In 1995, the Ruapehu volcano erupted and ejected rocks, water, and gas across several square kilometers, creating the most common but least publicly known volcanic hazard. All of this happened without triggering a local warning system. Because of a major ski resort nearby, researchers are reviewing the 1995 event to help ensure that future volcanic eruptions do not take visitors by surprise.
by the Ruapehu Surveillance Group, written by Ian Nairn and B.
J. Scott, Institute of Geological and Nuclear Sciences, Taupo, New Zealand
A spectacular explosion beneath Ruapehu Crater Lake on September 23, 1995, ejected lake water, rocks, and scoria bombs onto the summit of the snow-covered Ruapehu volcano and sent lahars down its outer slopes (see photo on front cover). Eruptions continued into October, completely emptying the 10 million m3 lake, and dispersing 0.1 km3 of ash and scoria downwind.
While the volume of ejecta was relatively small, the eruption was big news in New Zealand because Ruapehu is the site of the only major ski resort on the nation's North Island. The slopes near the volcano attract as many as 10,000 skiers on some days, and chair lifts and tows carry skiers within 2 kilometers of the summit crater (see figure ). In fact, several lifts and tows cut across areas that were overrun by lahars during eruptions in 1969 and 1975.
Fortunately, no one was hurt by the 1995 eruption since it occurred when the ski slopes were closed. But it dramatically demonstrated volcanic hazards at Ruapehu to a largely unaware public. A larger explosion, like those that occurred in 1969 and 1975, would have produced larger, faster lahars that could have flowed into other valleys. That possibility has set researchers to redesigning the Ruapehu lahar warning system.
Ruapehu crater normally contains a warm lake, about 500 m in diameter, and situated at 2500 m elevation. The lake is surrounded by permanent snow and ice fields. Lahars produced by explosions through the lake have posed the major hazard during eruptions in 1861, 1895, 1903, 1945, 1969, 1971, and 1975, and nearly all of these eruptions produced lahars in the valleys draining from the summit. For instance, the catastrophic failure of an ice and debris dam on the rim of the crater in December 1953 sent a 1.6 million m3 lahar down the Whangaehu River. That lahar destroyed a rail bridge 38 kilometers downstream and caused a passenger train to plunge into the flooded river, killing 151 people.
The Ruapehu Crater Lake, which was about 150 m deep prior to 1995, traps chemicals and stores much of the heat released from the underlying magmatic/hydrothermal system. Through the Ruapehu Surveillance Group's volcano monitoring program, researchers track variations in lake temperature, water chemistry, and lake level/overflow, in addition to ground deformation and seismicity. The surveillance network includes a lahar warning system, whereby ski field alarms are triggered when large volcanic earthquakes are followed by destruction of a hut on the dome summit of Ruapehu, about 0.5 kilometer north of the lake.
In early 1995, temperature fluctuations in Crater Lake had been accompanied by minor steam eruptions. In August 1995, a report on lahar hazards identified areas on the Ruapehu ski slopes which were at greatest risk from eruptions like those 1969 and 1975 events, when large explosions ejected lake water, mud, and rocks onto the summit of Ruapehu.
The main 1995 eruption sequence began on September 18, when a volcanic earthquake was followed by the largest lahar on the Whangaehu River since 1975. Bad weather prevented eruption observations. A smaller eruption during the night of September 19–20 produced another Whangaehu lahar, and nine hours later, the temperature of Crater Lake was 48°C. The Crater Lake was overflowing, which was unexpected, given that a considerable amount of lake water had been ejected into the Whangaehu lahars.
These events—as well as fluctuations in lake chemistry, an increase in the width of the crater, and the appearance of fresh scoria on the lake shore—all indicated that there was magmatic activity beneath the crater and that lava was being extruded into Crater Lake. The volcano alert level was raised on September 21, with visitors warned to avoid the crater area.
No further significant volcanic earthquakes were recorded until September 23. The ski fields had closed at 4 p.m. (local time) and were largely deserted by 4:57 p.m. when an 18-second-long sequence of three explosions of increasing size occurred. Black spear-headed slugs were ejected more than 1 kilometer vertically and laterally. Rocks, mud, scoria bombs, and lake water were projected onto the summit, white base-surge clouds rolled across the summit, and lahars were generated in the Whangaehu, Mangaturuturu, and Whakapapaiti valleys. A steam plume rose 12 km with minor ash fallout to the northeast. The Whakapapaiti lahars entered western areas of the Whakapapa ski field within 90 seconds of the initial explosion.
