Nevado del Ruiz

The Nevado del Ruiz. also known as La Mesa de Herveo (English: Mesa of Herveo (the nearby town)), 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. The impact of such an eruption is increased as the hot gas and lava melts the mountain's snowcap, adding large quantities of water to the flow. 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 global 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.

Geography and geology

Nevado del Ruiz, which lies about 129 kilometers (80 mi) west of Bogotá, is part of the Andes mountain range. The volcano is part of the Ruiz–Tolima volcanic massif (or Cordillera Central), a group of five ice-capped volcanoes which includes the Tolima, Santa Isabel, Quindio and Machin volcanoes. The massif is located at the intersection of four faults, some of which still are active.

Nevado del Ruiz lies within the Pacific Ring of Fire, a region that encircles the Pacific Ocean and contains some of the world's most active volcanoes. It is the third most northerly of the volcanoes lying in the North Volcanic Zone of the Andean Volcanic Belt, which contains 75 of the 204 Holocene-age volcanoes in South America. The Andean Volcanic Belt is produced by the eastward subduction of the oceanic Nazca Plate beneath the South American continental plate. As is the case for many subduction-zone volcanoes, Nevado del Ruiz can generate explosive Plinian eruptions with associated pyroclastic flows that can melt snow and glaciers near the summit, producing large and sometimes devastating lahars (mud and debris flows).

Like many other Andean volcanoes, Nevado del Ruiz is a stratovolcano: a voluminous, roughly conical volcano consisting of many strata of hardened lava and tephra including volcanic ash. Its lavas are andesitic–dacitic in composition. The modern volcanic cone comprises five lava domes, all constructed within the caldera of an ancestral Ruiz volcano: Nevado El Cisne, Alto de la Laguna, La Olleta, Alto la Pirana, and Alto de Santano. It covers an area of more than 200 square kilometers (77 sq mi), stretching 65 kilometers (40 mi) from east to west. The mountain's broad summit includes the Arenas crater, which is one kilometer in diameter and 240 meters (790 ft) deep.

The summit of the volcano has steep slopes inclining from 20 to 30 degrees. At lower elevations, the slopes become less steep; their inclination is about 10 degrees. From there on, foothills stretch almost to the edge of the Magdalena River north of the volcano and the Cauca River to the west. On the two major sides of the summit, headwalls show where past rock avalanches occurred. At times, ice on the summit has melted, generating devastating lahars, including the continent's deadliest eruption in 1985. On the volcano's southwest flank is the pyroclastic cone La Olleta, which is not currently active, but may have erupted in historical times.

Eruption

At 3:06 pm, on November 13, 1985,[31] Nevado del Ruiz erupted, ejecting dacitic tephra more than 30 kilometres (19 mi) into the atmosphere. The total mass of the erupted material (including magma) was 35 million tonnes—only 3% of the amount that erupted from Mount St. Helens in 1980. The eruption reached a value of 3 on the Volcanic Explosivity Index. The mass of the ejected sulfur dioxide was about 700,000 tonnes, or about 2% of the mass of the erupted solid material, making the eruption atypically sulfur-rich.

The eruption produced pyroclastic flows that melted summit glaciers and snow, generating four thick lahars that raced down river valleys on the volcano's flanks. It also destroyed a small lake that was observed in Arenas crater several months before the eruption. Water in such volcanic lakes tends to be extremely salty and contain dissolved volcanic gases. The lake's hot, acidic water significantly accelerated the melting of the ice; this effect was confirmed by the large amounts of sulfates and chlorides found in the lahar flow.