Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 10, October 1995
Ruapehu (New Zealand)  Late September-early October
       eruptions rival those in 1945

Ruapehu
North Island, New Zealand
39.28S, 175.57E; summit elev. 2,797 m
All times are local (= GMT + 12 hours)

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 (Bulletin v. 20, no. 9). 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/sec (8.3 km/hr). 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 m^3/second; the
local velocity, 15 m/second. 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
10^5 m^3; the peak flow, at ~34 m^3/second. 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 10^5 m^3; its peak flow, at ~21
m^3/second. 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 7) were filled with non-steaming water that had
apparently cooled. The ice cave itself appeared largely intact.

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 7 and 8). 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 7).

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 degrees C (on
15 August it had been roughly 20 degrees C cooler, figure
8). There was a strong smell of SO2. The volume of water
escaping at Outlet was estimated visually at  1 m^3/second
(on 15 August it was only ~50 liters/second). This
exceptional output was the largest seen in 24 years.

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 8). 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 10^6 m^3, 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 micrometers/sec 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 (Bulletin v. 20, no. 9),
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 Ngauruhoe) 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 km^3.
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 3 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.

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 (Email:
bscott@gns.cri.nz); Bureau of Meteorology (see Tengger
Caldera).

Correction: The most recent analysis indicates that there
were 18 hydrothermal eruptions recorded between 0600 and
1640 on 20 September. Table 2 in Bulletin v. 20, no. 9
indicated ~15 small phreatic eruptions witnessed.~