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Volcano Hazards in the San Salvador Region, El Salvador

-- Excerpts from: J.J. Major, S.P. Schilling, D.J. Sofield, C.D. Escobar, and C.R. Pullinger, 2001,
Volcano Hazards in the San Salvador Region, El Salvador U.S. Geological Survey Open-File Report 01-366

San Salvador Volcano - Past Events

San Salvador volcano has erupted intermittently for more than 70,000 years, and historical observations of eruptions date back nearly 500 years. However, only three eruptions have occurred since the early 1500s, and those eruptions consisted of a series of small explosions of low viscosity basaltic magma and emplacement of basaltic lava flows on the flanks of the volcano. Most of the information about San Salvador's past behavior comes from studies of deposits produced by prehistoric events. Many details of past eruptions as well as the precise age of the volcano are unknown, but it is clear that the volcano has exhibited a wide range of eruptive behaviors -- from highly explosive events to relatively quiet lava flows.

The bulk of the volcano was constructed more than 70,000 years ago. The oldest rocks exposed at San Salvador underlie deposits from a 72,000-year-old eruption of Coatepeque, a caldera that is located about 50 kilometers west of San Salvador. These old rocks of San Salvador volcano consist of blocky basaltic to andesitic lavas and tephras, and they are exposed at the bases of El Picacho and El Jabali and in scattered outcrops around the volcano. El Picacho and El Jabali consist entirely of layered volcanic rocks that dip away from the center of the volcano, suggesting that these two peaks are remnants of an ancestral cone, which is called the San Salvador edifice. Presently, El Picacho and El Jabali mark the location of an older volcanic edifice that has been deeply incised. Beyond the volcano, outcrops expose sections of a series of andesitic and basaltic tephra-fall deposits. Some of these fall deposits are only a few centimeters thick and are separated by erosional surfaces, suggesting that they are deposits from many small explosive eruptions that occurred during a long interval of time.

A distinctive gray, dacitic pumice-fall deposit, known as the G1 unit, marks a significant episode of explosive activity at San Salvador volcano. The G1 deposit is widespread and locally is more than 1 meter thick within 10 kilometers of the volcano's crater. Near El Picacho, this distinctive fall deposit is interlayered with pyroclastic-flow and surge deposits, and multiple flow and surge deposits extend about 6 kilometers from the summit. Rock fragments within these flow and surge deposits consist of the older San Salvador lavas. The nature and volume (about 2 to 8 cubic kilometers) of the flow and surge deposits indicate a large explosive eruption that may have largely destroyed the older San Salvador edifice and formed the crater now defined by the peaks of El Picacho and El Jabali. Although the exact timing of this event is unknown, stratigraphic relations with tephra-fall deposits from eruptions of Ilopango caldera, which is located on the eastern outskirts of San Salvador city, help constrain the time of this eruption. The G1 unit lies between Ilopango caldera tephra-fall deposits Tierra Blanca 3 (TB3) and Tierra Blanca 4 (TB4). On the basis of paleosols, buried soil horizons, formed on these tephra deposits, TB4 has been estimated to be about 40,000 to 50,000 years old.

Eruptions subsequent to the G1 fall deposit produced tephras and lava flows that now largely fill the crater formed during the G1 eruption and form a cone called El Boquerón. El Boquerón is composed of multiple blocky lava flows interlayered with tephra-fall deposits, all of which are chemically distinct from the lava flows of the ancestral San Salvador edifice. Lava flows from El Boquerón spilled over the north, east, and south rims of the San Salvador edifice. In addition to eruptions from the central crater, several smaller explosion craters, cinder cones, and lava flows erupted on the volcano's flanks.

