The Nevado del Ruiz, also known as La Mesa de Herveo (English: Table of Herveo) or Kumanday in the language of the local pre-Columbian indigenous people, is a volcano 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 epoch, with three major eruptive periods. The current volcanic cone formed during the "present" eruptive period, which began 150 thousand years ago.

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. 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.
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, although these have retreated significantly since 1985 because of atmospheric warming. 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.

Information based on http://en.wikipedia.org/wiki/Nevado_del_Ruiz