The seamount shown in figure 29C rises to a sharp peak at 

 540 fathoms. This symmetrical feature was discovered 

 220 miles west of Alijos Rocks off Baja California. Its slightly- 

 irregular and upwardly concave sides have an average slope 

 angle of about 12 degrees. The terrace which is present on the 

 north slope at about 800 fathoms may be the result of wave 

 erosion, but this is very questionable. Echo extension sug- 

 gests that this terrace is of rather limited horizontal extent 

 normal to the recorded profile. 



The largest seamount of all was discovered in the south- 

 west Pacific about 1000 miles west of New Zealand in a 

 sparsely sounded region (see fig. 30A). This feature rises 

 from a sea floor depth of 27 00 fathoms to a minimum depth 

 of 600 fathoms. The extensive summit area is irregular and 

 there is little evidence of wave truncation. The north slope 

 has a declivity of 14 degrees as compared with 21 degrees 

 for the south slope. Both slopes are slightly concave. 



Another seamount (fig. 29D) was crossed near Hiva Oa in 

 the Marquesas Islands. The presence of this seamount is 

 suggested by soundings on published charts. It is reproduced 

 here because it is located in a region where volcanic sediments 

 have been found and because it is associated with the Marque- 

 sas Islands Atolls which are believed to be built on the sum- 

 mits of submarine volcanoes. For both of these reasons, it 

 is quite certain that this feature is an extinct submarine 

 volcano; therefore this seamount is useful for establishing 

 criteria by which submarine volcanoes can be identified 

 from their profiles. Especially noteworthy is the concave 

 form of the flanks. The steeper northern side has an average 

 declivity of 13 degrees with a maximum slope of 21 degrees 

 near the summit. The somewhat gentler south flank is marked 

 by a series of pinnacles. A plateau extends to the north of the 

 seamount. 



All of the seamounts that were observed have an appear- 

 ance which suggests that they are of volcanic origin. Such an 

 origin is suggested by their simplicity and symmetry of form, 

 by their slightly concave slopes, and by the average angle of 

 slope in excess of 10 degrees and not exceeding 21 degrees. 

 In addition, the seamounts off Baja California are located at 

 no great distance from the volcanic Guadalupe Island and in 

 a region of known volcanicity. Yet it is possible that the sea- 

 mount shown in figure 29B is a south-facing fault escarpment 

 rather than a volcanic mass. The slight asymmetry and the 

 straightness of the south slope suggests this alternate inter- 

 pretation. 



58 



