Smithsonian Institution Global Volcanism Network Bulletin v. 20, no. 1, January 1995 Ruapehu (New Zealand) Small phreatic eruptions in crater lake Ruapehu 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.