SECT. 2] TOPOGRAPHY OF THE DEEP-SEA FLOOR 269 



crust must be depressed in a moat that lias been filled predominantly with lava 

 flows. The moat filling is called an archipelagic apron. The surface of the moat- 

 fill is usually smooth and, in most places, the existence of a smooth region 

 surrounding an island group may be taken as evidence of an archipelagic apron 

 (Fig. 30). However, the surface material is sediment and the smooth surface 

 and extremely gentle slopes indicate that the mode of spreading of erosion 

 products is by turbidity currents. The smooth surface of an archipelagic apron 

 is known as an archipelagic plain. 



The Bermuda Islands are surrounded by a thick ring of sedimentary and 

 volcanic material. However, this thick ring extends much further than the 

 smooth surface of the Bermuda archipelagic plain. It may be that a formerly 

 smooth apron was deformed tectonically, forming abyssal hills. On the other 

 hand, it may be that the sediments shed by Bermuda since its origin, which 

 was at least prior to late Cretaceous time, have been insufficient to produce a 

 smooth surface to cover the irregular surface of the volcanic blister and, thus, 

 the archipelagic plain has not yet completely covered the archipelagic apron. 



A startling recent discovery illustrates how little is known about the relief 

 of the deep sea. The Hawaiian Islands are the focus of a very large number of 

 sounding lines by naval vessels, and several reconnaissance studies have been 

 made by oceanographic ships with recording echo-sounders. Nevertheless, a 

 major element of the sea-floor topography completely escaped notice until a 

 survey was made with the closely spaced sounding lines positioned with j)recise 

 electronic equipment not previously available. Narrow asymmetrical mountains 

 2 km high and 100-200 km long have been found in abundance on the Hawaiian 

 arch. They form a distinct pattern, trending northeastwards on the east side of 

 the Hawaiian Islands and southwards on the west side. Similar mountains do 

 not occur on the arch where it curves around the end of the Hawaiian structure. 



k. Tectonic activity of the ocean-basin floor 



The ocean-basin floor is generally aseismic. The number of actual volcanoes, 

 or even volcanic islands, is smafl. For this reason, it is commonly generalized 

 that the basin has been a stable area in geological time. Modern oceanography, 

 however, has shown that the basin is covered with volcanoes and broken in 

 many places by enormous faults, so that it cannot always have been stable. 

 The question raised is whether the present is abnormally quiet tectonically or 

 whether geological time is adequate to produce the volcanoes and faults in a 

 relatively stable basin. The answer depends to a large degree on the time 

 available. Demonstrable history of the ocean-basin floor, based on samples, is 

 very brief. Cores penetrate only a small distance and could hardly be expected 

 to reach very ancient sediments. Dredge hauls of seamounts might sample 

 material of any age, because most submarine elevations are swept clear of 

 sediment. To date, Cretaceous fossils are the oldest ones sampled. But fossils of 

 any kind are rare in dredge hauls. The basalts dredged have not often been 

 dated, and in many cases are undatable owing to their highly weathered condi- 

 tion. The potassium-argon method is capable of dating fresh basalts but the 



10— s. Ill 



