The eruptive history of San Vicente volcano is not well known, and there is no definitive record of historical eruptive activity. The last significant eruption occurred more than 1,700 years ago, and perhaps long before permanent human habitation of the area. Nevertheless, this volcano has a very long history of repeated, and sometimes violent, eruptions, and at least once a large section of the volcano collapsed in a massive landslide. The oldest rocks associated with a volcanic center at San Vicente are more than 2 million years old. The volcano is composed of remnants of multiple eruptive centers that have migrated roughly eastward with time. Future eruptions of this volcano will pose substantial risk to surrounding communities.
Volcanic eruptions are not the only events that present a risk to local communities. Another concern is a landslide and associated debris flow (a watery flow of mud ,rock, and debris -- also known as a lahar) that could occur during periods of no volcanic activity. An event of this type occurred in 1998 at Casita volcano in Nicaragua when 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 2,000 people. Historical landslides up to a few hundred thousand cubic meters in volume have been triggered at San Vicente volcano by torrential rainstorms and earthquakes, and some have transformed into debris flows that have inundated villages down stream. For example, a debris flow in 1934 on the north side of San Vicente destroyed the village of Tepetitan. Destructive rainfall-and earthquake-triggered landslides and debris flows on or near San Salvador volcano, west of San Vicente, in September 1982 and January 2001 demonstrate that such mass movements in El Salvador have also been lethal.
This report describes the kinds of hazardous events that occur at volcanoes in general and the kinds of hazardous geologic events that have occurred at San Vicente volcano in the past. The accompanying volcano-hazards-zonation map shows areas that are likely to be at risk when hazardous events occur again.
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