Santo Tomas is a stratovolcano with numerous volcanic vents and fissures (Philippines Commission of Volcanology, 1981). There was a report of an eruption or "displacement" in the region on 4 January 1641, described as an outburst of water accompanied by an earthquake. That is also the date of the major eruption of Parker volcano in southern Mindanao that impacted much of the Philippines. The Asin Hot Springs are about 12 km NNW of the summit, but their connection to the volcano is unknown.