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 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.