Nevado del Ruiz is a 5231 m stratovolcano (cone shaped volcano made up of layers of lava and ash) in the Los Nevados National Natural Park, at the northern end of the Ruiz-Tolima Volcanic Massif in Columbia, about 129 km west of Bogotá. It has a track record of producing devastating pyroclastic flows (flows of hot gasses and ash) and lahars (flows of water mixed with volcanic material), and consequently is monitored closely by the Instituto Colombiano de Geología y Minería (INGEOMINAS).
Nevado del Ruiz. United States Geological Survey.
On 5 March 2012 INGEOMINAS reported a significant increase in seismic (Earthquake) activity on Nevado del Ruiz, which persisted till 11 March. Seismic activity is often a sign that magma is rising within volcanoes, which in turn may indicate that an eruption is going to happen. On 8 March a gas plume was seen rising 1.4 km above the summit. A visit to the summit reveal ash on glaciers near the summit, though this was thought to be due to an earlier eruption. Raised levels of sulphur dioxide (SO₂) and a thermal anomaly (hotspot within the mountain) were detected. Later that day a remote camera observed an explosion on the summit, and fresh ash was detected near the headwaters of the Gualí River, southwest of the summit.
In 1985 an eruption on Nevado del Ruiz produced 35 million tonnes of volcanic material, including 700 000 tonnes of sulphur dioxide. This triggered a series of lahars that swept northwest through a number of local communities, killing over 23 000 people.
Hazard Map for Nevado del Ruiz, showing the extent of the 1985 lahars. United States Geological Survey.
In 1845 a series of lahars followed the Lagunillas River for about 70 km, devastating river communities and killing over 1000 people. In 1595 a major eruption triggered a series of lahars that travelled over 100 km along the Gualí and Lagunillas Rivers killing over 600 people. Similar lahars today would kill tens of thousands of people.
The Ruiz-Tolima Volcanic Massif runs north-to-south parallel to the Parallel to the Pacific Coast of Columbia. It is fed by the subduction of the Nazca Plate beneath the South American Plate. As the Nazca Plate it sinks into the Earth's interior it is heated by the planet's interior heat. This melts the plate, some of which then rises through the overlying South American Plate, forming volcanoes in the Andes along the full extent of the South American west coast.