The Taupo Volcanic Zone – Part 3

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.

credit: National Park Service. Valley of Ten Thousand Smokes

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.

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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.

Weathered ignimbrite at Cathedral Cove, Coromandel. Foto by Bruce Stout.

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.
http://www.wired.com/wiredscience/2011/02/the-1886-eruption-of-mt-tarawera-new-zealand/

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.

Bruce Stout

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.

http://petrology.oxfordjournals.org/content/51/12/2465.full.pdf

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.
GEOLURKING


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.

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110 thoughts on “The Taupo Volcanic Zone – Part 3

  1. Here you go, part 3 is up. And this will be more mind nutrition. Hope all displays correctly. If not it will have to wait till my noon break at work.
    Thanks Bruce, i love your posts!

    • Another great bit of typesetting and layouting work Spica! I can’t find any faults at all!! I guess that makes us a working team now 😉
      BTW, Xmas sounds great!

  2. Oh yeah, scare the carp out of me.

    I’m sitting here lazily poking around the new post to see how Whakamaru ties into it and “wham!” The new post is up. I scurry around trying to figure out how I hosed this up and it turns out to be an intended post….

    Okay… The Hatepe eruption was say… 35km³ of magma. The Oruanui Eruption was … 500 km³. Both of those critters were Taupo.

    But Whakamaru? 1500km³. >>yikes<<

    That’s Yellowstone class… and only about 357,000 years ago.

    From Zircon analysis of the Whakamaru Group ignimbrites, they put the start time at about 250ka before the eruption. That makes it about 0.0060 km³/yr accumulation rate of stuff that made it out of the caldera.

    Oruanui.. if we take the end of Whakamaru as being the start of the next phase, was about 335ka later (*22590 BC ±230), that give us 0.0015 km³/yr accumulation rate for stuff that makes it out of the caldera.

    The relatively diminutive Hatepe Eruption, at around 22ka later, has a rate of about 0.0016 km³/yr of stuff that makes it out of the caldera.

    On average, 0.0030 km³/yr for all three, 0.0015 km³/yr for just the last two.

    Hatepe was 1832 years ago… at the low end, maybe 2.8km³ of stuff has accumulated that could make it out of the caldera, high end, 5.5km³

    My logic for including the gobsmackingly scary Whakamaru, is that it is overlapped by the later Taupo system. Effectively, Taupo is a continuation of it.

    Now, the weird part… the Whakamaru Group ignimbrites show evidence of zonation… unlike the Hatepe material. Zonation means that the material had time to fractionate down in the chamber.

    Here be the critters…

    And what I have been digging through this evening… all because of Mr Evil, Mk-II mod-0

    "The Magmatic Evolution of the Whakamaru Supereruption, New Zealand, Constrained by a Microanalytical Study of Plagioclase and Quartz" Saunders et al (2010)

    http://petrology.oxfordjournals.org/content/51/12/2465.full.pdf

    "Calderas And Geothermal Systems In The Taupo Volcanic Zone, New Zealand" Wood – Institute of Geological Nuclear Sciences Ltd, Wairakei Research Centre Taupo, New Zealand

    http://www.geothermal-energy.org/pdf/IGAstandard/WGC/1995/2-Wood.pdf

    Linked before but worth doing it again:
    "Lack of volatile gradient in the Taupo plinian-ignimbrite transition: Evidence from melt inclusion analysis" Dubar et al (1993)

    http://www.minsocam.org/ammin/am78/am78_612.pdf

    • Lurking, you are so good at datamining.. I’ve been collecting material for years for these posts and in a day you come up with all this AND THEN you manage to put it in context in the space of an hour or two… wow. impressive. I was already thinking yesterday, it would be good to do a follow-up post at some stage to collate all the stuff that has come up in the comments.. Maybe we could make a joint exercise out of it?

      • Not a prob. When and how I dig depends on my schedule… Shoot an email at Spica and have her forward it to me, we can correspond then. Or, just get Spica to shoot my addy at you… (either way, it should stick in something and not fester.)

