Sarigan is roughly triangular in shape, with a length of and a width of and an area of . The island is the summit of a stratovolcano which rises to an altitude of above sea level at its highest peak. The volcano is topped by a caldera, 750 meters in diameter, with an ash cone and two lava domes, which produced lava flows which reached the coast. No eruptions have been recorded in the historical period, although a swarm of volcano-tectonic earthquakes took place here in the summer of 2005.