Nevado del Ruiz is a massive, glacier-covered volcano in central Colombia covering 200 square kilometres, with glacial ice 50 metres thick on average and up to 190 metres thick on parts of the summit plateau and under the Nereides Glacier. On the one hand, the steady release of meltwater from its glaciers provides fresh water for the Cauca and Magdalena rivers, forty surrounding towns, and up to three million people for drinking water and agriculture; on the other hand, water released too quickly can become a major problem. This occurred in 1985 when a volcanic eruption caused massive amounts of glacial ice to melt at once, causing colossal mudslides (or lahars) that can travel up to 100km. Although these mud flows have been known since the 16th century, the 1985 eruption caused a devastating mud flow that killed almost 25,000 people in the town of Armero, marking one of the worst volcanic disasters in history.
From a climate change point of view, rapid melting means a reduction of the glaciers at a rate of 25 metres a year, which has scientists and government officials concerned, because if the glaciers continue to retreat at such a rate, there will be a major impact on water supply for people and for the environment in the future.