192 BULLAKD AND MASON [CHAP. 10 



5. The Magnetic Properties of Submarine Rocks 



The geological interpretation of magnetic measurements depends on the 

 large differences that exist between the magnetic properties of different types 

 of rock. Basic rocks in particular tend to be more magnetic than others, and 

 any situation involving local excesses or deficiencies of basic rocks will lead to 

 anomalies in the earth's magnetic field. Little information has been published 

 on the magnetic properties of submarine rocks ; this is partly because of the lack 

 of representative material to study. 



In the interpretation of marine surveys it is often found necessary to take 

 larger values for susceptil)ility than those given for continental basalts (by, for 

 example, L. B, Slichter, in Birch et al., 1942). This is justified on the grounds that 

 basic igneous rocks commonly possess a permanent, or remanent, magnetization 

 several times greater in intensity than that induced in the earth's field ; if the 

 remanent magnetization is approximately aligned in the direction of the field, 

 then the rock possesses an apparent susceptibility several times greater than 

 the measured value. Thus susceptibilities ranging up to 0.015 e.m.u. have been 

 used for basalts. At the other extreme, the unconsolidated sediments of the sea 

 floor are generally thought to be too weakly magnetized to j)roduce measurable 

 anomahes at the surface. 



A recent study of some basalts dredged from the escarpment of the Mendocino 

 Fault in the north-east Pacific (R. G. Mason, unpublished) has revealed excep- 

 tionally high intensities of remanent magnetization, ranging up to 0.3 e.m.u./ 

 cm^, and correspondingly large ratios of remanent to induced magnetization. 

 The former, expressed in terms of apparent susceptibility, range from 0.003 to 

 0.6 with a median value of 0.035, the latter from 2 to 400, with a median value of 

 20. The results are presented in Fig. 10 as a histogram which shows, for com- 

 parison, the results of an investigation of more than 600 lavas from INIull. The 

 corresponding figures for the Mull lavas are 0.001 to 0.5, with a median value of 

 0.010, for apparent susceptibility, and to 130, with a median value of 3, for the 

 ratio of remanent to induced magnetization. The lavas from the Mendocino 

 Fault are thus, on an average, more than three times as magnetic as those from 

 Mull. 



Not all submarine lavas are so highly magnetic. Much of the material obtained 

 in dredge hauls is badly weathered and, as might be expected, is considerably 

 less magnetic than the average unweathered rock. Matthews (1961) has found 

 apparent suscej)tibilities ranging from 0.002 to 0.025, with a median value of 

 0.010, for weathered basalts from the eastern North Atlantic; similar results 

 have been obtained for weathered basalts from the Pacific. 



The relation between the magnetic properties of the Mendocino Fault 

 basalts and their mineralogy has been investigated by J. M. Hall (unpublished). 

 In genera], intensities of remanent magnetization increase with decrease in 

 grain size of the magnetic minerals, particularly in the range 50 [jl-5 [i. With the 

 exception of the most magnetic of the si)ecimens examined, which is rich in 

 maghemite (y-Fe203), these are mostly impure magnetites less than 50 [i in 



