Volcanic cones concentrate large masses of rock at high elevation and sometimes these great structures catastrophically colapse, creatinga a hazard zone sometimes measured in tens to hundreds of km². The 1980 activity at Mount St. Helens has demonstrated that a cone collapse and their associated lateral blasts are one of the most dramatic types of volcanic hazard. Several hundred meters high "megablocks" of the volcanic edifice can slide long distances from their source. These avalanches will be gradational with pyroclastic flows. Emplacement of these avalanche deposits cover the spectrum from simple slope failure, without major explosive activity, to avalanching with major phreatomagmatic and magmatic explosive activity due to sudden pressure release in a shallow magma chamber.
The crater geometry of Siete Orejas, with its specatacular opening toward the south, is highly suggestive of collapse of the volcanic edifice. We do not know if lateral blast resulted from this collapse because the downslope areas have not been mapped. Many other Central American volcanoes (e.g., Zuñil, Pacaya) show symmetry suggestive of cone collapse. A gigantic episode of cone collapse is reported to have occurred at Popocatepetl volcano (Mexico) about 50,000 years ago.
At Tacaná we did not recognize direct evidence of prior cone collapse, but a north trending set of faults is observed to crosscut the dome series of the volcano. These N trending faults appear to be normal, downdropped on the west. This could relect sliding of the volcano in a seaward direction, and suggest the possibility of collapse of the volcanic edifice. Collapse and/or avalanche of the cone would affect extremely large areas due to the immense slope gradient involved, and also generate many of the previously discussed hazards.
A lateral blast can be produced as a result of earthquake triggered collapse of a volcanic flank, resulting in the explosure of a near surface magma body. The sudden pressure release would trigger a violently explosive directed blast. A lateral blast could occur on any flank of the volcano, but in the short range it may be more likely on the NW flank due to the existing faults present on the volcano and the location of the 1986 fumarole. This lateral blast could be similar to those observed in the May 18, 1980 eruption of Mount St. Helens. Such a blast on the NW flank would create severe potential hazard of block and ashflows, pyroclastic flows, and extensive mudflows and flooding downstream Río Agua Caliente and Río Coatán. Because of the topographic slope of the basement of Tacaná, we consider that cone collapse, if it ever occurs, is more proable in a direction toward the west or southwest (downslope).
(Mercado and Rose, 1992)