RDRC-91.SCI.IMMEDIATE SCIENCE REPORT
K-044; Mount Erebus Eruption Mechanism Study
Antarctica New Zealand November-December 1990
Abstract of Scientific Work Achieved
During the field season, 25 days of seismic telemetry and TV video recording were made of the eruptive and seismic activity of Erebus volcano. The lava lake was in the form of two main pools, which were convecting and fuming without strong explosive activity. No eruptions were witnessed while we Here on the mountain, although members of the party were at or above the upper hut on 7 days.
A seismic refraction survey was made on the summit plateau over a 1.4km distance between shot points in the Side Crater and near Truncated Cones. Three contiguous 330m spreads of the Nimbus seismograph were shot from each end of the line, using shots up to 10kg in one meter deep holes. The arrivals were weak but readable. They have been interpreted by ray–tracing as showing a 1.3km/s sub–permafrost layer up to 100m thick overlying a refractor with lateral velocity variations between 1.6 and 3.7 km/s. The maximum delay time relative to a homogeneous 4 km/s model is only 0.21s, and shows that the delays of 1.5s previously found in the waves from explosions in the lava lake do not originate under the summit plateau. The elimination of this possibility makes it more certain that the delay originates in the lava lake due to reduction of the velocity in lava due to vesiculatian.
Prior to removing the Victoria University television station, infrasonic microphones, and preamplifiers, and the NIPR long period seismometer from the mountain, the 3 MSF telemetry stations in the summit area were serviced. The El station seismometer has shorter period and lower sensitivity than standard, but the only spare was itself faulty. NSF equipment abandoned between the main and side craters after destruction by the 1984 eruptions was removed and the mountain left tidy. The magnetic induction loop was also pulled out of the ground were possible, and all equipment and non–burnable rubbish was returned to Mew Zealand, with much appreciated help from Scott Base staff.
Studies of the physics and mechanisms of volcanic eruptions are not well advanced world-wide due to the rarity and brevity of eruptions and the danger to equipment from the volcano, the weather, and human interference. Yet they are essential if we are to reliably predict disastrous eruptions. Erebus offers a rare opportunity because it is continuously active, has a lava lake acting as a window to the magma chamber, is only mildly dangerous to equipment and personnel, and has no risk of water damage or human theft and destruction. Its situation in an aseismic region ensures that all the earthquakes have volcanic significance, and the relative lack of electronic and atmospheric pollution in Antarctica enable excel1ent telemetry of data, and sampling of gas and heat output. The Antarctic Treaty has enabled International Cooperation and the sharing of costs and data to an extent which would have been next to impossible on most volcanoes. No other active alkaline volcano in the world can be studied so efficiently.
Thus our study of eruption mechanism is important worldwide, as well as because Erebus is a very large volcano of considerable importance for the understanding of the geodynamics and structure of Antarctica.
Our work has covered the distribution in space and time of volcanic earthquakes, explosion earthquakes, tectonic earthquakes, earthquake swarms and tremor, explosion infrasonic waves, magnetic induction signals from eruptions, infrared temperatures, eruption velocities and volumes of lava bombs, and the velocity structure of the erupting magma column. Also our TV surveillance has been of considerable help to S–081 (USAP) and to K092 in their studies of erupted gases.
Scientific Endeavours and Achievements
To eliminate the possibility that the observed delay to seismic waves from strombolian explosions was caused by thick and/or very low velocity layers under the recording stations on the flanks of the volcano, a seismic refraction line was run between the warm ground in the Side Crater near El, and the somma rim at the edge of the summit plateau near CON station (at Truncated Cones). El is a first order trig station, and CON transmitting antenna is 20m from an aerial photography "field station" marker. Running the seismic line revealed a 272.7m mistake in the survey position of Con station, which has now been revised to 77 deg 32′ 04,68″ S, 167 deg 05′ 06.54″ E. The effect wi11 ripple through al1 previous seismic results.
We flew to the Fang Glacier acclimatization camp in 2 lifts on 23 November after servicing al1 the equipment at Scott Base, and reinstalling the NSF telemetry receivers in place of the Victoria University ones. The NSF equipment was provided by S–O81 so that Erebus could continue to be recorded by NIPR equipment.
During the 3–day acclimatization at Fang, trips were made to replace a faulty seismometer at MAC station, to retrieve batteries page 3 from the TV station for use in the refraction survey, and to check out the toboggan route to the lower hut. We commenced the seismic survey on 29 November, drilling all the holes at the south end of the line, and shooting spread one from the south end. Leaving the spread with the Nimbus seismograph in a heated insulating box inside a dome tent, the party moved explosives, drill, and shot-firing/transmitting equipment to the upper hut with the Grizzly. On 30 November, Nick and Brent drilled all the shot holes in the side crater and fired two shots. Ray and Alla recorded the first on spread one and shifted to spread 2 for the second. On 1 December, spread 2 was shot from CON, and the whole team shifted the spread and recording site to spread 3, which Nick and Brent shot first from the Side Crater, and then from COM to complete the line. Completing in 3 days was an excellent achievement by Nick and Brent, and I wished we had taken more explosive to extend the work. The seismic arrivals were weak, and radio switching noise affected the digital recordings until we changed to clearing the record memories after giving the a "fire in 5 seconds" command, Our preliminary result is stated in the abstract.
