Historic activity of Tacaná

Tacaná has no record of magmatic eruptive activity in historic time. It does exhibit bursts of fumarolic activity, spaced one quarter to half a century apart, accompanied by small phreatic eruptions, and preceded by a considerable increase in seismic activity.

Periods of fumarolic activity took place in 1855, 1878, 1900-1903, and 1949-1950. A fifth period of fumarolic activity started in 1986, and continues at the time of preparation of this report (1991). Nothing is known about activity prior to 1855.

All historic fumarolic events are characterized by an increase in seismic activity and generally low level fumarolic activity. The 1949-1950 event described by Mulleried was perhaps more energetic than the previously reported events, although this could be a function of better reporting.

The 1986-1987 Fumarolic Event

Aerial view of the 1986 fumarole on Tacaná's N slope. Photo by Bill Rose, June 1986.

Tacaná began showing seismic activity on December, 1985 with a series of shallow earthquakes centered within 20 km of Tacaná, no harmonic tremors were detected at the time. Some volcanic earthquakes were felt in February in Sibinal, accompanied by rumbling at the base of the volcano. Apparent volcanic tremors were recorded 2-3 times a day in February and March; events identified as possibly explosion shocks were detected on March 10. Tacaná had small phreatic eruption in February, May, and June of 1986. The May 8 eruption was preceded by an earthquake swarm starting on May 7, the earthquakes occurred at a rate of 1/min, accompanied by thunder-like noise. Much of the seismicity registered since December 1985 originated froma fault system NE of Tacaná and was not directly related to volcanic activity. A dozen small and 2-3 large tectonic quakes per day were registered by the end of May, with the majority being shallow and centered within 3 km NE of Tacaná.

The phreatic explosions of May 8, 1986 took place at a steam vent on the NW slope of Tacaná. It is located at an elevation of 3,600 m, very near the border on the Mexican side of the volcano. The vent was 20 m wide and continuously vented steam at high pressure with noise like a jet engine. The visible steam emission typically rose about 500 m above the vent. The vent is located along a radial fault (?) from the summit at the foot of a 30 m cliff. It is surrounded by an apron of muddy debris. Ground access for direct gas sampling was not possible. but visual observations indicate the temperature of the steam is not far about 100°C. There is a weak small of H2S in the vicinity of the vent. As reported above the ashes erupted from this vent are interpreted as phreatic. The gas vent is apparently a venting hydrothermal system, which also ejects mud, rocks, phreatic ash and some (magmatic?) sulfur. Remnants of similar vents are seen on aerial photos to the SE of the summit, in the area described by Mulleried. The 1986 vent is larger but otherwise very similar to the description given by Mulleried for the 1949-1950 vents. Generally low level seismicity continued through 1986. Numerous new small fumaroles were present by early October.

Sources of information on historic activity of Tacaná.

(Mercado and Rose, 1992)