        But… all I really do are keyword searches for pdfs. Its just a matter of knowing what words to look for that might yield some thing interesting. :D.

        Warning though.. I have yet to find a good dataset for Vostok or Dome C SO2. I’ve been looking for that for a few days now. I have Ecuador, but the resolution is a bit… crappy. I’ve seen it rerensed… just no data.

        • Next year in March we will go to NZ for the dream holiday of my life. I have already managed to talk my wife into visiting Taupo for some serious fishing…however after reading all this I am considering going to Lappland here in Sweden instead. Scared me to death 🙂 Usually I am plauged by unluck on my trips, always misplace my luggage, destroy my dslr:s etc-etc. So I am rather convinced that if we go to NZ there will most certainly be a huuuge eruption

          • Nah, go to Taupo. Its safe as houses.

            Remember, we live astonishingly short lifespans compared to geological processes.

          • Must remember to add an Aquila disclaimer:
            There is ALWAYS a risk, but it is minimal compared to what your bathroom can do to you.

  3. Some excellent food for thought, Bruce; many thanks.

    Tarawera is an eruption that has fascinated me and troubled me for a very long time. A bloody great basaltic dyke suddenly surfacing in the middle of a rhyolitic zone…The sudden onset of it, with only a few hours of felt seismicity, the size and violence of the eruption, and the eruptive rate (all over by breakfast time) – all a very nasty surprise.

    Got me thinking about what would happen if a similar dyke hit a large and well-primed almost-eruptible rhyolitic magma reservoir… could this explain the sensational eruptive rate seen with the Hatepe-style event you eloquently described at the start of this series?

    This raises the chilling possibility that large VEI events could occur with very little warning in the TVZ..

    Mike

    • That’s the really interesting bit about the whole thing. What signals would there be if something like this happens again? I think there were a couple of days of seismic activity before Tarawera, at least there are accounts of the lake level going up and down inexplicably and some ground-shaking, not to mention that strange waka (canoe) appearing. So I imagine with today’s monitoring we would get some forewarning as an earthquake swarm. The trouble is there are frequent swarms throughout the region (I can remember a couple of notable ones in Okataina over the last couple of years).
      So I think we should at least get a couple of days warning. But who is going to make the call as to whether a mafic intrusion (like the Tarawera eruption) is going to trigger a rhyolite eruption? That is going to be a tough one. Chaiten showed us that even very sticky rhyolite can ascend at speeds that were previously thought impossible. So probably we won’t know it’s going to be a rhyolite eruption until the pumice starts falling. In most cases it will be like the recent Cordon Caulle eruption of Puyehue: a large ash cloud and air fallout. This will inconvenience a lot of people and, depending on the wind, will put some townships at risk of heavy ashfall, but it shouldn’t necessarily mean there will be any fatalities as there should be sufficient time to evacuate, particularly if people are prepared.
      The Hatepe eruption started this way, with most ash blowing out to the east over Hawkes Bay and the Pacific.
      The next phase, and we would have to be REALLY REALLY unlucky for this to happen in our lifetimes, would be if (big if) the eruption then goes caldera on us. Hopefully by this time everyone would be outside of the exclusion zone and we can all sit back and watch the spectacle, hopefully with Pampers on.

      But it won’t mean the end of the world. And it won’t even mean the end of the North Island (I already have another post in mind on this!!). And if proper precautions are taken (exclusion zones, etc.) I don’t think it will even necessarily mean any great loss of life.

      • One thing to concider…

        The Icelandic dead zone. From those papers, there is evidence of high heat flux, and as I modeled for Lakiskagar and Veidivotn… that heat doesnt leave very quickly.

        All it may take is an inopportune wad of magma to makes its way upward… through material that is already near molten, making very little noise. That is until it makes the final push towards the surface.

