SECT. 2] 



OCEANIC ISLANDS, SEAMOUNTS, GUYOTS AND ATOLLS 



3. Seamounts 



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A seanioujit may be defined as a more or less isolated elevation of the sea 

 floor with a circular or elliptical plan, with at least 1 km {ca. 500 fm) of relief 

 and comparatively steep slopes. This definition fits both flat-topped and 

 pointed features. The former are a special class which will be discussed in more 

 detail under the heading "guyots". Seamounts are indicated by soundings 

 taken in the days of the Challenger Expedition, but it remained for Murray 

 (1941) to make the first adequate description of a group of seamounts. Now an 

 extensive literature exists and more than 1200 seamounts are known to be dis- 

 tributed in all the ocean basins (Figs. 1 and 2) ; roughly 200 have been surveyed 



80° 7P» _60° _ 501 40° 30* 20* 10' 0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100° 110° 120° 



«»• 



80° 70° 60° 50° 40° 30° 20° 10° 0' 



90° 100° 110° 120° 



Fig. 2. Seamounts, atolls, volcanoes and related features in the world exclusive of the 

 Pacific. Uncertainty of reported occurrences indicated by query. 



in sufficient detail to establish the shape, and bedrock has been dredged from 

 about 50. It seems clear that almost all, if not all, seamounts are submarine 

 volcanoes because the bedrock is always basalt, and the shapes and slopes are 

 characteristic of no other land-form. The only volcanic feature lacking in early 

 surveys of seamounts was a crater at the top. This occasioned much speculation. 

 However, the Precision Depth Recorder shows craters on many seamounts and 

 it is highly probable that the earlier types of sounders simply concealed the 

 crater in confused side echoes. 



The total number of seamounts (including guyots) shown on the distribution 

 charts is probably only a small fraction of the number that exists. The charts 



