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The Taupo Volcanic Zone – Part I

[link to volcanocafe.wordpress.com]
incredible images and diagrams with this fantastic article

This is the first installment of a series I have been planning to do for, um, about 16 years, on the TVZ and I should state right up front I am one of the least qualified people around to do this, being neither a geologist nor currently in New Zealand. What’s more I gleaned virtually all of my knowledge about the TVZ second hand and most of that while sitting on the other side of the planet, here in Germany. If anybody who is more knowledgeable about the TVZ has material to add or corrections, I would be most happy to hear it and will readily bow to their greater wisdom. Also the wider geological setting of NZ has already been touched on by Nathan in his post on the Auckland volcanic field so I apologize in advance for any redundancy on my part. Anyway, here goes, the TVZ:

Best bits firsts

To get straight to the juicy bit, the TVZ is currently the most prolific rhyolite-producing region on the planet (no doubt this will change, possibly at short notice – eek). But for the moment the TVZ is the bee’s knees. It is home to two active calderas, each of which can claim membership to the “mega-scary” club (Taupo and Okataina), and a number of other currently inactive calderas, each of which has produced a VEI 7 or greater eruption in the last 300,000 years. In fact there have been at least 34 ignimbrite caldera-forming eruptions in the last 1.6 million years. Take that, Yellowstone! (just joking).

Averaged out over the last 65,000 years (and, yes, in human terms, that’s a lot of years, ask the nearest Neanderthaler), Taupo alone, i.e. just the one volcano, not the zone, has produced 2 m³/s of fresh magma#. ( C.J.N. Wilson 1993, Stratigraphy, Chronology, Style and Dynamics of later Quarternary Eruptions from Taup volcano.) Set your watches. If that is not bad enough, what is more disconcerting is the apparent chaotic nature of major eruptions. Repose times don’t seem to have a lot of credence in New Zealand. But, more of this later on a separate installment on Taupo Volcano.

Less well known is that New Zealand is just the emergent tip of submerged continent, which kind of fits our national psyche. We’re actually one of the cratons that made up Gondwanaland but we were so thin and weak at the knees that we just slipped into the ocean when we departed from Australia 83 million years ago. The bulk of the continental crust making up Zealandia, the submerged continent that stretches from New Caledonia in the northwest to the Chatham Islands in the southeast, an area half the size of Australia, is composed of greywacke. For the linguists among us, greywacke is a word that originates from the mining industry in Germany which miners used to refer to the “grauer Wacke” loose sedimentary deposits that had to be removed before finding the profitable coal seams below. < [link to en.wikipedia.org] Well, I guess it is pretty grey and non-descript stuff, but, at least in New Zealand, greywacke is actually formed from eroded granite. And granite is pretty prevalent stuff. In fact, according to Wikipedia, 70% of the world’s continents are actually formed of granite, indicating just how much of the current continental crust was at some stage in the past in molten form (at depth) but this is the very stuff of which rhyolite volcanoes like Taupo are made.

However, the greywacke that makes up the bulk of New Zealand does not come from volcanic centers in Zealandia. Rather, it is most likely the eroded sedimentary remains of volcanic centers in Australia, Queensland to be precise, that probably formed in the first stages of the rift that finally drove Zealandia and Australia apart. Basically, vast volumes of eroded material were washed off the coast of Australia into the Pacific basin some 385 to 100 million years ago, forming new continental crust, a process known as continental accretion

Later, as rifting set in big time, the ocean encroached on the newly formed Tasman basin that split the eastern seaboard of Australia off from the remains of Gondwanaland and the fragment (Zealandia) drifted off towards the east, only stopping 23 million years ago when the spreading center in the middle of the Tasman inexplicably stopped. You can see the same kind of rift mechanism at work today in the Afar triangle. The main points to remember though, as they are important later when we get to the TVZ, are that Zealandia is a craton, composed mostly of the sedimentary products from the erosion of continental granite. However, it is thin crust, so thin that it lacks the buoyancy to poke its head above water like most other cratons. In fact it is conceivable that all of New Zealand was fully submerged 23 million years ago, however the jury is still out on this one. Possibly a few islands survived and it is from these islands that the bizarre flora and fauna of modern New Zealand evolved in the meantime.

So if only two things should be imprinted on your cerebellum by now it should be these: New Zealand is composed of thin crust, and this is made up of sedimentary rock composed of grauwacke, the product of the erosion of distant volcanoes in Australia.

All well and good. Zealandia could well have remained submerged for ever if it weren’t for that pesky plate tectonics thingy-me-bob. At some point, about 23 million years ago, the Pacific plate and the Australian plate changed from being a passive margin to becoming an active margin and the consequences for New Zealand were enormous. In fact without it, we probably wouldn’t be here today, or if so, us New Zealanders would be all crammed on the Chatham Islands and inbred beyond redemption.

