1996 fumarolic activity

Observations of Guallatiri Volcano on 19-20 July 1996 showed quiet emissions of white vapor from the summit crater. On the southern flank 400 m below the summit, there was a snow-free area which emitted vapour.
(GVP monthly reports)

1991 strong fumarolic activity

On 2 November 1990, 2 areas of strong fumarolic activity were observed at Guallatiri volcano. The most vigorous fumaroles were 80 m below the summit and produced a 200 m high plume, accompanied by jet-engine-like noises.

1985 probable eruption

Emissions of pulsating steam plumes producing billowing clouds rising 500 m vertically then drifting west were observed on 1 December 1985, but from 25 km distance. The eruptions occurred in intervals from 45-75 minutes. Originally, it was thought that Acotango volcano had erupted, but later studies and field observations revealed that most likely, Guallatiri volcano had a small eruption. The nearby volcanoes Capurata, Acotango, and Humarata did not show any signs of activity and their craters were covered with clean ice and snow.
Instead, vigorous fumarolic activity was found at Guallatiri volcano, indicating that this was the site of the 1985 eruption.
(Source: Smithsonian / GVP monthly reports)