San Cristobal is an active stratovolcano (cone-shaped volcano made up of successive layers of ash and lava) in the northwest of Nicarugua. The peak 1745 m high, and has been active more-or-less continuously since about 1520 AD. The current volcano lies at the northwest end of a chain of older volcanoes known as the San Christobal Complex.
Map showing the volcanoes of Nicaragua; San Cristobal being the northwesternmost. United States Geological Survey.
On Saturday 8 September 2012 San Cristobal began to rumble and emit an increased level of ash and gas in the morning. This was followed by a series of explosions and the eruption of a 4 km high column of ash, estimated by the Nicaraguan Institute of Territorial Studies to contain about 3221 tonnes of ash. The Nicaraguan Army has evacuated nearby villages, and the volcano is being monitored closely.
Column of ash over San Cristobal on Saturday 8 September, seen from Chinandega. AFP/La Presna/Eddy Lopez.
Nicaragua, and neighbouring Central American nations, are located on the southern margin of the Caribbean Plate, close to the margin with the Cocos Plate, which underlies part of the eastern Pacific Ocean. The Cocos Plate is being subducted along the Middle American Trench, to the south of the Nicaraguan coast, and passes under Central America as it sinks into the Earth. As the plate is subducted it is partially melted by the friction and the heat of the planet's interior, giving rise to liquid magma which rises through the Caribbean Plate to fuel the volcanoes of Central America.
Map showing the margin of the Cocos and Caribbean Plates and the volcanoes of Central America. Scripps Institution of Oceanography.
See also Large Earthquake off the south coast of El Salvador, Eruption on Rincón de la Vieja, Costa Rica, Oaxaca region of Mexico struck by second major Earthquake in two weeks, Understanding the subduction zone beneath Mexico and Volcanoes on Sciency Thoughts YouTube.
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