Tacaná as seen from the NE, on the slopes of the Sierra de Sibinal. Photo by Bill Rose, June 1986.

The summit of Tacaná is symmetrical and elongate, dominated by a series of exogeneous volcanic domes extruded along a NE trend. Their location appears to be controlled by a north trending set of normal faults. The faults appear to localize many of the domal events as well as the 1986-1987 fumarole.

The complex of domes indicates a style of activity similar to volcanoes such as Mount St. Helens, Santa Maria-Santiaguito, and Mount Pelée (Martinique). The domal character of Tacaná indicates a history of relatively passive events, similar to Santiaguito. The presence of domes could follow an explosive precursor event, the smae way the 1902 eruption of Mount Pelée was followed by an episode of dome growth.

Even though domal growth is a relatively passive event, it has significant associated hazards. Some explosive activity is usually closely associated with episodes of dome growth. Three types of explosive activity associated with dome growth have been described: (1) relatively small explosive eruptions, (2) larger explosive eruptions, (3) generation of pyroclastic flows as a result of gravitational collapse of domes. Explosive activity can take place before and/or after the domal growth starts. Santiaguito is an example of the occurrence of this type of sporadic energetic activity associated with domal growth.

Formation of domes on the flanks of Tacaná would lead to avalanching and block and ashflows, and possibly to pumiceous pyroclastic flows. Domal collapse may induce laterally directed nuées ardentes as has been observed at Mount Pelée.

An episode of dome growth could be similar to the 1981-1983 period at Mount St. Helens, or similar to the growth of Santiaguito since 1922. Such an episode will lead to the generation of block and ashflows and pyroclastic flows like those produced by Santiaguito. A dome growth event could occur on the summit or any flank of Tacaná, but may tend to occur along the NE trend of domal events as seen in the photo interpretation of Tacaná volcano on the previous page.

Alternatively, the position of the 1986-1987 fumarole suggests that domal activity could occur on the NW flank of the volcano. In this case the areas near the head of Río Agua Caliente would be most severely affected. The deposition of these materials would eventually trigger mudflows and floods downstream, as well.

The domal character of Tacaná, together with the observed plinian deposits suggest that Tacaná:'s past activity may be similar to the Santa María-Santiaguito 20th Century history. A violent plinian eruption of Tacaná (as evidenced by 3+ m thick ash deposits) predates some of the dome-forming events. We can infer that a catastrophic eruption took place sometime in the past and was followed by domal growth.

(Mercado and Rose, 1992)