. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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 hydroth
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 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. I 5 25 125 ;^ 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 t
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