Abstract
The Sukawa lahar deposits (L1-L7), at the western foot of Adatara volcano, are made up of clay-rich matrix-supported massive breccia containing abundant hydrothermally altered andesite. The lowest L1 deposit extends 10 km from the Numanotaira crater, has a volume of 8.4 × 106 m3, and covers an area of 2.8 km2. This lahar occurred at 2400 y. B. P. and was presumably generated by a break of the Numanotaira crater lake during a phreatic eruption. The L2-L7 deposits, whose individual volumes are less than 4 × 105 m3, fill a channel incising the L1 deposit. These lahars took place at 1600 to 860 y. B. P. The grain-size data and the ratio of drop to travel distance (H/L) indicate that the Sukawa lahars are typical cohesive lahars. The Numanotaira crater is now filled by muddy slurry, and opens directly to the Iwo-Sukawa River on the western flank. The possible cohesive lahars along the Iwo-Sukawa River should be added to the potential hazard of this volcano.
