The occasional minor phreatic activity . . . continued through late May, then stopped as Crater Lake cooled substantially. Since the observed 12 April activity, small eruptions occurred around mid-day on 16 April, and steam clouds were observed above Crater Lake at 1000 on 19 April and 0956-0958 on 3 May. Low-amplitude 2-Hz tremor was observed daily 12 April-2 May. Geologists visited the crater 3 May, 1 June, and 21-22 June. On 3 May, total heat flow remained elevated at 290 MW (compared to 120 MW on 25 January). Crater Lake temperature was 36.5°C, a decrease of 2.3° since the last visit 12 April. Distinct convection cells with yellow-green slicks were present above several of the N vents.

A volcanic earthquake recorded at 0515 on 29 April was preceded by 1 hour of seismic quiet. Low-amplitude 2-Hz tremor with a number of small discrete earthquakes continued through May. From 24 May until 0800 on 26 May, there were 50-100 small high-frequency tectonic events that ceased when strong 2-Hz tremor began. Tremor persisted until 31 May. By 1 June, Crater Lake temperature had dropped 11° to 25.5°C, equivalent to a surface heat flow of 160 MW. Weak upwelling cells with yellow slicks were seen in the lake's N end. Minor phreatic eruptions had occurred between 21 and 28 May, possibly associated with the 24-26 May seismicity. By the 21-22 June visits, phreatic activity that began 20 March had ended. Crater Lake temperature was 22.5°C, down 3° since 1 June. No central vent upwelling occurred and N vent convection had weakened. Since early June only minor tremor had been recorded and no volcanic earthquakes were detected.

Cumulative 3-year extension of a crater EDM line totaled 28 mm. Monthly surveys indicated that this inflation accumulated from a series of four short-lived inflationary-deflationary pulses (October 1985, January & November 1986, and August 1987) associated with significant Crater Lake heating episodes that culminated in minor phreatic eruptions. Negligible deformation recorded by outer stations suggests no lasting deep magmatic movements since early 1976.

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: I. Nairn and B. Scott, NZGS Rotorua; P. Otway, NZGS Wairakei.