          • A view that would completely change if it really happened, but right now I’d consider it a privilege to have something big happenig during my ridiculously short lifetime. Be it only to read what the daily fail would be able to write about a really fat event…
            I’m horribly keen on everything where you cannot be sure about anything because of an undeniable, even if highly improbable, potential sudden big and highly explosive thing…
            I was in San Francisco some years ago and couldn’t come down from imagining everything around me being sub-existend, like devoted to destruction, and it could be now or in 50 years. It made me feel strange about the people I met there, and I wanted to keep every detail in mind. And I felt like I could absolutely never settle down there knowing about the tectonic context. Strangest thing is Iceland made me the completely opposite effect. I felt at home like nowhere else. Like if there were rules that one could somehow “learn”, and if you respect them you’ll be ok. Would love to take some days in the tikkliest region Bruce describes. Sure that would be intellectually, scientifically and emotionally biggest moments.

          • Speaking of which, did you see that letter in the latest issue of Nature Geoscience about the tsunami risk on Lake Geneva?

          • Wow – fab series of posts Bruce, that last picture you posted saying gulp less than 8km – the scale in the image doesn’t really fit that then ? the red regions seem to show a large void just at the northern most bend, is that the reason for teh gulp ?
            I am still getting my head around 192 ships full of DRE jumping higher than felix every second (wow big numbers)

            one minor thing for Spica to fix – at the top of the post it mentions “response” times, but it should “repose” times

          • Hi everyone! Fantastic series Bruce – the photos are spectacular – thanks so much.

            ‘Response’ edited to ‘repose’ as spotted by Edward.

    • Thanks guys! That’s high praise indeed! I guess it comes from writing for a living.. but, as a translator, I normally just write other peoples’ ideas, never my own. This could be the start of something!!
      ha! Bouvet Island here I come!

  4. As you know (because i’ve mentioned it 50 times, my apologies…) i’m planning a big volcanic review for 2012. But I need to gather eruption start and stop dates. So can anyone tell me the latest date for the eruption of El Hierro?. I have found evidence of it continuing at least until June 3rd. But if anyone has any possible later dates please let me know 🙂

    • Hi Lucas, I can’t remember if I thanked you for the booklist, I did try but the comment got lost (not spam dungeoned…) Thanks for the booklist and yr reply to my question. Will definately let you know about the Teide book when it arrives…
      What are you using for your criteria for the end of the El hierro eruption? Last confirmed emissions such as de-gassing? Sorry I don’t have an answer for you, am just curious… 🙂 Have you checked back in the Earthquake report archive? Armand has always been pretty good on the “solid” information.

      • Just looked myself, I guess you have been looking at ER 🙂 I suppose one could infer that emissions continued for a while afterwards, but there’s no solid data that I know of…

      • There are very few earthquakes being recorded for El Hierro yet the seismograms still seem active. The quiet zone seemed to start at 8km depth when Bob was growing. I wonder if it is now growing quiet at 10km, though there is no sign of anything reaching the surface. There was a 2.7, no depth, at atlantico a few days ago. I would like to know if temperatures at Taranasoga are increasing or decreasing, and what is happening at the seamounts nearby. Funding is drying up for Spain though, so it will only be amateurs doing research there now, I guess.

        http://earthquake-report.com/2011/09/25/el-hierro-canary-islands-spain-volcanic-risk-alert-increased-to-yellow/

        • Hi Alyson,
          There have been some ramblings on this recently, Geolurking told me what he thought after I badgered on about it… Geolurking essentially (we were both drunk) said that what we have seen most recently is MAYBE Magma filling up available spaces in under El Hierro, hence the slight indication of magma movement on the traces…
          I wouldn’t read too much into the 2.7 EQ, Tenerife/Gran Canaria; they are not uncommon and they had one of 5.3 some years ago…
          The answers to the other questions would be interesting but as you say, there’s little chance of ever finding out 😦
          These are paraphrased amateur opinions/ thoughts… (standard VC disclaimer here)
          P.S. I wish WordPress tm would allow us to search for comments, I could have linked you straight to Lurking’s comment x

          • Hi schteve – I remember that discussion and the tracer still showing magma. The question as to what depth it went quiet at was left hanging, and it may still be 8km in which case it hasn’t risen to that depth again, or it may be getting quiet at a deeper level but there is no evidence of anything breaking the surface, so it isn’t of importance either way.

