goodSchist

So far as volcanic eruptions go, you can’t go past the spectacular images of the strombolian eruptions from Tungurahua (in the Andes). Large magmatic outpourings of glowing hot rock, contrasted against crisp white snow and black-as-night andesitic dacite make for eye-catching imagery. But spare a thought for the more sporadic strata volcanoes around the Pacific Ring of Fire.

Last night’s minor steam-driven eruption from the caldera lake, high atop New Zealand’s Mt Ruapehu, is the latest event in a long and regular series of volcanic activity. Mt Ruapehu, along with its sister volcanoes Mt Ngarahoe and Mt Tongariro, make up the Tongariro Volcanic complex, which is the terminal southern point of the Taupo Volcanic Zone and on a grander scale, the terminus of the Tonga-Kermadec volcanic arc (animated fly-through from the NOAA).
Being a “modern” “western” country, New Zealand has kept close tabs on all its volcanoes, and a history of volcanic happenings from Ruapehu makes for some interesting reading. A full month-by-month report relating to Ruapehu’s activity can be found on the Smithsonian Institution’s monthly report site. It’s a long, very pedantic report, so best skip through it to the interesting bits.

Below are the details of last night’s eruption, and a quick run down of the 1995-1996 eruption, which was the largest eruption from Ruapehu in the last half century.

25^th of September, 2007

The most recent event occurred last night at 2015 NZST (0815 UTC). The eruption consisted of a small ash cloud and two lahar, which annoyingly swept down the two ski-fields in the North Island (Turoa and Whakapapa). A climber was seriously injured and both of the major arterial routes (which stupidly part around opposite sides of the mountain - what could ever go wrong?) were closed for a short time (pdf link). Though not a major event, it did catch almost everyone by surprise, as there were no foreshocks. This is fairly consistent with the steam eruptions from Ruapehu, and I suspect the lower hydrostatic pressure in the crater lake, due to the lahar which occurred in March this year may have had a part to play in this one.

1995-1996 Eruptive Sequence

This was the big one. An eruptive sequence that produced some truly epic ash clouds and lahars, shutting down airports, roads and rail links on-and-off for a few weeks. You’ve no doubt seen some of the images above if you’re a volcanologist or merely interested in things which explode. The 1995 eruptions were preceded with the caldera crater lake rising and falling in temperature (from ~13^oC to ~50^oC) and a long series of seismic tremors (unlike last night’s eruption). On the night of September 23^rd (yes, 12 years ago almost to the day from last night), the eruption began in spectacular manner. Over the following 13 months, including an 8 month period of dormancy between November 1995 and June 1996, 60 million cubic meters of ash were expelled from Ruapehu. As the satellite view above shows, that’s quite an impressive output. New Zealand has a prevailing westerly wind, so an extensive eruptive history of the Taupo Volcanic Zone can be studied by looking at seafloor drill-hole material off the east coast (as a friend of mine is currently doing for his MSc project).

So, whether or not we’re in for an exciting eruption season on Ruapehu is yet to be seen. Hopefully it’ll hold off for the next few months so I can get some snowboarding in while the snow is good (yes, I snowboard on an active volcano, it’s extreme).

References:
Global Volcanism Program: Ruapehu
Global Volcanism Program: Tongariro
Global Volcanism Program: Tungurahua
Tungarahua photos found thanks to Kim at All My Faults are Stress Related
NOAA satellite ash cloud image from the http://www.volcano.si.edu monthly report

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