Things that go bump in the night!

Thursday, November 23, 1995

North island, New Zealand
Every year in my introductory geology course I cover topics on volcanic activity and on earthquakes, and every year, almost without exception, nature obliges me with some excellent practical examples. This term was no exception. The academic year had barely commenced when Mt. Ruhapehu, a volcano in the central volcanic belt of North Island, New Zealand, erupted with some spectacular phreatic explosions. Since the start of the eruption the New Zealanders have placed daily bulletins on the Internet, and one enterprising team mounted a minicam in the living room of the manager of the hotel on the lower flanks of the mountain. From this you can "sign on" and witness the activity above. Unfortunately, for those of us in eastern Canada and the United States, most of our working hours are hours of darkness in New Zealand, so you will have to be prepared for some late night or early morning views of the volcano. The relatively poor weather conditions (this is late winter in the southern hemisphere) also have hindered good viewing. To bring you up to date I have provided a few newspaper reports, a map and a summary of the recent activity. A separate article which follows the summary provides information on the minicam set up, and I have provided the URL's for you to do your own volcano watching on the far side of the world.

The news of the eruption first broke on September 24, 1995 (Web posted at: 1:00 p.m. EDT ; 1700 GMT)

Volcano blows at ski resort

MOUNT RUAPEHU, New Zealand (CNN): Skiers and hikers were shaken Saturday when a volcano erupted at a ski resort in New Zealand. Mount Ruapehu shot ash, rocks and steam into the air and sent three mud flows pouring down the 9,000 ft. mountain. There are no reports of anyone hurt or missing but skiers were evacuated and seven hikers were rescued. The volcano on New Zealand's northern island last erupted in 1988.

This first release was followed by a number of reports over the next few days, of which one from the Hobart (Australia) "Mercury" newspaper for Tuesday, 26 September 1995, is given below.

N.Z. braced for major blast

Scientists said yesterday a dangerous volcanic eruption was occurring at Mount Ruapehu in New Zealand as civil aviation authorities banned flights over the central North Island. "The kettle is boiling vigorously," said Department of Conservation scientist Harry Keys. "The more magma (molten rock) that approaches the surface, the bigger the eruption and the more voluminous the amount of material thrown out."

A resident of Ohakune township, at the foot of Mount Ruapehu, told Radio New Zealand the volcano was erupting violently and continuously. "The cloud is just billowing continuously ...just now it's going wild. The whole rim is just exploding," he said. The Civil Aviation Authority had closed the airspace up to 7600 metres (25,000 ft).

Civil defence authorities raised the alert level around Ruapehu to four on their five-point scale but a government spokesman said there were no immediate plans to evacuate the sparsely- populated region. He said a "general alert" was being issued, advising people to disconnect water tanks to avoid contamination from ash and urging motorists to drive with care on the main highway, known as the "Desert Road. "This is basically to get people to be on their toes, particularly people living within a 100 km radius of the mountain, and to keep tuned in to the radio for local civil defence instructions," he said.

The Institute of Geological and Nuclear Sciences said in a statement a continuous plume of smoke and ash was rising from many explosions. "The eruption is considered hazardous because there is clearly extreme risk to people on the upper slopes and in river channels which have, or may be, affected by lahars (mudflows). The ash plume east of the volcano presents a significant hazard to aviation," it said.

Keys said scientists were expecting a major eruption, probably within 12 hours. "The evidence is that we haven't seen the biggest yet," he said. Local ski fields were closed at the: weekend after earlier eruptions. Willy Sage, a pilot taking tourists for flights over the crater before the air restrictions were imposed, said rocks were being hurled up to 150 metres into the sky by eruptions every five minutes. "They are pretty big, some are as big as cars," he said.

Chris Griffin, general manager of the Grand Chateau hotel at the base of Ruapehu, said the mountain was "steaming away quite nicely" but there were no plans to evacuate 110 guests and 80 staff. He was checking the situation every half hour with emergency authorities.

Scott Lee, marketing manager for the nearby Whakapapa ski area, said about 100 people were being evacuated from 52 ski huts on the field and only a skeleton staff would remain.

Graeme Wheller 
Consultant Geologist,
Volcanex International Pty Ltd.

