Both White Island and Tongaririo have shown activity this week. White Island was last active in 1980, and is a beautiful example of an andesite volcano. Tongariro is a volcanic complex, comprising numerous adjacent vents. Technically, it includes Ngauruhoe (a volcano I have spent some time on, and wrote about back in January).
The first thing I should point out is that these three eruptions are not related. This is just one of those statistical anomalies that crop up with random scatter. All three volcanoes are associated with the subduction of the Pacific plate under the Indian-Australian plate, but their magma plumbing systems are completely unrelated. To put the magma plumbing in perspective, the crust is 35-40 km thick, and the distance between White Island and Tongariro (the closest two of these events) is about 220 km in a straight line. These two volcanoes lie at approximately the NE / SW extents of the Taupo Volcanic Zone (VTZ), and have the much larger features of Rotorua and Taupo separating them. Below is a map which plots the recognised features of the TVZ (taken directly from here and plotted in Google Earth), with White Island and Tongariro highlighted.
Importantly, the Monowai seamount – although associated with the same tectonic boundary (the Kermadec trench), is clearly not directly associated with activity over 1400 km away.
The Kermadec/Tonga trench is itself an interesting feature. At over 10 km deep in places, it is one of the deepest trenches known, and with a closure rate of 24 cm per year at the northern end, and perhaps as low as 4.5 cm per year at the southern, is something of an anomaly.
The eruptions we’ve seen from Tongariro and White Island have been small. The Tongariro event was particularly interesting as while there was some indication of rising activity 2-3 weeks ago, there was no immediate precursor activity. The seismic trace below is for the 24 hour window in which the eruption started. The drum is timestamped in the top right, and each horizontal trace represents 1 hour (numbered in hours before timestamp). All seismic traces here courtesy of GNS Science.
That lack of build up activity is good to highlight. One of the biggest issues volcanologists face is trying to provide some level of forecasting for eruptions. The simple fact is (and this event proves) that while we can monitor all we like, even with light-speed communication, eruptions are not nice orderly events which lend themselves to easy forecasting.
Below are two more seismic drums, from the last 24 hours at two separate locations on Tongariro.
The red lines indicate points where the signal was stronger than the drum could record. You can see the constant activity in the second trace 7-14 hours ago, which is likely to represent magma movement within the plumbing system, similar to that demonstrated at El Hierro for example. The alert level at Tongariro is 2 (minor eruptive activity), and its aviation colour code is down to yellow (down from red at the onset of eruption).
The White Island eruptions are somewhat more significant – not least because while Tongariro (exluding Ngauruhoe) has not been active for over 100 years, White Island was last active in 2001, following 25 years of activity. This is the seisimic trace from White Island:
And it’s looked like that pretty much constantly since Sunday.
So, with everything from rhyolite/obsidian eruptions, to the largest eruption seen on Earth in the last 5000 years, the TVZ is a fascinating study. It is related to a complex plate boundary system, as it transitions from subduction to transform (before reversing its subduction direction West of South Island), and on the whole a great place to do some geology.
There’s some great videos and images collated by Erik Klemetti over at the Eruptions blog which are well worth having a look at.