Ok, so now we know a bit about the tectonic setting, a bit about what the volcanoes in the TVZ can produce.. what’s left is all the juicy stuff, all the questions that come to mind, such as
Why is the TVZ so prolific?
Why is rhyolitic volcanism constrained in a band between Okataina and Taupo?
Why is the pattern of volcanism so chaotic?
Why are the repose times between big eruptions (sometimes) so short?
When you drive through the TVZ, you could be forgiven for not noticing that you are in the middle of hell on earth. Sure, there are very extensive geothermal regions and often steam just suddenly appears out of the ground at the side of the road, but most of the time you drive through miles and miles of planted forest or green farmland. In fact, without any geological knowledge you wouldn’t really know you are sitting on a couple of kilometers of volcanic debris, the products of the many ginormous eruptions that fill the Taupo graben.
This is what Lockwood and Hazlett refer to as landscape burying volcanism, as seen in the Valley of Ten Thousand Smokes in Alaska. According to Wilson (1995), the Taupo graben is filled with more than 10,000 km³ DRE that has erupted over the last two million years. No wonder it is hard to see.
Even the geologists took a while to identify some of the features, such as the Reporoa caldera, source of the 230,000 year-old Kaingaroa ignimbrite because these features are simply so overlain with the products of other massive eruptions. The 700,000 year-old Kapenga caldera, for instance, is completely buried.
So, why is the TVZ so prolific compared to other rhyolite centers around the world?
Well, there are a couple of answers to this. The first is obviously the combination of extensional tectonics, with the graben opening up at a rate of roughly 8mm a year, combined with the back-arc volcanism related to the relatively rapid subduction of the Pacific plate under the thin crust of Zealandia. The subducting Pacific plate delivers heat and volatiles and the extension of the thin continental crust of Zealandia means that the rising heat and volatiles from the saturated oceanic slab encourage the formation of large rhyolite chambers at relatively shallow levels. Remember that the crust of Zealandia is largely formed of greywacke, a sedimentary stone formed from the erosion of granite, i.e. recycled silica rich igneous rock. We can see this combination of subducting ocean slab erupting through continental crust at other sites of large caldera volcanism, such as the Altiplano in South America and the large calderas of the US (e.g. Long Valley). The difference in New Zealand is obviously that the continental crust is very thin, in places no more than about 15 km, compared to the Altiplano, where the crust is about 70 km thick.
However, there is another factor that may well be at play here. According to Wilson’s paper on the Oruanui eruption, there is an astounding homogeneity in the eruptive products from Taupo. In other words, the magma chambers did not form over a long period of crystal fractionation with various phases differentiating out within the chamber. This led him to surmise that perhaps the source rock for the massive Oruanui eruption was not the product of fractionation over a long period of time, but was quite possibly rejuvenated plutonic rock. In fact there is some indication that the source plutons may have formed contemporaneously with the Coromandel volcanics. Remember how I mentioned batholiths in part 2 of this series? Quite possibly, the Taupo Volcanic Zone marks the intersection of a former batholith related to Coromandel with the current back-arc related volcanism. And I would not be surprised, if a new batholith were forming right now, extending up into the Havre seamount. Pumice raft, anyone?
Now, this is a nice neat little hypothesis and is based on no more than a couple of sentences in Wilson’s paper on Oruanui and a comment in Campbell and Hutching’s book (see the first installment) but it might explain why rhyolitic volcanism is so sharply constrained between Okataina and Taupo, with andesitic volcanism prevailing north and south of this zone. This idea gains some credence from the fact that the oldest caldera, Mangakino, sits in the western part of the zone, more in line with the Coromandel volcanic series.
