Abstract

The mean coercivities of natural (n.r.m.), anhysteretic (a.r.m.), and isothermal (i.r.m.) remanent magnetization in 34 samples of submarine basalt from the Mid-Atlantic Ridge are 270, 250, and 370 oersteds respectively, showing their magnetization to be stable. The spectra of coercive force of n.r.m. and a.r.m. of these basalts are similar, although, because of small secondary components, there are small differences in the low coercive force range (0 to 100 Oe). The magnitude and sign of these differences are used to derive an empirical relationship between coercivity and the magnitude of secondary components, and to infer the polarity of primary components. The inferred polarities of samples from within the Median Valley of the Ridge are all normal, whereas both normal and reversed polarities are found from the adjacent mountains and plateaus. Experiments show that warming specimens from the Median Valley to 100 °C for 90 h causes the production of a new component with a lower intensity, and higher blocking temperature, but with the same direction as the original natural remanence. Similar effects may occur in the axial volcanic zone, and may have caused the decrease in intensity with distance from the axis. The magnetic properties of various other rock types from the Mid-Atlantic Ridge are also described.

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