Variation of magnetisation on White Island, New Zealand

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Abstract
Measurements of the magnetisation of andesitic rocks from White Island have been combined with an interpretation of an aeromagnetic survey of the island to show the high variability of magnetisation in the rocks. Sampling of the rocks on White Island shows that, as expected, fresh andesitic lavas are highly magnetised (up to 20 A/m) whilst the phreatomagmatic deposits and altered andesites are weakly magnetised (10 A/m, occur in three settings. They form a ring around the outside of the crater floor; there is a group of them between Shark Bay and Troup Head in the east of the island; and there are two areas of strong magnetisation associated with the active crater and a region c. 100 m due east of Donald Mound. The highly magnetised rocks within the crater are probably fresh andesitic flows and sills. Older andesite within the crater has probably been altered by the corrosive volcanic gases. On the northwestern coast of the island, magnetic anomalies indicate magnetisations >10 A/m that could be associated with rocks of the Western Cone.

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