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Report on Ruapehu (New Zealand) — January 1995

Bulletin of the Global Volcanism Network, vol. 20, no. 1 (January 1995)
Managing Editor: Richard Wunderman.

Ruapehu (New Zealand) Small phreatic eruptions in crater lake

Please cite this report as:

Global Volcanism Program, 1995. Report on Ruapehu (New Zealand). In: Wunderman, R (ed.), Bulletin of the Global Volcanism Network, 20:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199501-241100.

Volcano Profile |  Complete Bulletin


Ruapehu

New Zealand

39.28°S, 175.57°E; summit elev. 2797 m

All times are local (unless otherwise noted)


Some key descriptive and eruptive data on Ruapehu are summarized on figure 16 and table 5.

Figure (see Caption) Figure 16. Ruapehu Crater Lake activity and surface lake temperature at Outlet, 1985 through January 1995. Courtesy of IGNS.

Table 5. Summary of Ruapehu crater lake observations, January 1995. Courtesy of IGNS.

[Skip text table]
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°C
    outlet temp   18 Jan        46.5°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

"Ruapehu Crater Lake has been in a heating phase since ~25 November with the Outlet temperature rising from 17°C to 46.5°C by 18 January, and the ARGOS [satellite-relayed] temperature (at 20 m depth) rising from 17°C to 47.5°C during 25 November to 12 January. The 6.5°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 6m3 heating at a mean rate of 0.7°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 earthquakes were not interpreted as related to the activity at Crater Lake. Otherwise seismicity has remained low.

I. 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- to 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 g/cm3 and a melting temperature of ~113°C.

Geologic Background. 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 110 km3 dominantly andesitic volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically 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. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.

Information Contacts: P. Otway, S. Sherburn, and I. Nairn, IGNS Wairakei.