Ruapehu, New Zealand
Location: 39.3 S, 175.6 E
Last Updated: 11 September 2000.
Phreatic explosion at Ruapehu in 1992. Photo by
Christian Treber
.
Mt. Ruapehu is a stratovolcano on the North Island of New Zealand. It
has erupted about 50 times since 1861. At the top of the volcano
there is an acidic crater lake. Most eruptions are
phreatic
, involving
steam explosions caused by the contact of lava and water. The most
recent major phreatic eruption was in 1975.
September 27, 1999
A ground inspection on 17 September at the Ruapehu Volcano discovered an
actively convecting lake at 58 degrees C with sulfur slicks. Steam plumes were evident
and thought to be due to the cool weather and a hot lake emitting steam.
Between 21-26 August, elevated tremor culminated in a series of volcanic
earthquakes on 26 August. No eruptions occurred. On 13 September, increased
tremor levels resumed and are continuing.
This information was summarized from the Global Volcanism Program Volcano
Listserv.
October 13, 1997
Over the weekend of October 11-12, minor steam eruptions occurred in the
summit crater of Ruapehu. The largest of these eruptions occurred on
October 12 and threw mud and rocks about 500 ft (150 m) above the crater
lake. Some eruptions also produced tall steam columns. The lake level
in the summit crater of Ruapehu is dropping due to increased evaporation
from the heat and from the small steam eruptions. This is uncovering large
fumaroles on the crater floor. Increased activity at
Ruapehu is still likely in the near future.
This information was summarized from an
Institute of Geological and
Nuclear Sciences News Release.
October 10th, 1997
The alert level at Ruapehu has been raised from 1 to 2, indicating the
onset of eruptive activity. This occurred after a swarm of earthquakes
were detected beneath the volcano. The warning area now includes a 1 km
radius around the crater lake. Along with the earthquakes, small
geyser-like erutpions occurred on October 9th. The crater lake is
changing colors and steam is rising from its surface. All these
occurrances mean that a larger eruption at Ruapehu is likely.
This information was summarized from an
Institute of Geological and
Nuclear Sciences news release.
October 2, 1997
Two small eruptions occurred in the crater lake of Ruapehu on the evening
of October 2. These steam-driven eruptions threw mud and other debris
onto the shore of the lake. Scientists believe a larger eruption is
possible at Ruapehu in the near future.
This information was summarized from the
Global Volcanism Program Volcano
Listserv.
Pre-1997 Updates
The current increase in eruption activity began on Monday, September 18,
when a moderate vent-clearing explosive eruption occurred through the
lake. This eruption generated a flood and
lahar.
On September 24, volcanologists reported small explosions that
sent steam-rich plumes a few thousands of feet (hundreds of meters) high.
The effects of these explosions were confined to the area of the crater
itself. Later in the day, bigger explosions ejected steam-rich,
ash-bearing plumes to heights of 1,500-5,000 feet (500-1500 m) above the
volcano.
Conditions at the volcano intensified on September 25. The number of
volcanic earthquakes increased significantly and strong tremor (volcanic
earthquakes) was recorded for most of the morning. A strong eruption
made a column of ash and steam that
reached a height of about 6 miles (10
km). An ash plume drifted to the east for several tens of miles
(tens of kilometers). A substantial lahar flowed down the Whangaehu Valley.
Because of the increase in earthqauke activity and the involvement of new
magma,
the Alert Level for Ruapehu was raised to Level 4.
Mt. Ruapehu continued to erupt in late September, sending plumes of ash
and steam to a maximum height of 12 miles (19 km) and mudflows down the
flank of the volcano. Explosions in the crater threw large blocks of
rocks thousands of feet (hundreds of meters). It was the largest
eruption in New Zealand in the last 400 years. A volcano observatory,
nearby residents, and a nearby army village were evacuated. Roads,
railways, and air space near the mountain were closed. The status of the
volcano remained at level 4.
On October 2, the status of Mt. Ruapehu was reduced from level 4 to level3.
This change was based on a reduced level of activity at the volcano
including a reduction in tremor and volcanic earthquakes to pre-eruption
levels, decline in sulfur dioxide (SO2) emissions, and an end to ash
eruptions and the production of lahars.
Volcanologists at Mt. Ruapehu point out that the present eruption has not
ended and that some areas on the volcano continue to be hazardous. They
issued a warning that isolated explosive eruptions can still occur and
they established hazard zones on the volcano. Good weather permitted
field parties to investigate the volcano. They measured deformation of
the volcano, gas emission rates, and the erosion of ash by melting snow.
On October 3-4, earthquake activity remained at low levels. Eruptions
consisted of small steam-rich plumes containing little ash.
Volcanologists warned that lahars may be generated by rainfall
remobilization of material.
On October 5, volcanic tremor gradually increased following a magnitude 3
earthquake. Tremor was above pre-eruption levels. Weather did not
permit direct observations of the summit. However, ash did fall on the
flank of the volcano following the magnitude 3 earthquake, indicating
this event was accompanied by an eruption.
Earthquake activity increased on October 6. Tremor remain higher than
pre-eruption levels. Three magnitude 3.1 to 3.2 earthquakes occurred in an
11 hour period. One earthquake was associated with an eruption column
that reached 23,000-25,000 feet (about 7,000 m). The most
recent earthquake was associated with an eruption column that reached
more than 27,000 feet (about 8,000 m) and generated a small
lahar. Weather conditions prevented viewing of the summit.
