Hot, buoyant blocks rose to the surface of Crater Lake on two occasions during 17 June fieldwork. Both episodes were associated with increased seismicity. However, lake temperatures continued their decline of the past several months.

Filling of the lake's outlet pond was noted between 1242 and 1244, indicating a rise in lake level estimated at ~0.15 m (equivalent to a volume of ~3 ± 10⁵ m³). A steaming object surfaced above the main lake floor vent at about 1245 without noise or upwelling of discolored water. Within the next 2 minutes, several similar but much smaller fragments, all steaming, rose ~20 m from the first object. Through a telescope, the larger block appeared pale gray with deep surface cracks, similar to a pumiceous breadcrust bomb. Steam emerged vigorously from a single orifice on the large block. The smaller blocks sank within 5-10 minutes, but the larger one remained on the surface for ~15 minutes, and was still steaming, although less vigorously, when it sank. Lake level dropped during the episode. Data from a 1970 bathymetric survey indicated that depths to vent areas at the bottom of the lake were roughly 60-80 m (Hurst and Dibble, 1981) [but see 15:07].

A second, smaller, temporary rise in lake level began at about 1439. Two large steaming blocks broke the lake surface 1 minute later, soundlessly but with enough vigor to cause small waves to radiate outward. During the next 3 minutes, three much smaller blocks surfaced over an area at least 50 m across. Diameters of blocks were estimated at 0.2-1 m, and they rose to 0.2 m above the surface. The blocks again had a breadcrust bomb appearance and all steamed vigorously from one or two openings, but moved very little while at the surface. Sinking occurred in order of ascending size, with the largest block remaining at the surface for ~15 minutes. Geologists noted that similar activity in January 1980 ended a period of lake cooling and marked the onset of a period of phreatic eruptions.

A series of small to moderate volcanic earthquakes began early the same day (table 2). An earthquake that began at 1239 and continued until 1244 triggered a period of moderate volcanic tremor and coincided with the first block ejection episode. No volcanic earthquakes were recorded during the second episode. Before the 17 June seismicity, local volcanic earthquakes had last been recorded on 17 May (M 2) and 16 April (M 1.8). Although such earthquakes have been rare in 1990, small bursts of tremor, resembling volcanic earthquakes, have been common at times.

Table 2. Volcanic earthquakes at Ruapehu, 17 June-early 18 June 1990. Times mark peak signal amplitude for a given event. [JL magnitudes estimated by J. Latter from HZ digital records.]

Deformation measurements suggested minor inflation across the crater (figure 9, top). Lake temperature continued its gradual decline, to 23.5°C on 17 June, down from 46.5° on 6 February (figure 9, bottom). A 2% rise in the lake's chloride content accompanied by a slight decline in Mg values dropped the Mg/Cl ratio to 0.053, the lowest value yet recorded at Ruapehu. This trend suggested to geologists that the vent remained essentially closed, with the vapor phase (including HCl gas) rather than the liquid phase (including Mg++ ions) strongly dominating influx into the lake.

Figure 9. Crater lake temperatures at Ruapehu (bottom), and change in length of an EDM line (IJ) across the crater (top), January 1989-October 1990. Arrows mark eruptions.

Reference. Hurst, A.W., and Dibble, R.R., 1981, Bathymetry, heat output and convection in Ruapehu Crater Lake, New Zealand: JVGR, v. 9, p. 215-236.

Geological Summary. 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 dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: P. Otway, DSIR Wairakei.