The broad isthmus between Lagonoy Gulf and San Miguel Bay in SE Luzon is occupied by the isolated Mount Isarog volcano. The 1966-m-high andesitic stratovolcano is truncated by a 2.5-km-wide crater that is breached to the east along a narrow valley drained by the Quinarag River. A major debris avalanche deposit extends northwest to the coast and into San Miguel Bay. Pyroclastic flows originating from lava dome collapses have occurred during the Holocene, one of which has a calibrated radiocarbon date of about 5500 years Before Present. Currently the Maalsom vent displays gas seepages, warm springs, and steaming vents.