Following the end of the increased activity in the spring of 1985, Villarrica has remained in a low-level persistent intracrateral activity through early 1997. The activity is characterized by the presence of a boiling lava pond deep in the inner crater pit, interrupted by occasional ash emissions. The magma column within the volcano's conduit shows remarkable fluctuations, at times rising to the rim of the central pit and subsiding completely at other times, leaving the pit's bottom free from any active lava. Such fluctuations are occurring again in early 1997.
The first indications of continued intracrateral activity after the 1984-1985 eruption were an increase in microearthquake activity in late 1985. The earliest photo of the lava pond in the central pit is of March 1986 when activity was vigorous.
By early 1991, the surface of the lava pond had diminished remarkably, and a small new cone had formed at the bottom of the central pit. Photos taken in late January 1991 show the inner cone with a small (less than 10 m in diameter) incandescent vent. Activity reported in that period consisted in occasional ejections of incandescent spatter from the vent.
Observation by Werner Keller on 15-17 January 1996 disclosed the continued presence of active lava on the crater floor. In the Austral spring (that is, late 1995), occasional hydromagmatic vent-clearing explosions have occurred, leaving a light dusting of ash on the summit snow. The active lava pond shows oscillations of 2-3 m with dome fountains lasting up to 3 seconds. Fumarole temperatures of 58-60° C were measured about 10 m below the S rim of the crater.
Following an increase of the seismic activity during the first week of September 1996, small phreatic ash emissions occurred on 14 September 1996, alarming persons in the skiing areas on the volcano's lower flanks and prompting investigations by a team of Chilean volcanologists. The activity was declared as minor but could have been a predecessor of more vigorous activity, given that many of Villarrica's larger eruptions have begun during the Chilean spring (October-December) and were preceded by minor explosions. A few weeks later, a report was received from a group of climbers who had visited the crater on 11 September and failed to detect the lava pond. The crater was filled with a dense steam plume, and no significant gas odor was perceived.
The absence of the lava pond just a few days prior to the phreatic explosions indicates that the magma column had subsided, and water from the volcano's hydrothermal system had entered the conduit, leading to the explosions. Phreatic activity has often been observed during the Chilean spring and summer (September-March), probably coinciding with the snowmelt on the volcano.
A report by A. Köner from Kiel who climbed the volcano in mid-November 1996 indicates that the lava pond has reappeared. A distinct glow could be seen at the bottom of the inner pit of the crater although the activity was described as "less vivid than at other times in recent years". Bad weather prevented very clear observations, but the information means that the magma column has again risen to the height of the active vent. This coincides with a report in the September 1996 GVN Bulletin about continuing microseismicity after the phreatic events of September.
Local mountain guides reported vigorous activity of the lava pond in mid-October and early November 1996. The level of the magma column appears to have risen remarkably high during that period as ejections of incandescent spatter occurred at intervals of 3 seconds, and bombs and spatter frequently fell outside the summit crater. In early January 1997, the lava pond had completely disappeared again, and two fuming vents were exposed on the bottom of the central pit, at a depth of about 100 m below the pit's rim. Reports about the return of the lava pond in mid-January 1997 remained uncorroborated and were probably false. On 13 January, however, the collapse of the septum separating the two pits was stated, with a resulting large pit on the crater floor. Between 15 and 19 January, observations of the crater floor were hampered by strong fumarolic activity, and on the 21st, the crater was seen unchanged from the situation on 13 January.
On 21 January at about 2200, two small ash emissions from the summit crater were seen from Pucón, probably being caused by another landslide on the eastern crater wall as detected on 27 January. On 3 February, the crater floor was seen partially covered with talus, but a small open vent was actively degassing in the northern part of the crater floor. Another significant collapse of the northeastern crater wall occurred around 15 February, preceding the reappearance of incandescent lava on the crater floor two days later when two small boccas on the northeastern side of the crater floor were the site of small Strombolian eruptions. On 22 February, the crater floor had risen by several meters due to the infilling with new lava, and small lava fountains were active in the northeastern part of the crater floor. Observation on 24 February disclosed a further rise of the magma level within the summit crater and active convection on the lava pond.
Residents of Pucón reported the reappearance of the characteristic night glow above Villarrica's summit crater on 2 March after several months of absence. On 12 March, a light dusting of ash was seen on the snow-covered NE flank of the volcano. Strombolian explosions from a bocca on the crater floor were observed during a summit visit on 15 March (see photo below). Similar activity was reported for the remainder of March.
Crater of Villarrica, January 1996
(Full picture JPEG: 96K) Aerial view into the crater of Villarrica, 15 January 1996, taken by Werner Keller. Note several concentric ledges and fractures and nested pits. Ledges in the gas-filled pit result from growth of intracrateral cones in the early to mid-1990's; they did not exist in the late 1980's when the structure of the pit was much simpler. Note absence of snow near the crater, probably due to minor eruptive activity in late 1995 during the Austral summer season when there was little snowfall.
Dome fountain rising from lava pond on Villarrica's crater floor, January 1996
(Full picture JPEG: 36K) Close-up view into the crater pit of Villarrica, 17 January 1996. The active vent is filled by a convecting dome fountain. This photo was taken by Werner Keller.
The active pit of Villarrica, January 1996
(Full picture JPEG: 180K) View into the central pit of Villarrica's crater from its W rim, 17 January 1996. The vent containing the actively boiling lava pond is visible at the bottom of the photograph. Note scoria deposits on the lower pit walls. Some of the dark ledges are remnants of scoria cones that have formed and collapsed during the early 1990's. Note tephra layer on thin ice layer on the upper rim of the pit (near upper margin of photo), probably deposited by the eruptions of late 1995.
Pit of Villarrica's inner crater with active lava pond
(Full picture JPEG: 144K) View into the active inner pit, showing the active lava pond about 80 m below the photographer's (that is, Werner Keller's) viewpoint. Note concentric cracks in the scoria rampart surrounding the lava pond (at lower left), indicating recent subsidence of the magma column. Photo taken on 17 January 1996.
Ash emission from Villarrica, 14 September 1996
(Full picture JPEG: 96K) Ash emission from Villarrica's summit crater on 14 September 1996. The photo was taken from immediately below the summit crater rim where mountain guide Claudio Marticorena of Pucón had arrived with a group of tourists. The activity of 14 September 1996 consisted in four phreatic ash emissions which the climbers described explicitely as non-explosive. Note persons in foreground for scale. Photo by Claudio Marticorena.
Crater of Villarrica, January 1997
(Full picture JPEG: 96K) View into the inner pit crater of Villarrica, taken on 4 January 1997, by Víctor Sepúlveda (mountain guide of Pucón). The lava pond has subsided completely, exposing two boccas at the pit's bottom which display only gas emissions. Just one week after this photo was taken, the lava pond covered again the bottom of the pit.
New active vent, 15 March 1997
(Full picture JPEG: 54K) Strombolian explosion from active vent on the floor of Villarrica's inner pit crater, 15 March 1997, by Víctor Sepúlveda. The vent appears to lie in the central area of a crusted over lava pond. Comparison with photos taken in February 1997 shows a rise of the magmatic column by several meters in less than one month.