San Miguel volcano is one of the most active volcanoes in El Salvador; it has erupted at least 29 times since 1699. Historical eruptions of the volcano consisted mainly of relatively quiescent emplacement of lava flows or minor explosions that generated modest tephra falls (erupted fragments of microscopic ash to meter sized blocks that are dispersed into the atmosphere and fall to the ground). Little is known, however, about prehistoric eruptions of the volcano. Chemical analyses of prehistoric lava flows and thin tephra falls from San Miguel volcano indicate that the volcano is composed dominantly of basalt (rock having silica content <53%), similar to the lava erupted by Hawaiian volcanoes. The chemical composition of eruptive products and the lack of evidence of large cataclysmic eruptions suggests that prehistoric eruptions probably were similar in nature to the historical eruptions. Unlike San Salvador and San Vicente volcanoes, San Miguel volcano does not appear to have had a history of violent explosive eruptions.
Volcanic eruptions are not the only geologic events that present hazards to local communities. Landslides and associated debris flows (watery flows of mud, rock, and debris -- also known as a lahar when they occur on a volcano) that can occur during periods of no volcanic activity are also of concern. In 1998 at Casita volcano in Nicaragua, extremely heavy rainfall from Hurricane Mitch triggered a landslide that moved down slope and transformed into a rapidly moving debris flow that destroyed two villages and killed more than 2000 people. Torrential rains at San Miguel volcano in 1988, 1994, 1999 and 2000 triggered debris flows (lahars) on the northwestern slopes of the volcano, and they traveled downslope and damaged the main road that leads to San Jorge. Although modern landslides and lahars at San Miguel thus far have caused only minor loss of property and have been primarily a short-term inconvenience,destructive rainfall-and earthquake-triggered landslides and debris flows on or near San Salvador volcano in September 1982 and January 2001, and at San Vicente volcano in September 2001, demonstrate that such mass movements in El Salvador can also be lethal.
This report describes the hazards of landslides and lahars in general, and discusses potential hazards from future landslides and lahars at San Miguel volcano in particular. The report also shows, in the accompanying lahar-hazard-zonation map, which areas are likely to be at risk from future landslides and lahars at San Miguel. A report by Chesner (2000) focuses on a broader volcanic-hazard assessment of the volcano.
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