Geologic Background:
Ruapehu is a complex stratovolcano with five
Holocene vents on the summit and flanks. It was
constructed in at least 4 cone-building episodes. A
single active vent, Crater Lake, is located in the broad
summit region, but at least five other vents on the
summit and flank have been active during the Holocene.
The volcano is surrounded by a broad plain of
volcaniclastic debris.
Historic Activity:
- There have been frequent mild to moderate explosive
eruptions from the Crater Lake vent in historical
time.
- There have been at least 40 eruptions from 1861 to
1982. Eruptions have been reported in every year since
1966.
Recent Activity:
- From early 1983 to early 1988 there were periods of
inflation and deflation of the crater, rises and
decreases in the temperature of the crater lake. Changes
in the color of the crater lake were also been reported
regularly. During May 1985, February 1986, and August
1987 there were small hydrothermal eruptions that eject
water as much as 15 meters above the lake and steam
clouds upto 100 meters high.
- From March 20 to 27, 1988 small phreatic eruptions
were reported, and small eruptions continued into late
May after which none have been observed and the lake has
cooled. Moderate eruption on Dec. 8th resulted in steam
clouds and tephra fall. Ash fall on Whangaehu Glacier
resulted in small mudflow. Rising lake temperatures in
Jan. 1989. Continued rising lake temperatures and minor
phreatic activity in Feb. 1989. No discrete eruptions
during March and April 1989. Minor phreatic activity in
July 1989. Phreatic Activity in January 1990 ending in
early February. Eruption of hot buoyant blocks in crater
lake during mid-June. Decrease in lake temperature
through June 1991 and then sharp increase. Earthquakes
and evidence of eruption below lake. Small phreatic
eruptions within crater lake occurred during February and
March 1992. Lake temperature increased in December
1992.
- January-June 1993, activity at unusually low levels.
In early July crater lake temperatures increased sharply
and by late September had reached levels at which small
phreatic events had previously occurred. No phreatic
events have been reported by large steam clouds are now
common.
- Strong heating phase of crater lake began in
mid-January 1994. Convection cells in lake and minor
phreatic eruptions in March then cooling of lake thru
July with increased heating in late August. Another
heating phase occurred in Nov. following burst of
acoustic noise on Nov. 25. The lake had been heating
since Nov. 25, 1994 and by Jan. 18, 1995 the outlet
temperature had risen from 17° C to 46.5° C. On
January 13, Crater Lake was reported steaming vigorously
both before and after two small phreatic eruptions.
Following noteworthy "vent clearing" eruptions on June 29
and July 3, and phreatic eruptions in September, a series
of large eruptions began on Sept. 23. During the next
week discharge plumes were frequently reported by
aviation sources to have reached at least 10 km. Another
eruption occurred on Oct. 11, 1995.
- During night of June 15-16, 1996 high levels of
volcanic tremor were recorded. Eruption plumes were first
reported early on June 17, with a significant Strombolian
eruption late on the 17th. Eruptions were continued into
September.
- Increased seismicity and mud eruptions in early Oct.
1997.
- Increased seismicity and steam plumes in Sept.
2000.
- A tremor episode peaked on Feb 16, 2001.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 8, No. 2 to V. 14,
No. 11)
- Global Volcanism Network (V.15, No. 1 to V. 26, No.
7).
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