One problem of the ocean floor that has puzzled geophysicists 

 for many years is the pattern of features associated with island arcs. 

 Island arcs are found in the West Indies and in the Scotian arc 

 southeast of South America, but mostly on the western and northern 

 sides of the Pacific Ocean. The pattern consists of a curving arc of 

 islands about a thousand miles long enclosing an inland sea, usually 

 rather shallower than the typical ocean. Outside any typical island 

 arc, and running parallel to it, is a deep trench where the greatest 

 ocean depths are found. It was in the Marianas Trench in the 

 western Pacific that man eventually conquered the greatest known 

 depths. In i960, Jacques Piccard, son of Professor Auguste Piccard, 

 and Lieutenant Don Walsh of the U.S. Navy descended to a depth 

 of 35,800 feet in the bathyscaphe Trieste, a vessel designed by Pro- 

 fessor Piccard and his son. During a twenty-minute stay on the 

 bottom they confirmed by direct observation what dredge hauls 

 and photographs had shown us — that there is life even at these 

 enormous depths, where the pressure of the water is more than 

 seven tons per square inch. 



These deep-sea trenches, where the sea depth is almost double 

 the average, are rare gashes in the oceanic crust. We believe that 

 they may be regions which, when filled with sediments, are for 

 some reason subsequently thrust up as new mountain ranges. The 

 full story of the sea floor's trenches has yet to be told. 



In nearly all the oceans we find underwater mountains of one 

 kind or another. Some are called "seamounts," and may be mere 

 isolated bumps on the bottom, or they may reach up to the surface. 

 These are submarine volcanoes that form wherever the floor of the 

 ocean is weak enough to allow the molten magma to burst through. 

 Seamounts are found singly, in groups, or in long lines associated 

 with fracture zones. Underwater photographs reveal them in splen- 

 did variety, often showing vertical cliffs of exposed rocks with 

 corals, crinoids, gorgonians, and many other beautiful deep-sea 

 animals attached. At the base of these cliffs are scattered boulders 

 and piles of rocks or pebbles, and trapped in valleys and on ledges 

 are accumulations of sediment. 



Often the fine particles of sediment have been winnowed away 

 by the currents, and only sand composed of calcareous shells is left. 

 This material is ideal for currents to work and shape as sand ripples, 

 and many seamounts at a variety of depths have ripple markings. 

 Until quite recently geologists believed that ripple marks found in 

 sedimentary rocks indicated that the rocks were formed in shallow 

 water, since the currents of the deep ocean were thought to be too 

 mild to form them. However, underwater photography has shown 

 us that ripple marks are common in depths down to at least sixteen 

 hundred fathoms. 



Among the most unusual shapes of seamounts are those found in 

 the North Pacific. Dotting the sea floor are hundreds of flat-topped 

 structures called "guyots," rather like truncated cones, the tops of 

 which are submerged to a depth of several hundred feet and capped 

 with limestone. For a long time it was hard to see how the guyots' 

 tops could have been flattened, since they are far too deep to be 

 worn smooth by wave action. The history of the guyots, as we 

 now understand them, is very interesting. In the first stage, a 

 volcano erupts through the ocean floor and builds a cone reaching 

 far above sea level. Gradually, at the water line the guyot is eaten 



w. 





1 



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This echogram of a seamount rising 

 900 fathoms from the floor of the Caribbean 

 was made by an echo sounder. Seamounts 

 may be mere bumps on the ocean floor 

 or they may reach up to the surface. 

 The top of this seamount is 300 fathoms 

 below the surface, here represented by 

 the top edge of the illustration. 



199 



