Ruapehu's current eruptive period began with a vent-clearing blast on 29 June 1995 and a series of larger eruptions began on 23 September (BGVN 20:09). More recently available information (in Immediate Report RUA 95/06) highlighted 18 and 20 September observations summarized below. These are followed by brief comments on eruptions during October.

Activity during 18-20 September. An eruption at 0805 on 18 September was accompanied by a ML 3.6 earthquake; the eruption produced the largest lahar down the ESE flank since 1975. The ESE drainage is called the Whangaehu River. Two days later, at 0122 on 20 September, another eruption associated with a smaller earthquake (ML 3.2) also sent a smaller lahar down the Whangaehu River.

At roughly 0800 on 18 September the ski field manager heard what he initially thought was wind noise while he was inside a ski lodge building on Ruapehu's flanks, a spot 400 m N of the Whangaehu channel (Aorangi lodge at Tukino). He went closer to the river and saw a 12-18 m deep lahar in the narrow channel.

Later that day, a flood warning gauge 27 km downstream was triggered at 1123, suggesting the lahar moved at an average speed of roughly 2.3 m/s (8.3 km/hour). By around noon at Tukino the lahar was 40-m wide and had covered the snow up to 20-30 m above the Whangaehu valley floor. The lahar's surface rose about 11 m on the outside of one turn. A preliminary estimate of peak flow was >1,000 m³/s; the local velocity, 15 m/s. An early phase of the lahar had cut out 2-3 m of ice and snow formerly filling the valley.

The 18 September lahar arrived at a point 57 km downstream from Crater Lake (Karioi) at 1515, 7 hours after the eruption. Volume of the lahar at this point was estimated (by groups identified as NUWA Wanganui and ECNZ) at ~2 x 10⁵ m³; the peak flow, at ~34 m³/s. The lahar destroyed a hiking bridge, leaving only its 0.2-m-high concrete abutments on either side of the river.

The smaller 20 September lahar arrived at 57 km downstream (Karioi) 8 hours after the eruption; its size there was estimated at ~0.9 x 10⁵ m³; its peak flow, at ~21 m³/s. In an area above ~2,000 m elevation, the 18 and 20 September lahar deposits were separated by an intervening snow layer. Still higher, above ~2,400 m elevation, both lahars had emerged from the upper Whangaehu valley's snow and ice tunnel system. Lahars passing through and over the uppermost part of this system had produced considerable new crevasses and collapse features in the snow and ice. On 20 September, collapsed holes downstream of the large ice cave (located below the crater lake's drainage point at Outlet, figure 19) were filled with non-steaming water that had apparently cooled. The ice cave itself appeared largely intact.

Figure 19. (above) Survey points for deformation studies at Ruapehu (prior to the disappearance of Crater Lake). (below) Summary of deformation between stated stations and given time intervals. Courtesy of IGNS.

A helicopter was used to visit the crater on 20 September. A large column of steam rose from the waterfall immediately below Outlet. A large volume of lake water continued to spill over the waterfall even though recent eruptions through the lake had expelled substantial lahar-forming discharges. Ash from the 18 September eruption was plastered on some steep slopes. Ash from the 20 September eruption was plastered on the new snow around the lake margins. On the E side of the lake there was a N-trending, 100-m-long lobe of ash on the glacier surface. Scoria clasts found near Outlet (the largest, 20-50 cm across) formed a continuous layer trapped behind a low lava ridge. Their distribution suggested they were deposited by a passing surge rather than as impacting ballistics. Absence of snow on the surface of the scoria indicated they had probably arrived during the 20 September eruption and some clasts still had warm interiors. Sampled clasts were black in color, and consisted of an unaltered plagioclase-, augite-, orthopyroxene-bearing andesite. The lack of Fe-Ti oxides makes them similar to 1966 ejecta; in contrast, ejecta from 1971 and 1975 did contain minor amounts of Fe-Ti oxides. Three ash samples collected from within the crater contained lapilli up to 25 mm in diameter and composed of angular lithic material. Ash finer than 2-mm diameter was dominated by gray shiny spheroids and globules of sulfur with lesser amounts of gray comminuted lake bed material.

In the interval 15 August-20 September the deformation of the area about Crater lake was significant and indicated moderate inflation (figures 19 and 20). The deformation survey was hampered by snow and ice, which deeply buried most survey stations. Survey mark D had been bent 70 mm out of position immediately prior to the August survey, but eccentricity corrections enable a valid comparison with all former observations at D. Maximum changes took place in the E-W direction. These changes were similar to those computed by comparing the mean of the five surveys made earlier this year to the September survey (first column, bottom of figure 19).

Non-elastic inflation of the style seen was previously noted as much as 2 weeks prior to eruptions on 8 May 1971 and 24 April 1975. This short-term inflation (lasting weeks) was also seen on 12 occasions during 1980-91; these occasions were tentatively correlated with intense heating and minor eruptions. Still, the relation between inflation magnitude and the corresponding eruption remains uncertain.

