The Asama volcano (elevation 2568m), central Japan, erupted on September 1, 2004. Since then thick volcanic fumes prevented monitoring of the topography on the summit crater floor by standard optical methods. To detect geomorphic changes of the volcano, Airborne Synthetic Aperture Radar (AirSAR) measurements including interferometry were repeatedly carried out on September 16, October 22, December 15, 2004 and March 10, 2005. The comparison of AirSAR result in September with a digital elevation model (DEM) by laser profiling measurements in October 2003 revealed that a pancake shape lava mound (volume=0.9×106m3) was formed on the floor. The subsequent AirSAR measurement on October 22 indicated that the pancake shape mound had disappeared and a small pit hole was formed. The volume of ejecta amounted to 2.1×106m3. The AirSAR results in December 2004 and March 2005 suggested that the shape of the crater floor stayed generally unchanged during that time window. From these, we infer that lava extrusion during the 2004 Asama eruption occurred within a short period around September and October.
