VARIATION OF MAGNETIZATION ON WHITE-ISLAND, NEW-ZEALAND

Measurements of the magnetisation of andesitic rocks from White Island have been combined with an interpretation of an aeromagnetic survey o f the island to show the high variability of magnetisation in the rock s. Sampling of the rocks on White Island shows that, as expected, fres h andesitic lavas are highly magnetised (up to 20 A/m) whilst the phre atomagmatic deposits and altered andesites are weakly magnetised (<1 A /m). Most of the island is composed of layers of differing thicknesses of phreatomagmatic deposits and andesitic flows, so that the apparent magnetisations of the rocks, determined from the aeromagnetic survey, lie between 2.0 and 10.0 A/m. Highly magnetic rocks are concentrated in die floor of the crater exposed to the sea in the east of the islan d. These rocks, with a measured average magnetisation of >10 A/m, occu r 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 t he 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 D onald Mound. The highly magnetised rocks within the crater are probabl y fresh andesitic flows and sills. Older andesite within the crater ha s probably been altered by the corrosive volcanic gases. On the northw estern coast of the island, magnetic anomalies indicate magnetisations >10 A/m that could be associated with rocks of the Western Cone.