Report on Ruapehu (New Zealand) — January 1983
Scientific Event Alert Network Bulletin, vol. 8, no. 1 (January 1983)
Managing Editor: Lindsay McClelland.
Ruapehu (New Zealand) Pattern of changes presages activity
Please cite this report as:
Global Volcanism Program, 1983. Report on Ruapehu (New Zealand) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 8:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198301-241100.
39.28°S, 175.57°E; summit elev. 2797 m
All times are local (unless otherwise noted)
NZGS personnel visited Ruapehu on 3 and 24 January. Crater Lake was clearer than in December, and was not steaming on either day. On 3 January, no upwelling was apparent over the central vent, but moderately strong upwelling was occurring from one of the vents at the N end of the lake, radiating discolored water and yellow-gray sulfur slicks. Lake temperature measured at the outlet was 22°C, 7° higher than on 15 December. The NZGS ascribed this to the significantly increased upwelling from the N vent, but also to reduced meltwater inflow. Reduced heat flow from the main vent was credited with the steady color change from gray to blue-green, as had occurred in the past.
On 24 January, upwelling was slight over the main vent and moderate over the N vent. A considerable number of sulfur globules were floating in the outlet area, where lake temperature was 20°C.
Although concentrations of magnesium and chlorine were higher on 3 January than in December, and lower on 24 January, the Mg:Cl ratio remained stable at 0.110 and 0.109 respectively, reflecting the generally low level of activity in the lake.
The 3 January horizontal deformation survey showed that the November and December inflationary trend had reversed. The 20-mm extension of the 600 m-wide crater that had developed between the 17 September and 15 December surveys had disappeared. The tilt-levelling survey detected no significant changes since the previous measurements on 17 September. However, a second deformation survey on 24 January revealed renewed extension. The distance between two stations on opposite sides of the crater was only 10 mm less than on 19 October. Analysis of deformation measurements since September showed greater changes than had been estimated, the most rapid changes recorded in recent years.
According to an NZGS tentative interpretation, the changing conditions at the volcano (table 1) indicate "strain release as a deep blockage (at about 1 km) of the vent was overcome in the latter part of December. The main vent may now be in an open state, and will possibly allow magma or gas to rise relatively freely. The degree of activity at the N vent is complex and is related to the degree to which the main vent is blocked as well as to the supply of heat from depth.
|Interval||Deformation||Temperature Change (°C)||Main Vent Upwelling||North Vent Upwelling||Mg (ppm)||Cl- (ppm)||Mg/Cl- Ratio|
|17 Sep-19 Oct 1982||None||-4 (to 25)||None||Slight||1022||8850||0.115|
|19 Oct-09 Nov 1982||+8 mm||-4 (to 21)||None||None||988||8725||0.113|
|09 Nov-15 Dec 1982||+22 mm||-8 (to 13)||None||Slight||900||8240||0.109|
|15 Dec-03 Jan 1983||-50 mm||+9 (to 22)||None||Strong||918||8350||0.110|
|03 Jan-24 Jan 1983||+10 mm||-2 (to 20)||Slight||Moderate||850||7785||0.109|
"The pattern of low lake temperatures with no visible upwelling above the main vent combined with temporary inflationary expansion of about 20 mm is similar to that of July 1980 and August 1981. In both cases heating from the main vent recommenced 6-7 weeks later, followed by hydrothermal eruptions from the same site about a month after that. If the same sequence occurs on this occasion we can expect to see strong convection start from the main vent by the end of January or early February, and (provided excessive heat is not released by the N vent) eruptions may commence by early March. (However) . . . due to the now apparently relaxed state of main vent, the onset of renewed activity . . . may start with relatively quiet lake reheating."
Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the Murimoto debris-avalanche deposit on the NW flank. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. A single historically active vent, Crater Lake (Te Wai a-moe), is located in the broad summit region, but at least five other vents on the summit and flank have been active during the Holocene. Frequent mild-to-moderate explosive eruptions have occurred in historical time from the Crater Lake vent, and tephra characteristics suggest that the crater lake may have formed as early as 3,000 years ago. Lahars produced by phreatic eruptions from the summit crater lake are a hazard to a ski area on the upper flanks and to lower river valleys.
Information Contacts: P. Otway, NZGS, Wairakei; I. Nairn, NZGS, Rotorua.