The summit eruption of Unzen volcano started at Jigokuato (J) and Kujukushima (K) craters on November 17, 1990, after 198 years quiescence. The volcanic activity intensified at Byobuiwa (B) crater, about 150 m west of J, on February 12, 1991, and a new dome was extruded at J on May 20, 1991. Seven domes were extruded until early summer 1999. Preceding the major volcanic activity, a geomagnetic change of about 20 nT was detected by repealed surveys at the summit during the period of 1986 to the end of 1990. To elucidate the cause of the geomagnetic change, closely placed and continuous geomagnetic observations with proton magnetometers have been conducted near the summit of the volcano since January 1991. In association with the volcanic activity at B, geomagnetic changes amounting to about +18 and -5 nT were observed at close points in the northern and southern directions of B, respectively, from February to March 1991. During the early stages of the new dome extrusion, large geomagnetic changes of more than 80 nT were simultaneously observed at several points near J from May 12 to June 3, 1991. The changing rate of about 5 nT/day at mid-May 1991 decreased exponentially to about 0.1 nT/day by the end of 1991. The total change reached to more than 160 nT at three points closer than 400 m from J from early March 1991 to the end of June 1992, although it was only 10 nT at a point of about 500 m south of J. Efforts to clarify the causes of each geomagnetic change resulted in no systematic simple model through this eruption sequence. These magnetic changes were probably the result of many processes such as thermal demagnetization, removal effects of magma intrusion, strain/stress, rotation and displacement of magnetic materials and perhaps also electrokinetic effects.