After a rest day and a windy day, we brought down 3 more batteries from the TV station, and then on 5 December retrieved the buried electromagnetic loop Hire shown on L & S map 1253. Only about 60% of it could be pulled up, because it had been chopped up, displaced, and further buried by the eruptions of 1984. Our field assistant judged that NSF cables and buried batteries, abandoned at the summit after damage in the 19S4 eruptions, had higher priority. We had no authority from NSF, and it usurped a second trip to the summit planned by a Scott Base recovery team, but majority ruled, and we spent 2 days returning about 200 kg of Carbonnaire batteries to the upper hut, and spiral–4 cable to the 1ower hut. Again this was a remarkab1e achievement by Nick and Brent.
On 8 December we visited the Sauna Cave 470m from El in Azimuth 199 deg.E at spot height 3569.3m on L & S map 1253. Access is via a 15m deep shaft at the base of an impressive ice tower, and the rock cave (an old lava tube) underlies the source of the fumarole feeding the tower. The rock roof in the upper reaches of the cave is uncomfortably hot to touch, and the air is good and hot. Later that day we set out to retrieve the TV station but the 16 day spell of fine weather ended suddenly with blowing snow and dark cloud.
December 9 and 10 were also poor, but the CON transmitter was brought in and the VUW preamp/VCO removed from it. Reinstallation on 11 December showed that the mountain was clear except at the lower hut, and we were able to retrieve the TV station, except for one load left at the toboggan terminal in Ray's Gully when Brent reported his ring finger frozen nearly to the first joint. He had a chemical hand–warmer with him, but didn't appreciate the problem until he took his glove off. We applied the rapid warm–up treatment, and confined Brent to the heated hut. The finger blistered badly, and the nail came off, but it has since healed satisfactorily.
December 12 was bad, and we rescheduled our descent for the 14th, and began a 24 hour watch. Clearance at 2:30am on 13th page 4 enabled us to complete the TV retro, and remove the 2 VUW preamp/VCO's from the Ei station, and so we were ready to descend on 14th. However, an orographic cloud cap Formed, and persisted until the 20th, when Gentle 10 miraculously found its way through the clouds, and took all 4 of us off without any gear. Two more flights brought dawn most of the gear and rubbish, but much of the VUW equipment remained on the mountain, even though it had been carefully sorted and flagged.
Brent, Alla and Nick barely had time to pack their own gear before their scheduled departure on 21 December (held over to the next morning due to lack of a backup aircraft), but the major task of packing the VUW equipment from the mountain and the lab for shipment to another volcano remained to be done. The cost to VUW and myself of removing the abandoned NSF gear was now apparent. DSIR Antarctic kindly rescheduled my return flight and by working all hours, including three flights with S-081 (at their expense) to recover the remaining equipment at the lower hut, and the camp at Fang, I completed the packing with an hour to spare before my return f1ight on 27 December.
The Erebus program remains one of excellent international cooperation. but this season Phil Kyle of S–081 went up Erebus after our descent, and Katsu Kaminuma could not arrange funding-tor a Japanese collaborator.
Often disguised by our field event program, has been the work of maintaining a major recording facility in the Scott Base lab, and of processing the data from a whole years recordings. The processing is shared between Prof Katsu Kaminuma of N1PR Tokyo, who locates most of the earthquakes, and myself and Mr O'Brien (M.Sc. candidate) who have been studying the explosion earthquakes and video records.
A list of publications with VUW authors is attached. A book in the Antarctic Research Series of A.G.U. to be entitled Volcanological Studies of Mount Erebus is being prepared under Prof Kyle's editorship. The provisional list of titles and authors is attached.
The impacts of the VUW contribution to the IMESS and IMEEMS programs have gone with the exception of the degradation of the ice caves described by Giggenbacb (1975). By enlarging the opening to install seismographs in 1974, I altered the air/steam balance, and thus the conditions required to maintain the ice tower around the opening. Considerable natural ablation of the small glacier above the upper hut has contributed to this. The summit area has been tidied by the removal of NSF wire and batteries abandoned after the 19S4 eruptions.
Future Research.page 5
The study of seismicity and eruptive activity will be continued by Prof Phil Kyle with support from NSF, and by Prof Katsu Kaminuma of the National Institute of Polar Research. At present, the NSF telemetry receivers/demodulators and the Japanese analogue recording equipment continue to operate in Scott Base., recording from a six station seismic net at El, CON, MAC, HOO, ABB and BOM, which is entirely owned and operated within NSF. On the completion of the McMurdo Science Lab, the receiving/recording equipment might be moved to that lab. The trend there may be towards a permanent Erebus Volcano Observatory (EVO).
The proposed future program of Dr.R.R. Dibble is to reinstall his TV surveillance station, and infrasonic microphones, augmented with 3 seismometers on another volcano showing continuous strombolian activity, to check the conclusion that the seismic waves of explosion earthquakes are delayed by low velocities in vesiculated lava below the event. Yasur volcano (Vanuatu), Arenal (Costa Rica) and Pacaya (Guatemala) are suitable volcanoes, but negotiations are not yet complete.
Management of Science in the Ross Dependency
Antarctic Division are to be congratulated on the improved management of Scott Base and the field events, and the excellent cooperation with USAP achieved by the new SENSREP system. The cleanliness of the base area is also most impressive. However, the threatened departure of all the seismic recording programs from the Scott Base laboratory disturbs me.
Everyone concerned has been helpful to our program, but the consideration accorded us by Malcolm McFarlane and John Alexander in faci1itating the field work, Peter Kraak and Bruce McGregor in operating the recording equipment, and Pat Nolan in stores and freight were especial1y appreciated. Special thanks also to Phil Kyle of S–081 for generous help in bringing the VUW equipment down from Erebus in time for me to pack it for shipment to a new volcano, and to the Vice Chancellor's Committee, VUW grants committee, and Institute of Geophysics for financial support.