Similar activity occurred the following day except the level of
background seismicity increased significantly, reaching levels just below
the highest levels observed during this eruption. Volcanologists pointed
out that there was no sustained eruptive activity. One spectacular
eruption produced a column that climbed rapidly to 25,000 feet (7,500 m)
and threw lava blocks, water, lake-floor muds, and volcanic ash from the
crater. Some blocks were thrown more than 0.6 mile (1 km) from the
crater.
On October 9, volcanic tremor was at moderate levels. Sixteen small
volcanic earthquakes were recorded in the early morning hours. Due to
poor weather conditions there have been no visual confirmations of any
eruptive activity.
On November 30, 1995, the scientific Alert Level at Mt Ruapehu was lowered
from 3 to 2. This reduction in Alert Level is based on a declined in the
last week to ten days in activity at the volcano. However volcanologists
remind people living near the volcano that this does not mean that
eruptive activity has completely stopped. Alert Level 2 indicates that
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
continue surveillance at a level which is commensurate with activity at
level 2. Volcano-tectonic earthquakes are now occurring at similar in
number and size to those which existed at Mt Ruapehu when it was last at
level 2 (mid-September 1995). The amount of sulfur dioxide released by
the volcano is about 1000 tons per day, well down from the highs which
have exceeded 10 000 tons per day. Although the Alert Status has been
lowered, volcanologists remind people that the present eruption has not
ended nor is it safe to approach the volcano. They note that isolated
explosive eruptions can still occur without useful warning signs.
On September 23, an explosive eruption sent lahars into the Whakapapa
Skifield. This occurred during Alert Level 2.
No significant eruptions have occurred at Ruapehu since November of
1995.
On April 21 a flood of mud and boulders moved down the Whangaehu River.
Examination of the seismic records revealed an event that lasted for several
minutes and was possibly a landslide. An overflight confirmed that a
portion of the north east rim of the lake basin had collapsed into the
Whangaehu Glacier and flowed down into the Whangaehu River valley. The
area that collapsed may have been weakened during the 1995 eruptions.
This type of failure has been a possibility in this area since September
1995 and further failures could be expected, especially
during periods of high rainfall. Based on the report of B.J. Scott,
Volcanology Programme.
June 20 1996 (Thursday)
1600 NZST (UT +12)
Overnight the intensity of seismicity at Ruapehu decreased to
levels that are lower than those recorded since prior to the
September-October eruption. Based on observations during an over
flight this morning the low levels are related to the very open
vent situation. The crater now has a flat floor at about
120-150m deep below pre-1995 lake overflow level. Volcanic gases
are freely streaming through the rubble on the crater floor,
to feed the eruption column. As the gas velocity changes so does
the amount of ash in the eruption column. As the vent area is so
open there is no necessity for gas pressures to build very high,
hence the low level of shallow seismicity.
An eruption column of dark grey ash was generated at 1144h to
over 600m and only the weakest of seismic signals was recorded at
Dome. The active vent is continuing to produce weak-moderate ash
emissions, which are being blown off down wind and continue
to reach over 100-150km from the volcano.
There is no evidence that this week's eruption has altered the
nature of the eastern wall around the active crater. In places,
especially on the northern slopes of Ruapehu, some large volumes
of loosely compacted scoria and ash have formed, and may well be
remobilised during rainfall. The debris has not frozen into place
at this time. Usually rainfall at this time will fall as snow
above 15-2000m. However there remains a high possibility that
these deposits could remobilise. The catchments most likely to be
affected are the Whakapapaiti clockwise around to the
Mangatoetoenui.
A COSPEC flight to monitor the levels of sulphur dioxide in the
plume was completed late on Wednesday, and data reduction
indicates a SO2 flux of 4100 T/day.
The alert level remains at 3.
BJ Scott for CJN Wilson (Volcanology Programme Leader) at the VOLCANO
mailing list.
July 8, 1996
On July 8, 1996, activity at Ruapehu intensified with an eruption from
the vent in the south part of the summit crater. The last moderate-sized
ash eruptions occurred from mid to late June. Discrete events began
early Monday morning with Strombolian explosions occurring every two or
three minutes. Lava bombs are been ejected to 300-1,500 feet (100-500 m)
above the vent and some were landing as far as 2,700 feet (900 m) out
from the vent. These discrete explosive events continued for about 12
hours and then began to decline. The eruption is producing an ash poor
plume to about 15,000 feet (5,000 m). The plume is blowing to the
north-north east. Very minor ash fall is occurring and will probably not
cause any problems. However, the gas and steam rich plume and will
produce a VOG (volcanic fog) which, if it starts raining, make some acid
rain.
Earthquake activity has been at levels similar to those recorded during
the June 17-18 events. This level and style of activity is consistent
with an alert level of 3.
The Institute of Geological and Nuclear Sciences provides the latest Science
Alert Bulletins and Ash Prediction Maps for Mount Ruapehu, New Zealand.
More information on
Ruapehu
is in Images of Volcanoes.
The latest pictures
of the
eruption are posted at the Volcanoes homepage of
Michigan Tech University
which has a mirror of the REAP homepage in New Zealand. Michigan Tech is
also posting
new
releases.
Sources of Information:
Institute of Geological & Nuclear Sciences Limited RUAPEHU ERUPTIONS homepage
Science Alert Bulletins, Institute of Geophysical and Nuclear Science,
New Zealand.
Simkin, T., and Siebert, L., 1994, Volcanoes of the World: Geoscience
Press, Tucson, Arizona, 349 p.
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