Another thing that I find puzzling about the TVZ is the huge prevalence of silicic volcanism and relative absence of more mafic volcanism. According to Hochstein (1993), basalt makes up less than 1% of the eruptive products from the TVZ. But, if the crust is so thin and spreading, why doesn’t the underlying mantle wedge just shoot up right through the myriad fault lines that run through the region? Here, too I suspect that rhyolitic magma chambers are at work, effectively acting as a spongy reservoir that accommodates the numerous mafic injections from below. Indeed, mafic injections appear to have been the trigger of the massive Oruanui eruption and have probably played a key role in most of the other 34 ignimbrite events in the zone. However, just occasionally, the mantle does find its way to the surface, like the plinian basaltic eruption at Tarawera in 1886.
This implies, though, that the crust in the TVZ must be just this kind of spongy reservoir if it is to stop mafic intrusions reaching the surface. One magnetotelluric study indicated that there was indeed a layer of connected melt underlying the entire zone (Heise, Bibby, Caldwell 2007). Personally, I have my doubts about this and also about the results of seismic tomography, but, to be honest, these are based primarily on my vast wealth of ignorance about the subject than anything else. Whatever the case may be, I think it can be assumed, given the huge heat flux in the region, that there are large bodies of crystal mush or something close to it in the region which could act as such a spongy barrier to mafic intrusions reaching the surface.
Another thing we should not forget when trying to visualize what the hell is going on down there in, well, hell, is that the zone is not made up of nicely ordered strata but is bent and contorted and blasted every six ways to Sunday. Consequently, any body of crystal mush is likely to be similarly irregular, with domes, copulas and other strange shapes involved in any chamber that forms. This complex underground morphology could be significant when trying to understand eruptive activity.
Another neat little observation made by Spinks and Aocella in their paper <http://www.geol.canterbury.ac.nz/people/kari/2005%20Spinks%20et%20al.pdf> is worth mentioning in this regard as it explains why silicic volcanism is constrained to the middle of the TVZ. The basic idea is simple. They correlated the extension regimes in the TVZ to the currently active calderas and, lo and behold, both Taupo and Okataina are placed in precisely those places where a kink in the main fault line that runs down the zone results in very little shear. Or, to put it another way, the shear caused by the NE movement of the Australian plate and the SW movement of the Pacific plate frustrates the formation of large magma chambers in the shallow crust, except in those places where a kink or bend in the main fault effectively renders the shear null and void.
Now, this is another neat little idea that initially seems to be countered by the monogenetic calderas (isn’t that a great term?) of Rotorua and Reporoa, both of which formed off the main fault (more on this later). Yet, on second thoughts, monogenetic calderas that are not on the main fault zones would not be subject to shear to the same degree anyway, so in a way, these calderas kind of indirectly corroborate the theory. Further, the correlation of not just one but two distinct sites of repeated activity in the zone, namely, Taupo and Okataina, and the sheer volume of volcanic activity at both centers rings bells with me.
Moreover, the Taupo caldera seems to have emerged from southwards movement of activity away from Maroa and Whakamaru, so the same kink in the fault line may well explain those two massive calderas as well. I mention this because such “tectonic constrainment of caldera formation” (Spinks and Acocella’s phrase) could be seriously relevant to other extensional regimes, like in Iceland, where shear may play a role in frustrating the formation of large bodies of rhyolite or, conversely, the absence of shear facilitates large ignimbrites (Thorsmörk ignimbrite, perhaps?) I’ll just toss that idea out there for others to play with.
Why is volcanic activity in the zone so chaotic? Why are the repose times sometimes so short?
Well this is the bit that gets me really excited. If you look at the petrological record presented by Wilson and Charlier in yet another great paper <http://petrology.oxfordjournals.org/content/47/1/35.full>, the most amazing feature is that the eruptive products in the TVZ do not seem to fit a nice contiguous line of formation, repose, eruption, rinse and repeat. On the contrary. Many of the crystals erupted in one eruption began forming before the crystals of another eruption in the same center only to be erupted shortly afterwards in a quite distinct eruption – and we are not talking about residual amounts here either but quite substantial quantities so this cannot be explained simply by incomplete exhaustion of the magma chamber. You can imagine how complex the tephrachronological puzzle can get when you get older crystals overlying younger crystals from the same center. No doubt it was findings such as this that led Wilson to recently claim that some “super-eruptions” may have taken place over an extended period of time and not necessarily in one big cataclysm.
