Magnetic field changes on White Island, New Zealand, and the value of magnetic changes for eruption forecasting

For about 15 years after their commencement in 1968, magnetic measurements in the crater of White Island gave a good indication of the state of activity and the likelihood of an eruption episode commencing. During this period, magnetic, thermal, ground level and chemical measurements were all generally consistent with a model in which these surface changes were caused by increasing temperatures under the Donald Mound area, in the central sub-crater. In more recent times, there has not been such a relationship between the changes around the remnants of Donald Mound and the volcanic activity, which has become localised in the west sub-crater. It appears that changes under the crater, especially lowering of the ground surface and the water table at the west end of the crater, have meant that eruptions are no longer preceded by heating under Donald Mound. The west sub-crater is not easily accessible for repeat surface surveys. One way in which we might be able to monitor future volcanic activity at White Island would be by repeated aeromagnetic surveys, as the magnetic changes are so large that changes of hundreds of nanoTeslas are likely to be observed at practical flying heights.