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REPORT:
Volcano-Hazard Zonation for San Vicente Volcano, El Salvador


-- J.J. Major, S.P. Schilling, C.R. Pullinger, C.D. Escobar, and M.M. Howell, 2001,
Volcano-Hazard Zonation for San Vicente Volcano, El Salvador U.S. Geological Survey Open-File Report 01-367

San Vicente Volcano - Past Events

Details of the eruptive history of San Vicente volcano are poorly known. The last major eruption occurred more than 1,700 years ago,and probably occurred long before permanent human habitation of the area (~2,000 B.C.). Nevertheless, we do know that a volcanic complex at San Vicente has a history that extends more than 2 million years, and that it has exhibited highly explosive eruptions as well as emplacement of lava flows and lava domes.

Previous studies recognize the existence of at least a three-stage evolution of San Vicente volcano. The oldest rocks at San Vicente are between 1 million and 2 million years old and are associated with a series of pronounced hills that lie immediately west-northwest of the volcano. These hills delineate the edge of an annular feature thought to be the remnants of an older volcanic center, known as La Carbonera. Lava flows extruded over hundreds of thousands of years built the La Carbonera complex.

Sometime after about 1 million years ago, relatively quiescent emplacement of basaltic and andesitic lavas at the ancestral La Carbonera volcanic center was interrupted by a phase of major explosive activity. The explosive eruptive phase produced pyroclastic flows, pyroclastic surges and thick tephra fall. Deposits of this explosive phase of activity are rich in dacite pumice, a light frothy fragment of exploded magma having a silica content ranging from 63% to 68%, which indicates that gas-rich magma intruded the volcano and erupted violently. The earliest deposits from this phase of activity are separated from later deposits by a thick, well developed paleosol, a buried soil horizon, indicating that the explosive phase extended over many thousands of years. The compositions, textures, and distributions of some nonpumiceous tephra and surge deposits within the sequence of deposits related to this explosive phase of activity indicate that some explosions were phreatomagmatic and involved interactions of magma and water. The timing of this explosive phase of activity is unknown. Development of a pair of thick, well developed paleosols within this sequence of deposits as well as the construction of the modern San Vicente volcano after this explosive eruptive phase ended suggest that this period of explosive activity occurred many tens to hundreds of thousands of years ago.

The modern San Vicente volcano was built after the phase of explosive activity. Its edifice consists of two prominent cones that are composed largely of andesite lava flows. On the basis of their morphology, the easternmost cone appears to be the youngest, suggesting that the focus of volcanism at this center has migrated east-northeastward with time. Local deposits of nonpumiceous pyroclastic flows interlayered with the lava flows suggest that construction of the modern edifice included growth and collapse of small-volume lava domes.

Mass movements of sediment from the volcano by landslides, lahars, and pyroclastic flows, and reworking of that sediment by streamflow, have formed an apron of debris that has accumulated at the base of the volcano. Of particular significance is an extensive lahar-like deposit southeast of the volcano that contains many small hills, composed of volcanic rock and debris, known as hummocks. This deposit, found near and southeast of Tecoluca, extends at least as far as the Río Lempa, 25 kilometers from the volcano, and represents a debris avalanche and associated lahar that resulted from collapse of a massive segment of the volcano.

Most of the deposits in the apron of debris accumulated at the base of the volcano are probably many thousands of years old. They commonly are weathered and capped by a well developed soil horizon, and are overlain by the Tierra Blanca Joven (TBJ) deposit, the youngest tephra deposit from an eruption of Ilopango caldera. The TBJ deposit is more than 1700 years old.

Although there has been no historical eruptive activity at San Vicente volcano, lethal, and potentially lethal, volcano-related events have occurred several times. Known earthquake-and rainfall-triggered landslides and lahars occurred in 1774, 1934, 1996, and 2001. Others may have occurred in historical time, but are not recorded. The 1774 lahar occurred on the northeast flank of the volcano and affected the village of San Vicente. The 1934 lahar occurred on the north flank of the volcano and destroyed the village of Tepetitan, more than 6 kilometers from the summit of the volcano. Landslides and lahars on the south flank of the volcano in 1996 damaged the major roadway between Tecoluca and Zacatecoluca. Landslides triggered by an earthquake in February 2001 occurred on the north and northwest flanks of the volcano, but they did not transform into lahars that flowed down valley. However, these landslides dumped more than 200,000 cubic meters of sediment into channels that drain the volcano, and potential remobilization of that sediment poses an increased risk from destructive floods and lahars to the downstream communities of Guadalupe and Tepetitan. In September 2001, a rainfall-triggered lahar from the northwest flank of the volcano, which possibly formed in material loosened by the February 2001 earthquake, damaged the town of Guadalupe.


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08/19/08, Lyn Topinka