Ruapehu, New Zealand
Phreatic explosion at Ruapehu in 1992.
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.
- 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.
- 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.
- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- No significant eruptions have occurred
at Ruapehu since November of 1995.
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- 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.
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- 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.
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- 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.
- 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.
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- 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.
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