Anyway, rather than taking a nice easy route around the plate margin, the Pacific and Australian plate margin decided to just run straight through the flimsy bit of continental plate called Zealandia. In fact you can basically divide up the current plate margin into three sections:

1. the northern section, where the Pacific ocean plate subducts under the Australian plate (Kermadec trench / Hikurangi trough
2. the central section, where continental crust to the east rams into continental crust to the west, forming the Southern Alps in the process
3. the southern section, where Australian oceanic crust subducts below old Zealandian crust now located on the Pacific side of the plate margin.

To complicate matters, this is not a pure head-on collision, but oblique, with the Pacific plate heading WSW and the Australian plate heading NNE, resulting in a good deal of shear along the plate boundary (this point too will become significant when we get to the TVZ). In fact the ratio of horizontal shear to vertical motion in the central section where the Southern Alps are forming is about two to one with the west coast of the South Island slipping NNE two meters for every 1 meter the Pacific side rises in height.

To complicate matters still further, the globe is not just a nice planar surface but nicely curved. Given the dimensions of NZ (more than 1500 km in length) this curvature also results in some interesting distortions which are highly significant if you want to fully understand the origin of the TVZ.

Basically, the Alpine fault is the crunch zone where the plates are pushing and slipping past one another. There is an enormous amount of deformation involved here, with a lot of folding and mountain building going on. However to the north, on the upper part of the North Island, the Pacific plate is subducting smoothly (well, ok, not so smoothly) under the Australian plate, ripping off and swallowing the leading edge of the Australian plate in the process. This is the East Cape which represents not only the accretionary edge of the margin but also, due to friction and the weird far-field geometry involved, is slowly getting eaten and drawn down into the trench along with the Pacific plate. Geonet used to have a great animation of this but unfortunately I can’t find it anymore. This, however, is crucial to the formation of the TVZ, for this mechanism is precisely what drives the back-arc rifting that forms the TVZ. Moreover, it is not just back-arc rifting but an extension zone with a slight rotary component that has its focus somewhere to the south of Ruapehu

Consequently, you effectively have two primary mechanisms for volcanism working in unison in the TVZ: subduction and rifting (did I mention that the Zealandia craton was anyway pretty thin stuff?). This combination is what makes the TVZ so prolific at generating the grey stuff that ignimbrite sheets are made of. The crust in the TVZ is extremely thin. Some estimates put it at just five kilometers but I would be wary of overly simplifying the schematics of it. 15 to 20 km is more likely.

North of Okataina, the Whakatane graben is opening up at quite a dramatic rate of about 2cm a year. ( Lamarche, Barnes et all 2006) However, volcanism here has been restricted to andesite, volcanic arc type volcanism (stratovolcanoes such as Edgecumbe and White Island). South of Taupo the same volcanic arc type andesite volcanism is evident (Tongariro and Ruapehu). It is the bit in the middle that is so frightening. Here thin crust is getting heated from below, resulting in massive magma chambers and associated caldera volcanism. I count 17 calderas in the above picture, almost all of which have formed in the last 400,000 years. This caldera volcanism is what the next installment will concentrate on.

Oh, there is something important I forgot. Batholiths. Nobody ever talks of batholiths.

Batholiths are weird things. Probably the largest volcanic features on the planet outside of large igneous provinces and mid-ocean ridges but you don’t get them at mid-ocean ridges because they are, by definition, only a feature of continental crust. But, like mid-ocean ridges, they are made of melt, and to get melt in continental crust you need a heat source and/or a source of volatiles (primarily H2O and C2O) which is why you will find them forming behind plate margins where a subducting plate can provide the needed ingredients to induce large scale melting in the crust. Now, just because they are composed of melt does not mean they are going to erupt. Far from it. The vast majority of this melt remains locked in the crust and slowly cools off to form granite. If the tectonic forces are right, they will be later lifted and eroded and provide some excellent rock climbing opportunities a few million years down the road. But batholiths can be huge. I went to Sardinia this year. The Costa Smeralda is a beautiful place and composed of granite as far as you can see. How much of a later batholith is actual melt deep in the crust at any one time escapes the extent of my reading. Maybe someone can fill me in here.

Whatever the case is with batholiths, it is obvious that the presence of a large volume of melt at depth in the crust is conducive to magma genesis at shallow levels, either as a source of melt in its own right, rising in diapirs, or indirectly, as a heat source for shallower magma chambers. I think it is a pretty safe bet to assume there are batholiths involved under Sumatra, the Altiplano, the Sierra Nevada in California, and New Zealand is no exception. The Southern Alps have lifted up and exposed a region called the Median batholith that originated when Zealandia first rifted away from Australia. More recently, as the Pacific/Australian plate margin switched from passive to active, much younger batholiths have formed, leading to the beautiful series of Coromandel volcanics, which, according to a tantalizing footnote in my favorite book on New Zealand (see below), just might play a role in Taupo as well. … but more of that in the second installment.

Bruce Stout
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