Variable-intensity eruptions continued during late July, August, and September after the last report (BGVN 21:06). The Alert Level between 22 July and 5 August remained at 3 (significant local eruption in progress). On 6 August the Alert Level was lowered to 2 (minor eruptive activity).

During 22-23 July, 2-7 discrete events were recorded every hour, and brown or light gray-brown plumes were observed. Pilot reports indicated that plumes reached heights of ~6 km. Minor ashfall was seen in downwind areas. Volcanic activity increased during 1500-2200 on 23 July, with moderate ash eruptions, explosive events, and volcanic tremor. The activity then declined, and only about one volcanic event was recorded each hour until 0600 on 24 July. After that time, tremor levels increased again, and moderate ash eruptions recommenced with some ash plumes rising to altitudes of 5,500-7,600 m. Minor ashfall was observed in Turangi (~35 km N) and Taupo (~70 km N). A COSPEC flight measured a SO₂ flux rate of ~9,000 metric tons/day. From 1600 on 24 July to 0200 on 25 July, seismicity suggested 3-7 ash eruptions every hour. After then only weak ash eruptions took place. Detectable ashfall was reported from the Hastings-Naiper area on 25 July.

A ground observation at 0900 on 26 July found that an apparently ash-free gas column rose from the crater. This period of quiescence was ended by a moderate eruption at 1253, which sent an ash column to 9-10 km altitude and was accompanied by very emergent volcanic earthquakes. Ashfall was observed near the volcano. Beginning at 1653, seismicity increased again and discrete ash plumes formed. On 27 July, 3-4 periods of relatively strong seismicity suggested that the eruptive activity continued. There was a white gas plume from the N section of the crater, and weak ash emissions from the S vent of the main crater. Ashfall was observed at Whakapapa in the morning.

On the morning of 28 July, pilots reported weak eruptive plumes up to 4,600 m altitude. A significant local ashfall was reported on the N slope around 0900. On the morning of 29 July, ground observations indicated only a white steam-and-gas plume around the summit area. On 30 July, intermittent tremor increased to low-moderate levels at 0100, then returned to low levels at 0900. This increased tremor accompanied very small ash eruptions with plumes dispersed at low elevations by strong winds. Pilot reports suggested a small amount of ash in the eruption plumes. An ash-bearing plume was also observed from Whakapapa at 1120.

A moderate ash eruption took place at 1930 on 31 July, accompanied by the resumption of moderate-level intermittent tremor. Eruptions also occurred at 2200 on 31 July, and at 0300 and 0630 on 1 August. Strong N winds kept the plumes at low elevations and caused most ash to fall on the S slope of the volcano. Ashfall at the Turoa Skifield from the eruption at 0300 on 1 August resulted in its closing. Moderate gas emissions with intermittent ash eruptions were observed on 1 August. A helicopter inspection at 1100 revealed that the active pit crater in the S of the basin might be deepening as intermittent ash eruptions excavated the tip of the magma column. Jetting onsets of ash emissions ejected blocks vertically; most fell back into the vent. No incandescence could be detected at the base of the eruption column. All these features suggested that the active vent was deep.

Tremor was low after 1100 on 1 August, but elevated during 0100-0600 and 1600-1800 on 3 August, and during 0300-0600 on 4 August, indicating that some minor ash emissions might have taken place. Tremor increased to moderate levels between 2000 on 4 August and 0700 on 5 August and appeared to have accompanied ash emissions. Very light ashfall was observed at Napier on the night of 4 August, and on the Desert Road the next morning (at 1100). A discrete earthquake was recorded at 0604 on 5 August near the Dome station; it appeared similar to the earthquake that resulted from a landslide on 21 April (BGVN 21:04). However, Tranzrail staff reported no sign of a high river level after inspection of the gauge at 1030. Tremor increased to low- moderate levels during 1500-1800 on 5 August, then returned to background.

A minor enhancement of eruptive activity occurred on the evening on 1 September consisting of two short-lived periods. The first was accompanied by an ash eruption that was reported by pilots and personnel at Flight Control Services in Taupo. The eruption plume trailed downwind (E) about 90-100 km. From 2301 to 2330 the second seismic sequence occurred, and an eruption produced minor ashfall over the Turoa Skifield area.

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: B.J. Scott, Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.