2008 年 117 巻 5 号 p. 894-900
A geomagnetic and topographic survey of the Main Crater Lake (MCL) on Volcano Island, Taal volcano, the Philippines, was conducted to identify the eruption mechanism of the volcano. A mound at the lake bottom was found near the northern part of MCL, which was not present at the 1986 survey. Called mound M, it was formed between 1986 and 2008 during a period of new fumarole activity, which started on the northeastern shore of MCL in the early 1990's. A geomagnetic survey found no Total Magnetic Field (TMF) anomaly corresponding to the mound, which implies that mound M is non-magnetic. The site coincides with the location of a high-temperature area detected by the ASTER satellite during the early 2005 volcanic crisis. The mound M might contain chemicals (chloride or sulphide?) deposited from volcanic gas. Two continuously-recording proton magnetometers were set up at two geothermal areas on Volcano Island. Recordings of TMF variations obtained over six months on Volcano Island showed excellent stability and precision for monitoring the volcanic activity.