    • From memory, there was activity SW of Tangansoga in late June; IGN attributed this to dyke and sill formation.

      AVCAN or IGN should be able to tell you.

  5. Once again, another brilliant post. Fantastic stuff and superbly written. Lots of mulling to do later on – a nice glass of Clearview Merlot will no doubt help!

  6. @Thank you again, Bruce, for this fine post. I like a lot your mixt style of scientific and entertaining elements! 🙂

    And I am still interested in the Te Maari Craters at Tongariro. The lower one seems to be sort of a maar or tuff cone. http://www.gns.cri.nz/Home/Tongariro And the upper one, is it also a tuff cone (or maar in formation, due eg. of a high ground water level)? Or is it sort of a lava dome?

    I’ll send you some screen shots I took via volcanocafe address.

  7. Dunno man… the dead zone is the dead zone. No matter if it’s here or there.

    If I’m right about dead zones, then what we see could just be peripheral noise around where globules are lurking.

    This region isn’t a “smooth” connection between two slowly departing plates, It’s being pushed up more on the east side by the descending plate, and the west side is being pushed into that plate. And right down the middle you have a cutting torch working on making a back-arc style basin

    That’s a lot of additional stress to deal with and could contribute to all the noise.

    A generic heat flow run from a 28km caldera/vent that erupted 900°C material (sample data from the papers points at roughly 850° material)

    Duration 1744 years of cooling and a generic 20°C/km geothermal gradient.

    Any refining info would help… such as the actual gradient if you know it.

    • Sorry, I don’t know the gradient and never recall anyone talking about it. But it must be available somewhere because of all the geothermal power projects. Probably it varies widely in the region.
      Re the dead zone, compare these two piots:
      1. 10 years 0 to 10 km depth:

      2. 10 years 10 to 20 km depth:

      What do you think?

      • Personally… I think it may illustrate material impinging on the roof. But I’m not a geologist.

        Later, when I have time, I’m gonna look at using your links for some plotting. Right now I have to go investigate a smoking piece of equipment 85 miles away…

        (they claim they let the smoke out… dunno yet)

      • Yup… they let the smoke out. It’s happy now.

        Step-son, not so happy. The parking brake of a 20 ton steel pavement roller he was operating went out though the “locked” lamp was on and the engine was shut off. It started to roll. He frantically tried to get it restarted so that he could engage the hydraulics and put it into reverse or at least steer it… no such luck. At about 40 to 45 kph, he bailed off of it as it left the pavement and onto the grass. Managed to get his foot run over by the rear drum. Fortunately, it was soil and not pavement when this happened, so the foot was not crushed. X-rays show no broken bones, and he probably only has soft tissue damage.

        The smartest thing he did was to have them call me first… his mother would have been a total nut-case by the time I got home to take her up to see him at the Hospital.

        While there, I ran a across an FHP trooper (Highway Patrol) who was just as pissed off as I was at the parking lot. He was wandering all over the place for a spot. I noticed at the main entrance that there were several law enforcement vehicles from varying jurisdictions… I’m thinking one of them had been injured and they were all there to check on him. “Brothers in Arms” sort of thing.

        I hope he’s okay. My Step-son will be. Right now he is frustrated at not being able to move around… but happy to be alive. It could have been a whole lot worse.

        • Best wishes to your step-son. Very lucky in the circumstances – on the bright side, he will always be able to say “Did I ever tell you about the time I was run over by a roller?” and have everyone’s attention!

