Smithsonian Institution Global Volcanism Network Bulletin v. 20, no. 4, April 1995 Ruapehu (New Zealand) Crater lake temperature drops 10 deg C from 13-year high Ruapehu North Island, New Zealand 39.28S, 175.57E; summit elev. 2,797 m The following was extracted from the IGNS Ruapehu Immediate Report (RUA 95/02). Peaks on the crater lake temperature versus time curve have often correlated to small vent-clearing eruptions (see figure in Bulletin v. 20, no. 1). "Crater Lake has been in a heating phase since late November, reaching the highest temperature (55 deg C) in 13 years by 12 February, but a 10 deg C decline since then and a reduction in volume suggest this phase has peaked. Minor phreatic eruptions have been occurring since early January but appear to have become infrequent, or may have even ceased, during February. Despite the relatively high heat output, the recent activity has so far followed the cycle of heating and cooling typical of Ruapehu since at least 1985." There were several reports of steam clouds and other phenomena after 20 January. A hiker on 24 January described the crater lake seen through the clouds as "a seething surface" that made "roaring sounds" lasting 1 to 2 minutes. Two or more observers on 29 January described the crater lake, which was visible for almost 2 hours, as "pale gray, almost white" and two, 1.5 m (or smaller) upwelling and splashing episodes were seen. The report also mentioned "pure yellow styrofoam-sulfur" littering the Outlet area. The water temperature, measured with two calibrated thermometers, was 51.4 deg C. Hikers in cloudy weather on 30 January witnessed a "small hydrothermal eruption up to 10-20 m." Hikers in cloudy weather on 5 February heard sloshing noises from the crater lake followed by two "loud explosions." On 15 February observers saw a 3 km tall, stationary steam plume over the crater lake; on 25 and 27 February observers also saw steam clouds. These clouds were undoubtedly steam, but they may have arisen from "atmospheric enhancement" due to a rise in relative humidity rather than from definite eruptions. Their interpretation thus remains ambiguous. A ground inspection on 2 March failed to confirm any significant surging took place around the shore of Lake Wade. In the interval 31 January-early March there were few discrete earthquakes and mainly background tremor was detected on the volcano's Dome seismograph. On the other hand, there were short intervals of strong, high frequency tremor, an unusual occurrence for Ruapehu. Although in the latest crater visit on 2 March all deformation survey stations were accessible and clear of snow, most of the length changes seen since 13 January were insignificant (<= 5 mm). Station I (see map, Bulletin v. 19, no. 12) appeared to have moved 18 mm ENE relative to all other stations since May 1994--a motion consistent with moderate deflation seen in the past 10 months, but also possibly due to displacement by local snow loading or other factors. Mg and Cl analyses of lake water were made on 18 and 29 January, and on 2 March, but showed relatively change. The Mg/Cl ratio changed only about 4% (shifting downward from an 18 January value of 0.036 to a 2 March value of 0.035). The Mg/Cl ratios were interpreted to indicate that the heating event was driven by convective flow of lake water through the upper portion of the vent. Thus, the heating event was regarded as mainly due to fluid flow rather than heat input from magmatic sources within the edifice. Information Contact: P.M. Otway, Institute of Geological and Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.