17. TRENCHES 



R. L. Fisher and H. H. Hess 



Man's knowledge of the deepest parts of the oceans, the trenches that nearly 

 ring the Pacific and border parts of the Atlantic and Indian Oceans, goes back 

 about a hundred years. Early workers pointed out the arcuate shape of these 

 features, and their common association with island arcs, mountain ranges and 

 vulcanism. More recently, geophysical investigations of gravity, crustal 

 structure, heat flow and earthquake activity indicate that trenches are surface 

 expressions of large-scale processes acting deep within the earth. 



1. Previous Work 



By 1900, ships engaged in scientific reconnaissances or searching for cable 

 routes had established that the greatest oceanic depths lie near land, seemingly 

 closely related to the island groups of the western Pacific, to the Indonesian 

 islands, the Antilles or the Andean mountain chain. Murray's bathymetric 

 chart (Murray and Hjort, 1912) showed deeps in nearly all of the presently 

 known trench areas. The only notable omissions are the Middle America 

 Trench off western Mexico and Central America and the South Sandwich 

 Trench in the South Atlantic Ocean. Subsequent bathymetric exploration has 

 refined Murray's map, joining some separate deeps he plotted and named, but 

 it has not revealed any trenches off Antarctica between New Zealand and 

 South America, or between Mexico and southeastern Alaska. Recent work by 

 Vitiaz, reported by Udintsev (1959), confirms the presence of a series of small 

 deeps, shoaler than the usual trench, from western New Guinea to near Rotuma, 

 northwest of the Fiji Islands. Another chain of trenches, southwest of or 

 "interior" to the Solomon, Santa Cruz and New Hebrides groups, extends 

 3800 km from southeastern New Guinea to beyond the Loyalty Islands. 



Prior to the 1920's, oceanic soundings were made by wire or line and lead, 

 and, though each measurement consumed several hours, most soundings 

 obtained a small sample of the bottom material. Modern echo-sounders record 

 thousands of de]3ths in a similar j^eriod ; however, conclusions as to bottom 

 materials and processes are commonly drawn by inference, from the shape and 

 character of the trace recorded or from minor relief on the trench walls and 

 bottom. 



Vening Meinesz developed a method for measuring gravity at sea by pendu- 

 lums installed in a submarine. In a series of cruises beginning in 1923, he 

 demonstrated that negative isostatic anomalies characteristically are associated 

 with trenches, and that nowhere else are such large negative values found 

 (Vening Meinesz, 1948, for summary). Vening Meinesz' trench surveys were 

 most detailed in the East Indies, but he, and later Ewing and Hess, made 

 similar surveys in the Caribbean area (Ewing, 1937, 1938; Hess, 1938). Matu- 

 yama (1936) made gravity surveys in the Japan Trench. Since World War II 



[MS received May, 1961] 411 



