GEOLOGICAL OCEANOGRAPHY 



Trough with a depth of 105 fathoms in the English Channel are dis- 

 continuous submarine valleys presumably cut out by the strong tidal 

 currents prevailing. 



The most common sediments found on the shelf are sand, mud, 

 and gravel-pebbles, cobbles and boulders. In addition, rare sediments 

 are bombs, ash, or pumice of volcanic matter; seaweed and other 

 marine plants; oysters mostly found in brackish water; and sponges 

 which grow on the shelf in tropical areas. 



The depth of the sediment layer varies in different locations. 

 Off the east coast of the United States, the thickness of the sedi- 

 ments under the shelf is about 17,000 feet. According to G. E. Murray, 

 the coastal plain and inner shelf at the Gulf Coast of the United 

 States and Mexico have a great wedge of sedimentary strata which 

 thickens to about 6.5 nautical miles under the present coast line. 



The lowering of the sea level that happened during the ice 

 periods when a greater part of the continents was covered with ice 

 masses seems to be the main factor to explain many of the present 

 conditions of the continental shelves. Formerly, it was believed that 

 the shelves were cut by wave action at present sea level. However, 

 most shelves join the coast without great cliffs and have outer 

 limits that are too deep to be caused by wave action. 



Proof of the former subsidence of the sea level may be found by 

 the presence of coarse sand, presumably deposited when the sea 

 level was lower, on the outer shelf and by the existence of river 

 channels which have been found across the North Sea and the 

 English Channel. According to R. A. Daly, the sea level was lowered 

 about 246 feet during the periods of maximum ice advance. 



As one-half of the ice on the continents has melted so far, an 

 increase in the world temperature will eventually flood the coastal 

 plains around the world. It has been observed that the sea level 

 throughout the world is rising about 2Vi inches in a hundred years 

 because of the melting of glaciers. 



THE CONTINENTAL SLOPE 



The area between the continental shelf and the deep-ocean floor 

 is marked by steep slopes. The steep upper part is called the contin- 

 ental slope; the lower part with gentler slope has been named the 

 continental rise or deep-sea fan. According to Shepard the steepness 

 of the slope from the shelf outer margin to a depth of 1,000 fathoms 

 averages 4° 17'. The greatest gradient has been found in the Bartlett 

 Trough off Santiago, Cuba, with a slope of 45 degrees. 



Although most continental slopes are continuous, others are 

 interrupted by continental borderlands, areas with terraces or basins. 

 These borderlands are at greater depths than the valleys of the con- 

 tinental shelves but at lesser depths than the deep-sea floor. 



It is believed that the continental slopes were originally formed 

 by fracturing of the earth's crust. The numerous earthquakes appear- 

 ing on or near the continental slopes substantiate this belief. Renewal 

 of fracturing and submarine landslides causing sediment-laden 

 density currents to flow near the bottom prevent the accumulation 

 of large sediment deposits on the slope. 



After submarine cables were laid across the ocean, it was found 

 that breaks appeared in them on the continental slopes. These breaks 

 are believed to be caused by landslides, the sediment on the slope 

 becoming liquified, or turbidity currents flowing along the bottom. 



Submarine Canyons 



The winding V-shaped depressions with many branches that 

 extend down most continental slopes are similar to the canyons on 

 land are called submarine canyons. 



One hundred years ago the assumption was made that submarine 

 canyons were old river valleys, that had subsided below the sea level. 

 Since the canyons extend often to the bottom of the continental 

 slope, this theory has been abolished. Several geologists believe 

 that the principal cause of the existence of the canyons is fractur- 



ing of the sea floor. The winding nature of the canyons with its 

 branches prompted other scientists to find a different explanation. 

 The opinion has now been widely accepted that submarine canyons 

 were cut by the turbidity currents flowing near the bottom. 



Marginal Plateaus 



Separated from the continental shelf by a section of the conti- 

 nental slope, the marginal plateaus are areas with a shelf-like appear- 

 ance. Often the surface is irregular in profile such as those of the 

 marginal plateaus off Recife and Rio de Janeiro, Brazil, and west of 

 Angola. The Blake Plateau located off the east coast of the United 

 States is rather flat with depths ranging between 400 and 600 fath- 

 oms. The bottom consists of rock or a similar substance which is 

 difficult to core. There are many explanations for the existence of 

 this plateau. One opinion suggests that due to slow subsidence dur- 

 ing many centuries, the latter shelf sank to lower levels, but sedi- 

 ment deposits did not build up because of the powerful current of 

 the Gulf Stream. 



The Continental Rise 



The sediment-covered plains bordering the continental slope are 

 called the deep-sea fan or the continental rise. Its main features 

 are the gentle slope and smooth bottom. In most areas the continen- 

 tal rise is bordered by the abyssal plains; in others, for example off 

 California, the beginning of the abyssal hills marks the lower limit 

 of the continental rise. 



THE DEEP-OCEAN FLOOR 



The deep-ocean floor, a part of the earth's surface, consists 

 mostly of a sediment layer on top of the earth's crust superimposed 

 on the earth's mantle. Inside the earth's mantle is the core. 



The earth's mantle has a volume about 10 times greater than 

 the combined volume of the core and the crust. Its inner surface, 

 the core-mantle discontinuity, lies about halfway between the center 

 and the surface of the earth, and its outer surface, the crust-mantle 

 discontinuity, also called "Mohorovicic Discontinuity", lies about 6.5 

 nautical miles under the ocean basins and between 19 and 27 nautical 

 miles under the continents. 



The earth's crust is divided into a top layer of lighter rocks, 

 mainly granite and granodiorite, called "sial" for silicon and alum- 

 inum, and a much thicker bottom layer of heavy rocks, both peridotite 

 and basalt with peridotite of even greater density underneath, known 

 as "sima" for silicon and magnesium. The layers forming the earth's 

 crust are not evenly distributed over the earth's surface, but at the 

 margins of the continents the sial layer becomes thinner to seaward 

 or stops entirely. In the Pacific Ocean, the sima layer is exposed 

 over vast regions; in the Atlantic and Indian Oceans, however, a thin 

 stratum of sial covers the floor. 



By topographic features the deep-ocean floor can be divided into 

 two regions; the oceanic ridges and rises, and the abyssal floor 

 subdivided into the abyssal plains and the abyssal hills. 



A mid-oceanic ridge is an elongated elevation on the sea floor 

 with steep irregular slopes in the middle of the oceans. Bottom- 

 water temperatures taken in the east and west basins of the Atlantic 

 by early oceanographic expeditions ascertained that the shallow 

 area in the middle of the ocean, previously discovered by soundings, 

 was a ridge, named Mid-Atlantic Ridge. It extends from Iceland to 

 a southerly latitude of about 57 degrees. Points on the ridge are 

 almost equidistant from the outer edges of the continental slopes on 

 both sides of the Atlantic. The highest part of the ridge rises about 

 2,000 fathoms above the deep basins on the sides to about 1,000 

 fathoms below the sea level. The highest peaks in the ridge project 

 above water, and form islands as the Azores, St. Paul Rocks, Ascen- 

 sion, and Tristan da Cunha. The ridge is intersected by an important 

 break, called "Romanche Furrow", just northward of the equator. 