  8. Thanks for the articles Bruce, most interesting, I like the way you’ve led us through it; from the Tectonic setting via the Volcanoes and into the Magma… The follow- up has been fascinating to follow too, thanks all 😀

  9. Thanks, Bruce ( and Lurk, too.) for a very informative series..I’ve held that the next big eruption will be something that will completely catch us off guard. Taupo may be that…

  10. Interesting post, Bruce, and comments. Thank you. 😀

    Just a thought: if Whakamaru erupts and produces a larger caldera, this may overlap Lake Taupo so we end up with lake >2 x the size of Lake Taupo. (pure speculation only, no expertise etc)

    • This has actually happened before. (what follows is from my scratchy memory and to be taken with a grain of salt but I don’t have the time to research it properly at the mo):
      Wilson (?) posited the existance of a large body of water involved in the Oruanui (?) eruption as the eruptive products had obviously been quenched. By simply extrapolating the volume of ignimbrite and the inferred temperature drop, he came up with a larger lake than the current Lake Taupo. I vaguely remember seeing a diagram somewhere that suggests the proto-lake Taupo extended up into the Whakamaru basin. When I have time, I’ll follow it up for you.

  11. Awesome series – one of the most interesting series so far on VC, and also a series that raises more questions (which is always a good thing).

    So I have a few questions, which I feel like by now I should know, but don’t.

    1. How does rhyolite form? From what I know, Basaltic magma is typically magma that comes directly from the mantle or mantle wedge. I believe andesitic style magma is typically formed from the melting of oceanic crust. Where does this place Dacitic and Rhyolitic magma? I see a lot of stuff about “Evolved magmas”, but nobody on here has ever bothered to really explain what that actually entails, why an arc-volcanic system would have 3-4 different magma types in close succession, and how shield volcanoes can erupt in large rhyolitic eruptions.

    2. Is back arc rifting + subduction melt a formula for large rhyolitic caldera systems? Every large silicic caldera system I’ve found that is placed on a subduction zone seems to have some sort of extensional mechanism going on. As was previously covered in the Phillippines calderas, they sit within an extensional zone in an subduction arc volcanic area. Lake Toba also sits in a graben (which generally implies a large area where extensional forces are occurring). Similarly, the altiplano complex sits in a back-arc area, while other large caldera systems I’ve found have very similar characteristics.

    • Oh cool! A question I can answer! 😆
      a couple of years ago I was kind of flummoxed as to how rhyolite appeared on Iceland. That’s when I learned about fractionation. Basically that is where all the continental crust ultimately comes from. Over long periods of time, fractionation will lead to separation of silica-rich fractions in melt. This is where all our continents come from. And because they are lighter they have been buoyed about across the surface of the planet for as long as there has been crustal material.

      The rhyolite melt in the TVZ is generally all recycled… old crust that has been erupted, eroded, remelted, rinse and repeat.

      Lurking talked of the Whakamaru eruptives being fractionated. That was new to me. This means they had a long repose time in the crust, or at least long enough to fractionate. The Oruanui eruption by contrast appears to be all rejuvenated material. That is why I got so excited by the greywacke in Part I. This is the stuff that the rhyolite is made of. Just add heat and maybe some water.

    • I just realized I didn’t answer your second question.
      OK so once you have crustal material, which has ultimately formed over the entire age of the planet, then you have a huge source of silica-rich rocks to form granite/rhyolite. The only problem is that crustal rocks are cold and generally won’t erupt by themselves, you need a heat source. This generally comes from two main sources:
      1. Upwelling mantle (hot spot) shunts a whole lot of heat from deep within the earth to the surface and this melts its way through the crust (Yellowstone) or
      2. A subduction scenario which results in a mantle wedge forming. The mantle wedge is formed of ductile mafic material (basalt) and is also where volatiles (water and CO2 mainly) collect as they percolate up from the subducting slab. These volatiles are important as they lower the melting point of rocks and are also the gases that drive the explosive power of eruptions. It is this source of hot gas rich mafic material that ascends in blips and blobs and results in magma chambers forming in the crust. The rhyolite component is merely crust that has melted from this heat source (plus the bit added by fractionation if the repose time is long enough). Repeated dike instrusions can feed these magma chambers, bringing heat and volatiles and thereby adding to the volume of eruptible material.
      Now, as you point out, add an extensional regime (which also lowers the pressure acting on the chamber (lower pressure means more melt)) and you have a situation where large rhyolte chambers can form at shallow levels in the crust (deep ones don’t go caldera on us). Does that answer it?

