The Nevado del Ruiz , also known as La Mesa de Herveo 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. The current volcanic cone formed during the present eruptive period, which began 150 thousand years ago.

During the past 11,000 years, Nevado del Ruiz passed through at least 12 eruption stages, which included multiple slope failures (rock avalanches), pyroclastic flows and lahars leading to partial destruction of the summit domes. During the past several thousand years, eruptions of the volcanoes in the Ruiz–Tolima massif have mostly been small, and the pyroclastic flow deposits have been much less voluminous.

The volcano continues to pose a serious threat to nearby towns and villages. The most likely hazard is small-volume eruptions, which might destabilize glaciers and trigger lahars. Despite the significant retrenchment of the volcano's glaciers, the volume of ice atop Nevado del Ruiz and other volcanoes in the Ruiz–Tolima massif remains large. Melting merely 10% of the ice would produce mudflows with a volume of up to 2,000,000 cubic meters (70,629,333 cu ft).

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