After the 1964 eruption, activity at the volcano "gradually declined until arriving at zero in
December 1970" (Gonzalez-Ferr‡n 1972). When the crater was visited in February 1971, no
kind of activity was observed, not even fumarolic emissions.
Typical of Villarrica's 20th century eruptions, the activity of 1971 began with explosive activity from a newly opened vent on the bottom of the summit crater, on 29 October. This activity apparently marks the beginning of intracrateral activity which gradually filled the pit left after the 1964 eruption. On 29 November, the crater was filled with a growing cinder cone, and lava issuing from a subterminal bocca at the cone's base began to overflow onto the southwestern flank of the volcano. The flow melted a deep canyon into the icecap and interacted vigorously with the glacial ice.
Strombolian activity and intermittent lava outflow onto the southwestern flank continued through late December. By this time, the intracrateral cone had grown significantly, now looming about 30 m above the highest part of the pre-eruption crater rim. The lava flow reached down to an elevation of about 2000 m.
Without any obvious premonitory symptoms, Villarrica entered a paroxysmal eruptive phase at 2345 local time on 29 December 1971. The upper part of the cone was cut by a fracture trending N30ˇE with a total length of about 4 km, and two brightly incandescent sheets of lava shot out of the segments of the fracture to the NE and SW of the summit crater (which itself apparently remained blocked during the event). The lava jets were obliquely directed away from the volcano at angles of about 45ˇ, and reached about 400 m high. The fall of hot pyroclastics and simultaneous outflow of large volumes of lava caused rapid melting of the volcano's icecap on the SSW and NNE flanks and subsequent formation of lahars and meltwater floods in the valleys of ChaillupŽn, Turbio, Correntoso and Pedregoso valleys. These lahars and floods entered the lakes of Villarrica and CalafquŽn. While Gonzalez-Ferr‡n (1972) reports 15 people dead, other sources mention up to 30 fatalities. Campsites and inhabited areas were devastated.
The event produced some of the longest lava flows recorded at Villarrica in this century. The flow running down the ChaillupŽn valley reached as far as 14 km from the summit, and another flow in the Pedregoso valley extended 6 km from the summit. The total volume of lava emitted during the 29-30 December 1971 event was estimated at about 30 million cubic meters. The main tephra fallout was to the SE, leaving 200 square km covered with black basaltic ash whose volume was not determined.
The culminating phase of the eruption lasted about 12 hours after which the volcano returned to much lower levels of activity. Sporadic ash emissions from the SSW part of the eruptive fracture continued for some time, and in January 1972, activity was mainly fumarolic, although very weak Strombolian activity from a small vent on the crater floor continued for an unknown period. Generally, during the period 1972-1984, the volcano showed vigorous fumarolic activity.