Magnetic Effects in Volcanic Regions

Abstract

A study using Geomagnetic Deep sounding and Magnetotellurics was made of the Taupo Volcanic Zone in the North Island of New Zealand. The geomagnetic sounding measurements, were mainly made with vector proton magnetometers. A significant increase in the amplitude of vertical magnetic variations, of periods of 10 to 60 minutes, was found at a station on the Taupo Volcanic Zone, compared to stations to the west of the zone. Magnetotelluric measurements on and around the Taupo Volcanic Zone showed upper crustal resistivities of the order of 10Ωm under several parts of the Taupo Volcanic Zone. The deep sounding results indicated that the low resistivity areas were of considerable horizontal extent. The low resistivity regions were interpreted as being due to the combined effects of hydration and a higher than normal temperature in the crust, these being related to the volcanic and hydrothermal activity of the Taupo Volcanic Zone. In another part of the study, a series of total magnetic field measurements was made in the crater of White Island, an active volcano. The magnetic field changes were related to the volcanic activity and to measured level changes, and were shown to be due to thermal demagnetisation of the crater material.