A PARADOX IN VOLCANOMAGNETISM - DISAGREEMENT BETWEEN ANALYTICAL AND NUMERICAL ESTIMATES OF GEOMAGNETIC CHANGES DUE TO AN UNDERGROUND PRESSURE NUCLEUS

The reason why numerical and analytical estimates of geomagnetic changes due to an underground pressure nucleus lead to considerably different results has been an unsolved enigma in volcanomagnetism. In particular, the two estimates manifest completely different characteristics of geomagnetic change when the depth of the Curie point isotherm is taken as a parameter even though they are based on the same stress field produced by a point source of dilatation. This paper aims to clarify the reason for this disagreement by using a numerical approach to evaluate changes in the surface geomagnetic field produced by stress-induced magnetization inside a pressurized sphere. The comparison of two numerical models subjected to the same stress field made it clear that the disagreement between numerical and analytical models is caused by the stress-induced magnetization inside the source sphere. The analytical estimate made so far cannot be applied to a case for which the interior of the sphere is at high temperature. It is still applicable, however, to a case in which the temperature inside an inflation sphere is below the Curie point.