Mt Taranaki: Hmmm
Mt Taranaki is ~100 Km away from the rest of the volcanics in the North Island of New Zealand. Mythologically it’s a social outcast but geologically it makes me go “hmmm”. My submission for the Accretionary Wedge #6.
The Taupo Volcanic Zone (TVZ) in the North Island of New Zealand is the terminal point of the Tonga-Kermadec arc, the volcanic arc that traces the Pacific plate’s subduction beneath the Australian plate. It’s all pretty straight forward. The best way to show this is with the image below.
In the image, from south to north are Mt Ruapehu, Mt Tongariro and Ngauruhoe, the volcanics associated with Lake Taupo (a caldera lake), the volcanics associated with Lake Rotorua, White island and then a string of sub marine and sub aerial volcanics that make up the Kermadec arc. All of these are marked with red stars. The white arrow-line shows the subduction trench that’s the result of the Pacific subducting underneath the Australian plate. The orange triangle is the outline of the Taupo Volcanic Zone. Then there’s Mt Taranaki marked with a “?“. It is this particular andesitic volcano that makes me, and many, many others go “hmmm”.
It’s a rather large feature (see satellite imagery above. The dark green is the rough outline of the surrounding national park), having produced a classical almost-circular flank. It’s still active (last eruption, though minor, was around 1800), and it’s young, having commenced eruptive activity ~130 Ka. The really weird part is, it’s not geographically in-line with the rest of the TVZ volcanics, being ~100 Km west of the TVZ (even Maori legend makes note of this). And it’s not geochemically linked with the TVZ, being enriched in Potassium and other Large Ion Lithophile Elements. For a volcanic zone that’s popped up through an established continent, it’s also rather lacking in enriched, assimilated continental material, resulting in a fractionated elemental makeup (Here’s a thorough Journal of Petrology article detailing Taranaki and contrasting it with Ruapehu). But the big question that really gets to me, and one I haven’t found a satisfactory answer to is; why is Taranaki there at all?
You can check out other “geohmmms” at this month’s Accretionary Wedge.
Images in this article were taken from Google Maps and Wikipedia.
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7 Responses to “Mt Taranaki: Hmmm”
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February 24th, 2008 at 5:15 am
You have a great “hmmm” there. I have no real expertise in volcanism, but looking at the diagram I am reminded of the Cascades arc, and the fact that St Helens is placed noticiably west of the main arc. In our flat Central Valley in California we have a possible remnant of the Cascades arc in the Sutter Buttes, which lie well west of the ancient arc, and are in fact the only interruption to the topography of the valley for a great distance. I don’t know why. Hmmm?
February 24th, 2008 at 6:00 am
Clearly it’s a mantle plume!
February 24th, 2008 at 10:19 am
Andrew: I don’t think any other 5 word geology joke has made me giggle so much
March 11th, 2008 at 3:10 am
[...] the volcano scale, Chris wonders why Mt. Taranaki is so far west, relative to the rest of New Zealand’s volcanoes. I’ve only seen Taranaki from the plane, and [...]
June 15th, 2008 at 2:35 pm
[...] and why it is where it is. I’ve talked about Taranaki in a previous Accretionary Wedge “Things that make you go hmmmm“, it’s definately an oddly positioned volcano. But this time I’m going to talk [...]
April 18th, 2009 at 6:12 am
If you look there is actually a second volcanic trend to the kermadec arc behind the main trend which forms the TVZ. If we account for the curve of both arcs with the plate boundary, Taranaki lies along this second trend, and the basaltic andesite composition and structure (strato) points to arc-volcanism. Many Arc systems have two trends/lines of activity, see the marianas trench bathymetry for a great example.
This is due to dehydration of the subducting slab with depth. Metamorphic reactions which occur at various depths release fluis (minly water) into the overlying mantle wedge, causing it to melt and produce magm which eventually forms the arc system. The reason there are two lines of volcanism is that there are two reactions which release the fluid, and this occur at different depths (dehydration of amphibolite and dehydration of ecologite and also possibly serpentinite). The shallower reaction (amphibolite) forms the first trend, and the deeper reaction forms the secondary trend (distances can depend on the slab angle… in some areas no secondary trend occurs as subduction ocurs too steeply (i.e. the andes).
It looks like taranaki lies along the second trend…
April 18th, 2009 at 3:01 pm
Mole: There’s no doubt it’s a subduction-driven volcano due to being at the tail end of the Tonga-Kermadec arc. The “Hmmm” bit is the geochemical disassociation with the rest of the TVZ, the lack of a line of back arc volcanism leading up to Taranaki, and that Taranaki has a little volcanic arc of its own which trends SE/NW whereas the Tonga-Kermadec trends SW/NE. Below I’ve made little map to show the trend of Taranaki volcanism, of which Taranaki and the smaller Fanthoms peak are the modern active vents, while the Pouakai complex and the Kaitake range are the older, seemingly extinct vents:
And here’s the trend of the TVZ vs that of the Taranaki region:
I’ve been out of the loop for while, though, so I’m a little hazy on the region.