Smaller eruptions continued until September 25, when volcanic earthquakes and tremors intensified. Frequent explosions projected black plumes hundreds of meters above the lake surface, and white steam clouds filled the crater basin. Although this eruption style is common at emerging submarine volcanoes, it seemed incongruous on the summit of a snow-covered mountain. The plumes formed a 10-km-high column, and a highway to the east was closed due to ashfall.
After being progressively emptied after September 23, Crater Lake was finally dry after an 8-hour explosive eruption on October 11. Ash fell as far as 250 kilometers downwind. Later eruptions occurred from two deep, active vents in the crater basin, producing dark, ash-rich columns. Another voluminous ash eruption on October 14 was followed by intermittent explosions through late October.
Substantial lahars in the Mangaturuturu and Whakapapaiti valleys were generated only by the September 23 explosion, when laterally directed jets impacted on the snow-covered steep upper slopes of these valleys, followed by base-surge clouds rolling across the summit. Falling water, mud, scoria, and lithic blocks mixed with the snow cover and moved downslope. The Whakapapaiti lahar flowed with estimated velocities of 15–20 meters per second.
Later, heavy rainfall and spring snowmelt remobilized the ash and scoria atop the snow- and ice-covered upper slopes of Ruapehu to produce secondary lahars. Two Whangaehu secondary lahars had peak flows of 60 and 105 m per second, even larger than the September 23 lahar. Similar secondary lahars occurred in the Mangatoetoenui valley, flowing at sufficient speed to override a 25-m-high ridge near Tukino and destroying a walking track bridge.
Significant eruptions at Ruapehu ceased by early November 1995. Crater Lake has been slowly refilling, aided by the summer melting of the ice sheets of the crater basin. And as lake volume increases, the lahar hazard from renewed explosive eruptions also increases. The lake should reach overflow within 3–4 years if there are no large eruptions. As it did in 1953, such an overflow could lead to the collapse of the margins of the lake and a catastrophic lahar down the Whangaehu River.
The pre-1995 lake overflowed across solid lava that is now buried beneath the unconsolidated ash and scoria deposited on the outlet area in 1995. So now the rising lake will be closely monitored as it nears the pre-eruption level, particularly if erosion has failed to restore the outlet to the pre-1995 level.
The 1995 eruption provided new insight into volcanic hazards at Ruapehu and emphasized the need to redesign the lahar warning system. The high-velocity lateral jets of volcanic debris and gas that were produced by the unusually shallow explosions sprayed ejecta into the Whakapapaiti and Mangaturuturu valleys. The distance traveled by the debris in those jets made for a relatively short time between the initial explosion and the formation of lahars in those adjacent valleys. More importantly, the lateral jet that unexpectedly generated the Whakapapaiti lahar did not destroy the Dome Shelter, so the lahar warning system was never triggered. Firm snow conditions allowed the small Whakapapaiti lahars to reach high velocities and rapidly enter the Whakapapa ski field. Hence the warning system needs to be redesigned so that it triggers earlier and allows increased warning times in the ski fields.
Source: Eos, May 14, 1996, p. 189.
Since this article was written, Ruapehu began erupting again on June 17, 1996. An ash eruption similar to those of October 1995 ejected the small lake which had re-formed in the crater. A small lahar was formed in the Whangaehu Valley, and ash fell on towns more than 150 km to the north. Intermittent eruptions continued into August and, although relatively small, disrupted the 1996 ski season, with severe effects on the local economy.
I was born in 1944 and raised in Wellington, New Zealand, where I attended Victoria University for both undergraduate and graduate studies.
My interest in science was sparked by a chemistry set given to me as a young child, and by a couple of good teachers at high school. I became interested in geology at about 10 years old, when my parents took me along to night-school lectures and field trips. I remember being astonished at just how old rocks were.
My current research interests include analysis of the recent eruptions at Ruapehu, and how these events fit into the pattern of long-term volcanic activity in the past 20,000 years at Ruapehu and the adjacent Tongariro volcano.
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