SOME PACIFIC AND ANTARCTIC SEA FLOOR FEATURES 



General . During Operation HIGHJUMP the USS HENDER- 

 SON (DD785), under the command of Commander C. F. Bailey, 

 made continuous soundings along her track with a model NMC 

 Echo Sounder using 17-kc. sound pulses. Soundings less than 

 2000 fathoms were recorded automatically on a tape; soundings 

 made at greater depths were obtained aurally every hour. 

 Because almost all of the ocean bottom that was traversed is 

 deeper than Z000 fathoms, a detailed recorded profile was 

 obtained of only a few of the positive features such as tops 

 of seamounts and escarpments. Features displayed on the 

 HENDERSON fathogram that are of special geological 

 interest have been reproduced and are discussed in this 

 report. 



Seamounts. A number of new seamounts were discovered 



during the San Diego-to-Antarctic passage. Four of the 

 seamounts were found in the abyssal ocean off Baja California; 

 an especially large one was discovered in the southwest 

 Pacific off New Zealand. 



The two symmetrical seamounts shown in figure 29A were 

 located about 300 miles southwest of Allaire Bank off Baja 

 California. The larger seamount rises to a sharp peak at 

 about 860 fathoms; the smaller one rises to a peak at 1400 

 fathoms. The sides of these seamounts are fairly steep, 

 having a maximum average slope angle of 19 degrees.* 

 Extension of the bottom echo when crossing the seamounts 

 indicates steep slopes parallel to the ship's track and suggests 

 that the vessel "sideswiped" the seamounts rather than 

 passing directly over their highest peaks. 



The seamount shown in figure 29B was discovered off 

 Baja California about 130 miles northwest of Allaire Bank. 

 This one rises 1400 fathoms from the abyssal sea floor to a 

 peak at 900 fathoms. The north side of this feature is irregu- 

 lar and concave and has a slope of 12'degrees; the south side 

 is straight and featureless and has a slope of 16 degrees. 

 An examination of the pinnacle summit of the seamount reveals 

 no evidence either of terracing or of truncation. 



*In this report the apparent slope angles are corrected, 

 assuming a 30-degree effective half-angle of the sound beam. 

 In other words, the sound beam is assumed to be semidirec- 

 tional. However, the effective half-angle of the beam cannot 

 be accurately determined since it varies with many factors 

 such as gain setting and the roll of the ship. This correction 

 is significant only for slopes greater than 15 degrees and 

 the true slope is always greater than the corrected slope. 

 The corrected slope is obtained by correcting the fathometer- 

 tape slope for vertical exaggeration. 



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