So, how can this be explained? How can we at once get formation of melt with different chemical signatures from the same center at the same time, or at least at overlapping times, but also convective mixing and signs of chamber overturn which should prevent this happening, or at least mix it up a bit? At the same time, it appears most chambers are composed of relatively stable bodies of crystal mush that act like solids until the sudden decompression occurring during an eruption turns them into eruptible melt? Check this out:
<http://www.nature.com/ngeo/journal/v5/n6/fig_tab/ngeo1453_F1.html> If this suggests what I think it does (a long shot) how then do you get such homogeneity in a chamber if the semi-solid nature of the mush prevents mixing? The most likely explanation is that I have misunderstood something fundamental here, so any help would be most welcome!!
So, in sum, this array of seemingly contradictory developments boggles the mind, well, at least my mind. If you feel like having a stab at it, feel most welcome. There are a huge number of scientific papers out there on the subject. Perhaps some of it is explained by the complex morphology of underground bodies of crystal mush and the eruption dynamics that involves an interaction between decompression and sudden melt formation (kind of like water flashing to steam). I dunno. Perhaps the decompression melting during an eruption is a major factor in mixing up the chamber of all the stuff that doesn’t get erupted, i.e. large bodies of melt are left behind with associated mixing and convective currents, plus the turbidity of the caldera roof collapsing into the remaining bodies of magma?? I really don’t know. But it is fascinating to say the least.
Hazards of the zone
Given the above, the prospect of forecasting future activity in the zone is fraught with the risk of getting it wrong. There is obviously high heat flux throughout the region and it is almost certain that there is a relatively high fraction of melt in pockets of crystal mush throughout the zone. However, to what extent these pockets are isolated or connected is virtually impossible to determine. It seems that most activity on the fault zone is focused in those kinks of the fault where the shear is reduced, i.e. Taupo and Okataina, so these places would be the most obvious choices of future activity. However, it cannot be ruled out that another monogenetic caldera is forming as we speak and could pop up with relatively little warning, though the chance of this happening during our puny little lifetimes is tiny.
Oh, on this topic of forewarning, I should add that there is a basic distinction between large caldera silicic volcanism and basaltic volcanism. Lately, we have had great evidence of the seismic signal of basalt intrusions at both Eyjafjallajökull and at El Hierro. Well, rhyolite chambers aren’t like that as they form in situ. There is no need for physical movement of magma. The rocks the melt is made of are already there. All you need is heat. And Taupo has truckloads of it, well shiploads, really. So we might not see much of a seismic swarm before a major eruption. Hopefully though, the trigger will indeed be a mafic injection at depth, so we might see a seismic crisis similar to Eyjafjallajökull to give us some warning. However, whether that intrusion hits a body of crystal mush or not is anyone’s guess. And I would hate to have to be the one to call it in the TVZ. And if a large eruption did occur, this could impact large swaths of the North Island, even blanketing Auckland with a thick cover of ash. According to Phil Shane, TVZ rhyolite reaches the Auckland city region once every 3800 years on average. http://www.iese.co.nz/LinkClick.aspx?fileticket=DdpbIrnBNJc%3D&tabid=343
That said, past behavior shows that the majority of activity in the TVZ is small stuff and/or often just dome-building activity. So that is what we should expect first and foremost. However, even these events are not without serious hazard, particularly if lake water is involved.
Last but not least, I personally suspect the greatest hazard in the zone is not even volcanic. The last major event at Taupo was just 2000 years ago. It left a steep-sided hole in the ground that, over time, will experience major landslides and subsidence. I would hate to see the effects of such a slip in the lake and related seiche on Taupo township and the Waikato river with its countless hydroelectric dams. This small scale stuff might not grab the imagination quite like the Hatepe of Oruanui eruptions, but, as a direct result of it, it could pose the greatest hazard of all.
UPADTE: Updated with a comment by GeoLurking!
