DESCRIPTION:
El Salvador Volcanoes and Volcanics

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Map, Major Volcanoes of El Salvador

Volcanism has had important impacts on the region. As was learned with the 1982 eruption of El Chichon (Mexico), (and the even larger ca.260 A.D. eruption of Ilopango, in El Salvador), the region produces large explosive eruptions. ...

Most active volcanoes in this region occur in belts produced by subduction of Pacific oceanic crust beneath the southern edge of the North American Plate and the western edge of the Caribbean Plate. Large stratovolcanoes and silicic calderas are found here, but the region also contains many basaltic volcanic fields, particularly in the central valley of Mexico and along the Guatemala-El Salvador border. A few other active volcanoes in northern Mexico are related to extensional tectonics of the Basin and Range Province, which split the Baja California peninsula from the mainland.

Coatepeque Caldera is part of the chain of volcanoes resulting from Cocos-Caribbean subduction. Coatepeque lies astride the boundary between the eastern Guatemala and El Salvador segments of that subduction (Stoiber and Carr, 1973).

Coatepeque Caldera lies at the eastern foot of Volcaán Santa Ana and probably formed after Santa Ana had grown to nearly its current size (Carr and Pontier, 1981). Several rhyolitic and dacitic domes grew within the caldera, and several basaltic and andesitic vents developed just south and west of Coatepeque. V. Santa Ana, V. Izalco, and V. San Marcelino are part of the same volcanic center as Coatepeque Caldera, although they lie just outside the caldera boundary.

Santa Ana was particularly active during the late 1500's and again in the late 1800's. San Marcelino last erupted in 1772. Izalco, presently about 200 meters high, began to grown in (or before ?) 1770.

Ilopango is an elongate caldera bounded on the north and south by faults of a graben that parallels the volcanic chain throught El Salvador and parts of Guatemala and Nicaragua.

The Ilopango Caldera formed as a result of the Tierra Blanca eruption of 260 A.D. (volume of pyroclastic deposits = 10-20 cubic kilometers DRE) (Williams and Meyer-Abich, 1955; Sheets, 1979; Hart and Steen-McIntyre, 1983). Because Ilpango's shape is strongly controlled by regional faults, Williams and McBirney (1979) cite it as an example of a "volcano-tectonic depression". Islas Quemadas ("burnt islands") is a dacite dome that grew through Lake Ilopango in 1880.

The symmetrical cone of San Miguel volcano, one of the most active in El Salvador, rises from near sea level to form one of the country's most prominent landmarks. A broad, deep crater that has been frequently modified by historical eruptions, recorded since the early 16th century, caps the truncated summit of the basaltic volcano. Radial fissures on its flanks have fed a series of fresh lava flows, including several erupted during the 18th and 19th centuries that reached beyond the base of the volcano on the north, west, and southeast sides.

San Miguel volcano, also known as Chaparrastique, is one of many volcanoes along the volcanic arc in El Salvador. The volcano, located in the eastern part of the country, rises to an altitude of about 2,130 meters and towers above the communities of San Miguel, El Transito, San Rafael Oriente, and San Jorge. In addition to the larger communities that surround the volcano, several smaller communities and coffee plantations are located on or around the flanks of the volcano, and the PanAmerican and coastal highways cross the lowermost northern and southern flanks of the volcano. The population density around San Miguel volcano coupled with the proximity of major transportation routes increases the risk that even small volcano-related events, like landslides or eruptions, may have significant impact on people and infrastructure.

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.

The massive compound San Salvador volcano (1,893 meters) dominates the landscape west of the country's capital city of San Salvador. The Boqueron stratovolcano has grown within a 6-kilometer-wide caldera formed by collapse of the older Picacho and Jabali volcanoes. Three fracture zones that extend beyond the base of San Salvador volcano have been the locus for numerous flank eruptions, including two that formed maars on the WNW and SE sides. The summit of Boqueron is truncated by a steep-walled crater 1.5 kilometers wide and about 500 meters deep. It contained a crater lake prior to an eruption during 1917 that formed a small cinder cone on the crater floor. Most of the four historical eruptions recorded since the 16th century have originated from flank vents, including two in the 17th century from the NW-flank cone of El Playon, during which explosions and a lava flow damaged inhabited areas.

San Salvador volcano is one of many volcanoes along the volcanic arc in El Salvador. This volcano, having a volume of about 110 cubic kilometers, towers above San Salvador, the country's capital and largest city. The city has a population of approximately 2 million, and a population density of about 2,100 people per square kilometer. The city of San Salvador and other communities have gradually encroached onto the lower flanks of the volcano, increasing the risk that even small events may have serious societal consequences. San Salvador volcano has not erupted for more than 80 years, but it has a long history of repeated, and sometimes violent, eruptions. The volcano is composed of remnants of multiple eruptive centers, and these remnants are commonly referred to by several names. The central part of the volcano, which contains a large circular crater, is known as El Boquerón, and it rises to an altitude of about 1,890 meters. El Picacho, the prominent peak of highest elevation (1,960 meters altitude) to the northeast of the crater, and El Jabali, the peak to the northwest of the crater, represent remnants of an older, larger edifice. The volcano has erupted several times during the past 70,000 years from vents central to the volcano as well as from smaller vents and fissures on its flanks. In addition, several small cinder cones and explosion craters are located within 10 kilometers of the volcano. Since about 1200 A.D., eruptions have occurred almost exclusively along, or a few kilometers beyond, the northwest flank of the volcano, and have consisted primarily of small explosions and emplacement of lava flows. However, San Salvador volcano has erupted violently and explosively in the past, even as recently as 800 years ago. When such eruptions occur again,substantial population and infrastructure will be at risk.

Volcanic eruptions are not the only events that present a risk to local communities. Another concern is a landslide and an 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 on San Salvador volcano by torrential rainstorms and earthquakes, and some have transformed into debris flows that have inundated populated areas down stream. Destructive rainfall-and earthquake-triggered landslides and debris flows on or near San Salvador volcano in September 1982 and January 2001 demonstrate that such mass movements in El Salvador have also been lethal.

San Vicente volcano, also known as Chichontepec, is one of many volcanoes along the volcanic arc in El Salvador. This composite volcano, located about 50 kilometers east of the capital city San Salvador, has a volume of about 130 cubic kilometers, rises to an altitude of about 2,180 meters, and towers above major communities such as San Vicente, Tepetitan, Guadalupe, Zacatecoluca, and Tecoluca. In addition to the larger communities that surround the volcano, several smaller communities and coffee plantations are located on or around the flanks of the volcano, and major transportation routes are located near the lowermost southern and eastern flanks of the volcano. The population density and proximity around San Vicente volcano, as well as the proximity of major transportation routes, increase the risk that even small landslides or eruptions, likely to occur again, can have serious societal consequences.

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 v ery 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.

Santa Ana, El Salvador's highest volcano (2,365 meters), is a massive stratovolcano immediately west of Coatepeque caldera. Collapse of the volcano during the late Pleistocene or early Holocene produced a massive debris avalanche that swept into the Pacific, forming the Acajutla Peninsula. Reconstruction of the volcano rapidly filled the collapse scarp. The broad summit of the volcano is cut by several crescentic craters, and a series of parasitic vents and cones have formed along a 20-kilometer-long fissure system that extends from near the town of Chalchuapa NNW of the volcano to the San Marcelino and Cerro Chino cinder cones on the SE flank. Historical activity, largely consisting of small-to-moderate explosive eruptions from both summit and flank vents, has been documented since the 16th century. The San Marcelino cinder cone on the SE flank produced a lava flow in 1722 that traveled 11 kilometers to the east.

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