A study of the volcano-magnetic effect on White Island volcano, Bay of Plenty, New Zealand

Abstract

Previous study of magnetic field changes in the crater of White Island since 1968 (Hurst and Christoffel 1993, Christoffel 1989) has shown that changes have occured of unprecedented magnitude which indicate some correlation with eruptive activity. For this thesis a more comprehensive analysis of available data has been undertaken. Data from 61 magnetic surveys of the crater of White Island volcano between May 1968 and July 1993 were collected for Analysis, principally using contour maps of magnetic change and plots of the cumulative magnetic change at particular sites. This has shown that for most eruptions or series of eruptions detectable magnetic changes have occured, often with significant precursors. Close correlations exist between magnetic changes near the Donald Mound area of fumarolic activity and changes in the fumarole temperatures, fumarole chemistry, ground temperature and level survey data. Obvious inverse correlations occur between the magnetic changes to the north and south of the area of fumarolic activity, suggesting that changes in magnetisation has been occuring beneath the fumaroles at relatively shallow depths. These results confirm the mechanism for the magnetic changes proposed by Hurst and Christoffel (1973); that is thermal demagnetisation of rock by the hydrothermal system. Thermal demagnetising rock samples indicated that most change in magnetisation occured between 475 and 575°C. Assuming a spherical model for the source of the magnetic changes, the magnitude of the changes in magnetisation were calculated and were found to be small enough (with one possible exception) to have been produced by the hydrothermal system under observed conditions.