to calculations made by P. Kuenen, the average thick- 

 ness of the sediment layer in the ocean basins should be about two 

 miles. However, observations made with sound waves indicate that 

 the depth of the sediment layer in the Atlantic averages about 3,200 

 feet, and in the Pacific about 1,600 feet. E. L. Hamilton suggested 

 that the lower parts of the sediment layer have become solidified 

 into consolidated sediments. When sound waves are used to measure 

 the sediment layer, they are reflected by the interface between the 

 consolidated and the unconsolidated sediment layers. Sediment thick- 

 ness has now been measured along 150,000 miles of track. 



Sediment types can be arranged according to their origins- 

 terrigenous, biogenous, halmyrogenous, and cosmogenous. Terrig- 

 enous sediments originate from the continents, and are the results 

 either of mechanical and chemical breaking down of the rocks or of 

 volcanic activity. These particles in minute size can be widely dis- 

 tributed by ocean currents over the whole ocean floor and can be 

 found in red deep-sea clay. If these particles are of different sizes, 

 such as gravel and stones, they originate from moraines and were 

 transported by icebergs over the ocean areas. Terrigenous sediments 



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