Mt. Ruhapehu has the potential of being an extremely dangerous volcano. As the highest mountain in New Zealand, the snow- capped peak can produce lahars (volcanic mud flows) which flow into the valleys draining the volcano. This is exactly what happened in December 1953 when the Wellington - Auckland express went off the track when a bridge over a river was swept away by an unexpected lahar. 151 persons were killed in this accident.

The reports of the activity at Ruhapehu make frequent mention of volcanic earthquakes, the status of the summit lake (which has now vanished), and the tephra fall (a potential hazard for aircraft) from the eruptions which has fallen in the vicinity of the vent and drifted across a substantial area ofNorth Island (see map). A typical report is as follows, although this was selected because it illustrates one of the more active periods of volcanicity.

Saturday, 7 October 1995: 19:00 hours NZDT (UT +13) Ruapehu Volcanic Activity:- Situation Summary

As of 19:00 hours the situation is as follows. In the last 24 hours we have recorded 2 significant eruption earthquakes, at the following times:

7 October 11:14: ashfall and small lahar into the Whangaehu catchment

7 October 15:03: ballistic blocks to 1 km, ashfall and lahars into Whangaehu catchment

At about 10:38 hours, a very small eruption earthquake was recorded on the Dome seismograph and observers on the mountain reported activity in Crater Lake. Eruption slugs rose 50-100 m above the lake, and a tall eruption column rose much higher above the mountain. The level of volcanic tremor on the Dome and Chateau recorders showed marked but steady increases after that event until the15:03 event.

A moderate eruption earthquake (Magnitude 2.7 Richter) was recorded at 12:14hours and this event produced ashfall to the northeast across the summit plateau and a small lahar down the Whangaehu River catchment.

At 15:03 hours today there occurred the largest eruption earthquake to be recorded since the recent eruption sequence of Mt Ruapehu commenced. Seismicity built up over a period of 4.5 - 5 minutes and then remained at a "peak level" for 8 minutes. Sustained tremor is continuing at an elevated level which is just lower than on Monday, September 25, but of larger amplitude than at any other time during the September/October eruption sequence. The accompanying eruption produced a spectacular eruption column that rose rapidly to over 25,000 ft (7.5 km) and was clearly visible under fine weather conditions. The eruption expelled lava blocks, water, lake-floor muds and volcanic ash from Crater Lake. The lava blocks were thrown over 1 km from the crater, while the water and lake floor muds generated lahars and floods into the Whangaehu catchment. No lahars were generated into the Mangaturuturu and Whakapapa catchments. Ash fell from the plume to the northeast of the volcano.

Weather conditions have been favourable today allowing two inspection flights to the crater area today. Both flights have revealed a Crater Lake is still present within the summit crater, although the lake is somewhat reduced in size with respect to previous inspections. The morning flight indicated that the eruptions of 6 October were accompanied by ballistic blocks that had been thrown at least 400 m from the lake. Small lahars had also been produced during the 6 October eruptions into the Whakapapaiti and Whangaehu catchments, and further ashfall had accumulated on the eastern flanks of the volcano. The afternoon flight showed that the 15:03 hours eruption had little effect on either the size of Crater Lake or the stability of the crater walls.

Discussion, conclusion and alert status

Intermittent explosive eruptions are continuing from the volcano during a period of increasing levels of volcanic tremor. We therefore advise that the Alert Level for Ruapehu remains at 3, with the important proviso that a rapid escalation to Level 4 could occur with little warning.

Alan V. Morgan

Footnote: As this issue goes on the Web, the Ruapehu eruption seems to have died down. The last entry extracted from New Zealand was on Thursday, November 30, 1995. It commences "...The level of volcanic activity at Mt Ruapehu has declined in the last week to ten days to levels that warrant lowering of the scientific Alert Level from 3 to 2. However this does not mean that eruptive activity has completely stopped. At level 2 the volcano is still very much alive, but is not erupting as vigorously as it previously was. The Institute of Geological and Nuclear Sciences will be decreasing the level of surveillance to a level which is commensurate with activity at level 2. As part of the process of lowering the Alert Level we have also reassessed the hazards and presented below is an assessment of the present hazards."