SECT. 2] TOPOGKAPHY OF THE DEEP-SEA FLOOR 251 



1000 miles wide between the continent and the Marianas Islands and it is 

 deeper than normal oceanic depth. The few seismic stations available suggest a 

 relatively normal oceanic crust. Likewise, the Aleutian Islands are separated 

 from the continent by an area of normal oceanic crust which appears shallow 

 because of a thick fill of sediments in the Bering Sea Basin. Nevertheless, the 

 island arc trenches lying along the convex sides of these arcs may be con- 

 sidered analagous to the marginal trenches observed along the Pacific coasts of 

 Central and South America and off the north coast of Puerto Rico (see Chapter 

 17). 



Trenches and island arcs were among the first surveyed features of oceanic 

 topography because they were barriers to submarine cables. The island arcs 

 include lines of active volcanoes and they are associated with more large earth- 

 quakes and more pronounced gravity anomalies than any other part of the 

 Earth's crust. The more or less arcuate trenches bordering them are the deepest 

 places in the ocean, which is particularly noteworthy because they are near the 

 margins of the ocean basins rather than in the middle. At present, the greatest 

 depth known is about 10,600 m in the Marianas Trench, i but very similar depths 

 have been found in the Kurile and the Tonga Trenches. 



A second class of arcs, the Antillean Arcs of the Atlantic, are generally con- 

 sidered as extensions of the Pacific tectonic belt into the Atlantic Ocean. The 

 West Indies and Scotia Arcs are more complex than the North Pacific Island 

 Arcs, and are more sharply curving ; and the deep-sea trenches associated with 

 them are more limited in extent, either due to the fact that they were originally 

 smaller or that they have been fiUed. The Indonesian area is similar to both 

 the great island arcs of the North Pacific and the more contorted Antillean 

 Arcs. The Banda Sea appears to be quite similar to the Antillean Seas, whereas 

 the Java-Sumatra area has many similarities to the Japan and Kurile-Kam- 

 chatka area. Again, the Philippine Arc seems to fall in a class very close to the 

 Japan Arc but lies landward from the Marianas Arc. 



The typical Antillean Arcs and typical North Pacific Island Arcs are 

 associated with deep-sea trenches. However, there is a tliird general occurrence 

 of deep-sea trenches that has been mentioned before, i.e. along the continental 

 margins of Central America and South America and along the eastern margin 

 of the Melanesian sub-continent. 



The epicontinental marginal seas of northeast Asia, the Bering, Okhotsk and 

 Japan, lying as they do between a volcanic island arc and the continent, are 

 rapidly filling with sediments supplied both from vulcanism and from erosion 

 of the adjacent stable land-masses. If subsidence is arrested and sedimentation 

 goes on, the Bering Sea may eventually reach a stage similar to the present 

 Okhotsk Sea, and the Okhotsk Sea may eventually become a shallow sea 

 similar to the Japan Sea. Further, the Japan Sea may eventually be filled to 

 such a degree that it greatly resembles the East China Sea, which lies landward 

 of the Ryukyu Arc. Kay (1951) has pointed out that the Paleozoic Appa- 

 lachian geosyncline was probably similar to modern island arcs of the North 



1 In 1962 a new record depth was measured from H.M.S. Cook. 



