Variable intensity eruptions continued at Ruapehu. Although during 15-16 June volcanic tremor reached the highest levels seen in the past six months (BGVN 21:05), tremor dropped to background levels early on 20 June. Tremor and local earthquakes both remained low until 26 June when they increased slightly. A more definite increase in the intensity of seismicity took place on 27 June. Ruapehu then began larger discrete eruptions (around 0400) followed by ash-bearing eruptions (around 1000).

Airborne observers saw the volcano at 1120-1140 on 27 June and reported weak emissions that rose ~100 m above the crater. There appeared to be two source vents in an area N of the former crater lake's outlet area (on the crater's E side). The westernmost vent produced vivid white fumes; the easternmost vent produced dark gray ash. Ashfall had accumulated on Mitre Peak. Variable eruptions continued and after 1310 they grew larger. A ~6-km-tall ash column developed and ash fell to the E and SE.

The eruption continued the next day (the 28th), but appeared in conjunction with less seismicity and as a relatively gas-rich plume carrying minor ash. Tremor then was described as 5-10% of that recorded during the mid- June peak. Later, at 1144 on 28 June, a dark gray plume rose to over 600 m. Associated seismicity at the Dome station was weak. Weak to moderate ash emissions were detected 100-150 km downwind. Concern was raised about the mobility of near-source ash deposits.

A substantial amount of SO₂ escaped from the volcano. Correlation spectrometer (COSPEC) measurements of SO₂ flux rose from 4,100 metric tons/day (t/d) on 19 June to 5,100 t/d on 28 June (revised from an original estimate of 3,800 t/d), and remained high on 10 July at 6,000 ± 1,500 t/d.

Predominantly gas-rich plumes were seen from the ground on 1 July. Low seismicity, similar to that of 28 June, prevailed until 0400 on 2 July. At that time there were increases in both low-amplitude, high-frequency emergent events (the sort previously correlated with small ash eruptions) and larger high-frequency impulsive (A-type) events (interpreted as located at shallow depths beneath the summit). Bad weather on 2 July prevented detailed ground observations, but commercial aircraft reported ash-bearing plumes rising up to 600 m above the summit and blowing E.

The next day (2 July) white plumes were seen to 300 m above the summit. The day after (4 July) pilots reported a dark ash plume at ~3 km altitude extending 40 km downwind. At 1520 on 4 July, clouds cleared from the summit and observers saw a dark ash column rising up to 300 m above the summit; it deposited ash on snow in the ski area. On the Dome seismograph a large number of impulsive seismic events (larger than seen in the past few days) appeared to coincide with the ash column.

A report on 5 July stated that only relatively minor emissions had taken place since the last moderate-sized ash eruption (the event of 27 June). Intermittent minor eruptions still continued on 5 and 6 July, but on the later day, calm weather conditions allowed relatively high ash-bearing columns to develop.

An interval of increased seismicity, reaching levels seen in mid-July, began at 2030 on 7 July and preceded discrete inferred eruptions at 0115-0730 on 8 July. These eruptions took place at 2 to 3 minute intervals. An hour-long, intense burst of shallow seismicity ending about 0830 was followed by another interval of discrete explosions that diminished after about 1300. A helicopter flight at 0950-1030 documented an ash-poor plume to about 4.6 km altitude; an afternoon flight at 1330 also took place. Together, these flights confirmed that Strombolian eruptions ejected lava bombs up to 500 m above the vent.

Although seismicity dropped and then fluctuated in the last hours of 8 July, it resumed for intervals on 9 July. Eruptions continued on 9 July but bad weather and decreased, typically low seismicity prevailed during most of the next week.

At 1435-1735 on 16 July the volcano discharged an ash column to over 6-km altitude. Other eruptions followed; from 1735 to 0516 the next day there were 15 large explosive events, most with well-recorded air-wave phases. Tremor of variable amplitude and several hours of elevated seismicity prevailed. Aviation reports noted plumes at 3.3-4.6 km.

On 17 [July] seismicity dropped beginning at about 0330 and remained low for the next several days. Despite the lull in seismicity, in the afternoon on 18 and 19 [July] plumes rose 100-300 m above the summit.

On 20 July Ruapehu produced the largest outbursts since those in mid-June. Early on 20 July the seismicity and tremor again increased significantly; by about 0700 that day the tremor was replaced by small eruption earthquakes with airwave signals. Although pilots noted ash rising to 6.1-7.5 km altitude, some sustained moderate eruptions rose as high as 10.7 km and included bombs thrown 1.4 km above the crater and fire fountaining. Continuing through the next day, the seismic signals included both episodes with 2 to 7 discrete events per hour and episodes with continuous tremor and fewer discrete events. On 21 July the eruption's E-blown plumes, which were brown in color and thought to contain ash but not be ash-rich, ascended as high as 4.3 km.

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: Colin Wilson, B.J. Scott, B.F. Houghton, C.J. Bryan, S. Sherburn, T. Thordarson, and I. Nairn, Institute of Geological & Nuclear Sciences (IGNS), Private Bag 2000, Wairakei, New Zealand.