SECT. 1] 



THE MAGNETIC FIELD OVER THE OCEANS 



193 



diameter; it is probably because of the small grain size of the magnetic minerals 

 that these rocks are so magnetic. Since rapid cooling encourages fine grain size, 

 submarine lavas may be generally more magnetic than their continental 

 counterparts. The occurrence of maghemite is interesting. Maghemite is a 

 highly magnetic mineral believed to be generally formed from magnetite, either 

 by direct oxidation in a low oxygen environment or via hydrated compounds, 

 possibly by hydrothermal solutions at relatively low temperatures and pres- 

 sures (Mason, 1943). It is metastable and at higher temperatures and pressures 

 it is changed irreversibly to haematite (a-re203); it is possible that a deep- 

 ocean environment favours oxidation of magnetite to maghemite rather than 

 to the less magnetic haematite. 



In common with other ferromagnetic materials, a rock becomes non-magnetic 

 when heated to a temperature exceeding its Curie jDoint. The latter varies from 

 rock to rock, depending on the magnetic minerals involved, but is generally 



60 r 



MULL 



MENDOCINO 



0.2 I 5 25 125 625.10"^ 



DISTRIBUTION OF APPARENT SUSCEPTIBILITIES 



Fig. 10. The distribution of apparent susceptibilities of (a) basalts dredged from the 

 Mendocino escarpment and (b) basaltic lavas from Mull. (From R. G. Mason, un- 

 published.) 



less than 575°C. Since temperatures within the earth increase with depth, 

 there will exist everywhere a surface of magnetic discontinuity corresponding 

 to the Curie point isotherm. There is some uncertainty as to the depth at which 

 the Curie point might be reached ; on the basis of present information it seems 

 reasonable to assume that it will be between 20 and 50 km below the sea floor, 

 and, therefore, well below the base of the oceanic crust. Lateral variations in its 

 de]jth will lead to anomalies in the magnetic field, which, therefore, provide a 

 means for studying the thermal state of the upper mantle. 



6. The Magnetic Surveys of the North-east Pacific 



Much of the magnetic data obtained at sea is in the form of long unrelated 

 profiles recorded during passage of a ship. These are of limited usefulness 



