Nevado del Ruiz has undergone three distinct eruptive periods, the first beginning 1.8 million years ago. During the present period (beginning 11,000 years ago), it has erupted at least twelve times, producing ashfalls, pyroclastic flows, and lahars. The historically recorded eruptions have primarily involved a central vent eruption (in the caldera) followed by an explosive eruption, then the formation of lahars. Ruiz's earliest identified Holocene eruption was in about 6660 BC, and further eruptions occurred around 1245, 850, 200 BC and in about 350, 675, in 1350, 1541 (perhaps), 1570, 1595, 1623, 1805, 1826, 1828 (perhaps), 1829, 1831, 1833 (perhaps), 1845, 1916, December 1984 through March 1985, 1987 through July 1991, and possibly in April 1994. Many of these eruptions involved a central vent eruption, a flank vent eruption, and a phreatic (steam) explosion. Ruiz is the second-most active volcano in Colombia after Galeras.