Irruputuncu is a small stratovolcano that straddles the Chile/Bolivia border. It is the youngest and most southerly of a NE-SW-trending chain of volcanoes. It was constructed within the collapse scarp of a Holocene debris avalanche whose deposit extends to the SW. Subsequent eruptions filled much of this scarp and produced thick, viscous lava flows down the W flank. The summit complex contains two craters, the southernmost of which is fumarolically active. The first unambiguous historical eruption took place in November 1995, when phreatic explosions produced dark ash clouds.