Sure, they are looking for Geothermal Power sources… I’m not. But their work can be handy and eliminate me having to do anything fancy.
It has been shown (Sibson, 1984; Foumier, 1991) that because quartz becomes ductile at a lower temperature than plagioclase feldspar, the brittle-plastic transition occurs at a much lower temperature in granite than quartz diorite. Thus, under conditions of a high geothermal gradient (125°C/km), normal faulting and strain rate similar to that measured in the TVZ, [...] the transition occurs at ≈300°C in “wet” granite, and at ≈400°C in “wet” quartz diorite.
The brittle-plastic transition region is where the rock starts to deform from plastic flow (beginning of ductile region) in response to stress rather than being noisy by cracking and making quakes.
That’s from “ Basement Geology And Structure Of TVZ Geothermal Fields, New Zealand” C.P. Wood (1996) Wairakei Research Centre, IGNS, Taupo, NZ
Deriving (amateur estimated) geothermal gradients from borehole temperatures in potentially productive areas in the TVZ, I get the following.
Derived from Table 1 of “Geothermal waters from the Taupo Volcanic Zone, New Zealand: Li, B and Sr isotopes characterization” Millot et al (2012, revised manuscript)
Remember, this is not the average overall gradient, just the areas from the study that showed potential for geothermal power sites… in other words, the hot areas.
An ‘average of the averages’ of the temperature determined from the rims and cores of plagioclase melt equilibria in the “Plagioclase Zonation,Whakamaru Ignimbrite” document puts the Whakamaru material at about 837°C. This is probably a reasonable estimate for what most of the the eruptive material is at before the caldera events are initiated…. but it’s just a ballpark guess and relies on the “hunch” that Taupo is just a continuation of Whakamaru.
And again, a link to that document.
I do recommend that anyone interested check out Figure 11 of that document. It’s a schematic representation of how they think Whakamaru formed it’s magma chamber. It fits in with the basalt intrusion and heat addition quite well.
GL Edit: There is an equally salient comment (if not more so) that brings a my added comment above into better focus.
Oliver St John-Mollusc says:
November 21, 2012 at 10:09 (Edit)
Brilliant Geo! May I just add that quartz has two solidification points, 700C for quartzite and 550C for quartz (affected by pressure and water content as you mention). Looking at your borehole temperature gradient table in geothermally potential areas, it would seem that you have non-solidified quartz at depths no greater than about 1.8 to 2.8 km with ditto quartzite from 2.4 to 3.8 km. If memory serves, plagioclase feldspar too begins to crystallise at about 700C. As fractionation continues, more and more minerals crystallise and drop out of the solution which thus becomes more acidic. Therefore, the relative content of volcanic gases, primarily water, increases. Remember the temperature zone of plasticity for quartz, 300C in “wet” to 400C in “dry” conditions.
Now imagine the effects of a massive basaltic intrusion into such a layer. At 1300C, the whole zone from about 4 to 1 km depth would be “instantly” reheated to liquid state which because of the exsolution of earlier minerals and “gas” enrichment would become critically unstable as pressure skyrockets. With a “roof” no more than 1 – 2 km thick – BOOM!
What then happens to the surrounding areas of mush not yet critically reheated? The areas that were in the region of 500C to 700C or so? The mush would rush out through the central hole as ignimbrite and as the zone between 2 – 3 km is evacuated, the top collapses.
Maths? I leave that to those competent but seat of pants calculations would indicate that a basaltic intrusion could remobilise anywhere between 5 – 10 x the volume of mush. A 5 km diameter pancake 1 km thick would thus require ~2 – 4 km^3 of basaltic injection for a grand total of 25 km^3 of DRE which translates to >100 km^3 of ash and pumice. Let’s say another 4 km radius of non-critically heated mush rushes out as ignimbrite (even if not all escapes the crater). That’s about 75 km^3 of ignimbrite plus a hole at least 13 km in diameter.
I really do think Mike is on to something when he recalled the Tarwera eruption.