El Boquerón volcano exploded violently about 800 years ago in an eruption that was perhaps similar to, but smaller than, the eruption that produced the G1 fall deposit. Pyroclastic-flow and tephra-fall deposits, known as the San Andrés tuff, are found at the rim of El Boquerón, and these deposits have been correlated with similar deposits, known as the Talpetate tephra fall, on the western flank of San Salvador volcano. The Talpetate tephra has been dated archeologically at 1200 A.D. Together, the Talpetate fall and San Andrés tuff deposits have a volume of about 0.5 cubic kilometers, indicating that this explosive eruption was roughly one tenth as large as the eruption that produced the G1 fall deposit. Sedimentary structures in the Talpetate fall deposit suggest that it was produced by an eruption in which the magma interacted with water.

Some of the youngest volcanic features and deposits at San Salvador volcano are along the northern and northwestern flanks of the volcano. These features and deposits consist of explosion craters, cinder cones, and lava flows that are generally concentrated along two prominent fault zones. Several explosion craters formed between 260 A.D and 1200 A.D. They are older than the San Andrés tuff deposits, but overlie the distinctive Tierra Blanca Joven (TBJ) tephra deposit, the youngest regional deposit associated with eruptions of Ilopango caldera. Many of these explosion craters show evidence that they are products of eruptions in which the magma interacted with water.

Many monogenetic cinder cones and associated lava flows are on the northern and northwestern flanks of San Salvador volcano and beyond. Many of these cinder cones and lava flows are younger than the San Andrés tuff deposit, and thus are less than 800 years old. Three prominent lava flows formed on and beyond the northwestern flank of San Salvador volcano within the past 500 years. In addition to these volcanic events, landslide deposits younger than the Talpetate tephra are present on the northern and northwestern flanks.

The most recent volcanic activity at San Salvador began in June, 1917, when, following an earthquake, steam billowed from El Boquerón crater and several fissures opened along the northwestern flank of the volcano. Within a month of this activity, a lake in the crater of El Boquerón had boiled off, and small explosions formed a cinder cone called Boqueróncito. The eruption lasted several months, constructed Boqueróncito and extruded a lava flow on the northwestern flank of the volcano. After the 1917 eruption, fumarolic activity continued until the late 1970s.

Although no eruptive activity has occurred at San Salvador volcano for more than 80 years, lethal volcano-related events have occurred. In 1982, heavy rainfall triggered numerous landslides in El Salvador, and at San Salvador volcano a rainfall-triggered landslide from the flank of El Picacho swept along a channel and into the northwest part of the city. The landslide, having a volume of between 200,000 to 300,000 cubic meters, rapidly transformed into a lahar that traveled more than 4 kilometers from its source. Near the base of the volcano the lahar destroyed or buried several homes and killed between 300 and 500 people.

More than 30 volcanic events have occurred at San Salvador volcano in the past 40,000 years. Hence, the average apparent frequency of eruptions is roughly 1 per 1,300 years. The volcano has undoubtedly erupted more frequently, because some eruptions do not leave conspicuous deposits in the geologic record. If we separate eruptive events into discrete, datable time periods, then we establish the following apparent frequencies of eruptions: between 40,000 years ago and 260 A.D. about 13 identifiable events indicate an eruption frequency of about 1 event per 3,000 years. Between 260 A.D.and 1200 A.D. about 9 identifiable eruptive events indicate an eruption frequency of about 1 event per 100 years. Between 1200 A.D. and 1917 about 9 identifiable eruptive events indicate an eruption frequency of about 1 event per 80 years. These eruption frequencies are only statistically averaged values. Many of these eruptive events are likely associated with a single eruptive episode, such as eruptions of cinder cones and lava flows, rather than distinct events widely separated in time. The apparent eruption frequencies should not be interpreted to mean that the time between eruptions is necessarily decreasing. The apparent decrease with time is related primarily to better preservation of younger deposits. Nevertheless, even accounting for the imperfections of the geologic record, evidence clearly indicates that San Salvador volcano erupts sufficiently frequently, with an annual probability of perhaps 1 in 1,000, that potential hazards from future eruptions need to be taken seriously, as densely populated areas will be at risk.

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08/19/08, Lyn Topinka