The 20 September crater visit yielded the following lake observations. The lake's temperature was 48.5°C (on 15 August it had been roughly 20 degrees C cooler, figure 20). There was a strong smell of SO₂. The volume of water escaping at Outlet was estimated visually at 1 m³/s (on 15 August it was only ~50 l/s). This exceptional output was the largest seen in 24 years.

Figure 20. Plots of Ruapehu's cross-crater deformation, crater lake temperature, and Mg/Cl ratio for 1976 through late-1995. The cross-crater deformation is approximately E-W (between stations I and J, figure 19). Courtesy of IGNS.

Lake water sampled on 20 September showed clear increases in the concentrations of Mg, Cl, and SO₄ ions, and in the ratio of Mg/Cl (figure 20). The observed concentrations for 15 August and 20 September, respectively, were as follows: Mg, 584 and 713 ppm; Cl, 8,154 and 8,619 ppm; and SO₄, 26,600 and 30,600 ppm. Increases in Mg began in May and pointed to dissolution of fresh andesitic material into the hydrothermal system. Although previously it was not clear if the source of Mg was juvenile or older andesites, the increased amounts of Cl and SO₄ firmly established the input of fresh magmatic material.

SO₄ concentrations stand at the highest levels ever recorded at Ruapehu. In the absence of synchronous increases in K, and noting that Ca continues to be controlled by gypsum solubility, it is clear that the increases in SO₄ were not attributable to dissolution of secondary hydrothermal minerals. Instead the SO₄ increases indicated greater SO₂ flux into the lake. Assuming a lake of 9 x 10⁶ m³, the increase in SO₄ from 15 August to 20 September equates to a minimum input of ~700 metric tons/day of SO₂ into the lake. This behavior differs from that observed prior to the 1971 eruptions: The indication is that the quantity of magma involved in the current activity is larger than in the 1971. Taken with the rather moderate degree of cross-crater deformation seen, the quantity of SO₂ discharged into the lake indicates connection to larger volumes of degassing magma at depth.

Volcanic tremor remained at background from early July until early September; its amplitude was ~1 µm/s for signals centered around 7 Hz, and at this value or slightly lower for signals centered around 2 Hz. During a five day interval starting on 6 September, the amplitude of 2-Hz tremor increased. During the 24 hours prior to the 18 September eruption and earthquake (BGVN 20:09), predominantly 7-Hz tremor occurred, at one point doubling in amplitude. Later, ~80 minutes prior to the eruption and earthquake, tremor again increased by a factor of 2-3, with 2-Hz tremor becoming dominant. Although dramatic, Ruapehu often displays wide-ranging shifts in tremor amplitude and, in retrospect, the increased amplitudes seen would not have been a useful way to predict the eruption.

The 18 September earthquake took place at 0805, continuing for 6 minutes. Analog seismograms from the three local stations (Dome, Chateau, and Ngāuruhoe) were pegged, and the M 3.6 estimate was made based on amplitude recorded by the tremor-monitoring system. After the earthquake, predominantly 2-Hz tremor prevailed, remaining at or above the pre-earthquake amplitude. Later the same day (18 September), strong 1-Hz tremor occurred--for the first time at Ruapehu since the early 1970s.

Further minor earthquakes were recorded during the next few days. On 19 September seismometers registered a ML 2.2 earthquakes as well as four other discrete earthquakes; on 20 September there were ML 3.1 and 3.2 earthquakes followed by another interval of strong 1-Hz tremor until 0900.

October eruptions. At the time of this writing, IGNS reports for October are incomplete, but a brief survey of available "Science Alert Bulletins" and aviation reports suggested that minor eruptions continued and in mid-October moderate ash-rich eruptions took place. On 11 October a plume was seen in satellite imagery; on 12 and 14 October, pilot and associated aviation reports indicated ash to at least ~10 km altitude.

The 11 October eruption was described as near-continuous moderate eruptive activity that included hot ballistic blocks and lightning. Subsequent lower intensity eruptions presumably fed the plume so that its proximal end remained attached to the volcano. The eruption deposited ash in a blanket with a tentative volume between 0.01 and 0.05 km³. Thus, the steam-rich plumes seen in the 3 weeks prior to 11 October gave way to more ash-rich plumes during this eruption. A thin blanket of ash was also deposited during the 14 October eruption.

The absence of a crater lake was confirmed on 14 October. By 17 October, partly impeded views into the crater revealed steam and ash emitted from at least three vents, and a still-dry crater floor. COSPEC measurements around this time suggested the SO₂ flux was over 10,000 metric tons/day. A COSPEC flight on 21 October gave viewers their first look at a possible new lava dome, however, there were no subsequent confirmations of the dome in available reports.

Geological Summary. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: C.J.N. Wilson, B.J. Scott, P.M. Otway, and I.A. Nairn, Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand; Bureau of Meteorology, Northern Territory Regional Office, P.O. Box 735, Darwin, NT 0801, Australia.

Correction: The most recent analysis indicates that there were 18 hydrothermal eruptions recorded between 0600 and 1640 on 20 September. Table 7 indicated "15 small phreatic eruptions witnessed."