<Tr> <Th> First ascent </Th> <Td> 1936 by Cunet and Gansser </Td> </Tr> <P> The Nevado del Ruiz (Spanish pronunciation: (neβaðo ðel ˈrwis)), also known as La Mesa de Herveo (English: Mesa of Herveo, the name of the nearby town), or Kumanday in the language of the local pre-Columbian indigenous people, is located on the border of the departments of Caldas and Tolima in Colombia, about 129 kilometers (80 mi) west of the capital city Bogotá . It is a stratovolcano composed of many layers of lava alternating with hardened volcanic ash and other pyroclastic rocks . Nevado del Ruiz has been active for about two million years, since the Early Pleistocene or Late Pliocene, with three major eruptive periods . The current volcanic cone formed during the present eruptive period, which began 150 thousand years ago . </P> <P> The volcano usually generates Plinian eruptions, which produce swift - moving currents of hot gas and rock called pyroclastic flows . These eruptions often cause massive lahars (mud and debris flows), which pose a threat to human life and the environment . The impact of such an eruption is increased as the hot gas and lava melt the mountain's snowcap, adding large quantities of water to the flow . On November 13, 1985, a small eruption produced an enormous lahar that buried and destroyed the town of Armero in Tolima, causing an estimated 25,000 deaths . This event later became known as the Armero tragedy--the deadliest lahar in recorded history . Similar but less deadly incidents occurred in 1595 and 1845, consisting of a small explosive eruption followed by a large lahar . </P> <P> The volcano is part of Los Nevados National Natural Park, which also contains several other volcanoes . The summit of Nevado del Ruiz is covered by large glaciers . The volcano continues to pose a threat to the nearby towns and villages, and it is estimated that up to 500,000 people could be at risk from lahars from future eruptions . </P>

What type of eruption does nevado del ruiz produce