Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 1, January 1995

Ruapehu (New Zealand)  Small phreatic eruptions in crater lake

North Island, New Zealand
39.28 S, 175.57 E; summit elev. 2,797 m

The following information was extracted from the January Ruapehu
Immediate Report, by P.M. Otway and IGNS staff. Some key
descriptive and eruptive data on Ruapehu are summarized on table 4
and figure 6.

"Ruapehu Crater Lake has been in a heating phase since ~25 November
with the Outlet temperature rising from 17 deg C to 46.5 deg C by
18 January, and the ARGOS [satellite-relayed] temperature (at 20 m
depth) rising from 17 deg C to 47.5 deg C during 25 November to 12
January. The 6.5 deg C differential between Outlet and ARGOS
temperatures on 12 and 13 January appears to be common during a
heating phase. Note that the unusually high outflow of <=600
liter/sec is thought to be a combination of the strong thaw and
thermal expansion of the entire body of Crater Lake (9 x 10^6 m^3
heating at a mean rate of 0.7 deg C/day).

"Small phreatic eruptions have been occurring since ~11 January, or
earlier. No associated seismicity or acoustic noise has been
recognized, indicating typical open vent conditions."

S. Sherburn reported that there were two weeks in December without
seismic data, but between 24 December and 2 January a series of
moderate earthquakes took place ~12 km W of the summit (the
largest, M 4.3 and 4.4). These events were not interpreted as
related to the activity at Crater Lake. Otherwise seismicity has
remained low.

I. A. Nairn reported that on 13 January Crater Lake was "steaming
vigorously both before and after two small phreatic eruptions." The
first eruption took place at 1103 and the second ~40 seconds later;
both lasted ~10 seconds. The eruptions consisted of a sudden
upwelling near the center of the lake, reaching roughly 1-2 m in
height. Nairn also noted minor upwelling over several of the N
vents. The lake water contained considerable dissolved gas, and
bands of ~1-2 mm diameter hollow spherules of sulfur floated along
parts of the lake's edge. Large fragments (up to 20 cm across) of
vesiculated yellow sulfur both floated at the lake edge and formed
a strandline located near Logger Point. The strandline sat 2-3 m
from the lake edge and 0.5 m above its surface, and was thought to
have resulted from surges due to heavy rainfall (possibly on 11
January). The larger sulfur fragments contained vesicles with very
thin walls and diameters mainly in the 1-5 mm range. The vesicles
were mostly subspherical, and unconnected except where broken walls
touched adjacent vesicles. There were some solid sulfur spheres
within the larger fragments as well. Molten sulfur may have
detached from a pool at depth. The sulfur may have then been jetted
upwards reaching shallower depths where it subsequently degassed
and vesiculated. Molten sulfur has a density of 2.07 grams/cm^3 and
a melting temperature of about 113 deg C.

Ruapehu is a complex stratovolcano constructed in at least four
episodes and surrounded by a broad plain of volcaniclastic debris.
A single active vent, Crater Lake, is located in the broad summit
region, but at least five other vents on the summit and flank have
been active during the Holocene.

Figure 6. Ruapehu Crater Lake activity and surface lake temperature
at Outlet, 1985 through January 1995. Courtesy of IGNS.

Table 4. Summary of Ruapehu observations taken from the January
1995 monthly report. Courtesy of IGNS.

Observation     Date  Comment
visual lake
observations  13 Jan        Battleship gray, steaming vigorously,
                              with sudden, 1-2 m high upwelling
outlet temp   13 Jan        41.5 deg C
outlet temp   18 Jan        46.5 deg C
outflow rate   7 Dec        80 liters/sec
outflow rate  13 Jan        500-600 liters/sec
outflow rate  18 Jan        >600 liters/sec
seismicity    after 25 Nov  Generally low except for moderate
                              earthquakes mentioned in text.
tremor        after 30 Nov  Generally <=200 Watts at a dominant
                              frequency of about 2 Hz
deformation   6 May-13 Jan  No volcanologically significant
                              distance changes seen (line IJ
                              unmeasurable due to snow)

Information Contacts: P.M. Otway, S. Sherburn, and I. A. Nairn
Institute of Geological & Nuclear Sciences (IGNS), Private Bag
2000, Wairakei, New Zealand.