  12. As always, the best articles are those that make you ruminate along hitherto untrod paths. I like Mike’s point about the Tarawera eruption, a VEI 5 basaltic eruption which apparently happened with short notice. Now for some more speculation:

    If the Kapenga caldera is indeed the first and not simply just the oldest hitherto detected VLE, then it looks as if a) the TWZ is a new feature that is currently, geologically speaking, in the process of opening up, b) it is trending south with progressively larger calderas (Kapenga 700kA >> Whakamaru 254 kA>> Taupo 26.5 kA). If so, the Tongariru – Ruapehu – Tarawera area could be one to watch (cf Mike’s note on Tarawera).

    Another thought (caveat – non-expert speculation): If there is indeed a very large zone of magma underlying the whole TWZ, a new and possibly critical infusion might be difficult to spot. 1 km^3 of magma spread out over a circular area with a diameter of 40 km under a “normal” volcanic system would result in an uplift of approximately 40 cm. Easily detectible as a huge pimple (cf Uturuncu, Bolivia). Spread it out over a zone 250 x 50 km and it’s only 4 cm. Furthermore, in order to pick it up you’d need very widely spread reference points and also an observer who is alert to the possible implications.

    Since “we all know” that restless calderas oscillate many centimeters to several decimeters (Yellowstone 2000s, Campi Flegrei 1970s-80s) without there being any sinister implications, when do you blow the whistle on the TWZ? 10 cm? A metre? Over what time? Days? Months? Years? Decades?

    Again, thanks Bruce!

    • Couple of things here. Kapenga is not the oldest caldera complex. As far as I am aware it is Mangakino (could be wrong,.. need to research it). The progression is not simply North to South of the whole zone. The Taupo part of it does seem to have shifted southwards from Maroa through Whakamaru to Taupo but Tarawera (part of Okataina is way up in the NE section of the zone. Strangely, over much longer time intervals, there appears to be some eastwards stepping of the plate boundary. This is something for another post at a later time. As for the inflation, I really don’t know. Something makes me think that this might not be quite as prevalent here as we have seen at other systems. I know, this goes against accepted wisdom, but imagine a shot in the arm like the dike intrusion that fed the Tarawera eruption hitting a primed magma chamber in the zone.. you get the picture.

  13. I have thoroughly enjoyed this series of posts Bruce. Thank you so much. I know there was a lot of hard work put in to write these. As everyone says so many questions.

    • Thanks Diana, it was a lot of fun to do. The material was all just the culmination of all the questions I have asked over the years, so fire away! If I can’t think of an answer, then I am sure someone else here can.

    • I’d like to add my thanks to all the others. This is all so informative – and interesting and well written too! Thanks Bruce!

  14. 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.

    http://petrology.oxfordjournals.org/content/51/12/2465.full.pdf

    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.

    • man, I owe you more than just a couple of beers. Do they give honorary doctorates to people like you? I certainly nominate one from VC. This is great stuff.

      The other thing that intrigues me is how disparate the temperature gradients might be. Pumice is generally a fantastic heat insulator and if the TVZ is filled with 10000 km3 of the stuff I could imagine a contorted system of pockets of melt all over the place. Possibly, if the conditions are right, these isolated pockets might rapidly fuse together in one area to make one large chamber, or maybe this even happens during an eruption (due to decompression)

    • Yep, read that Lurk, that is probably as good of theory as it gets. Amazing geology there.
      kind of makes the Cascades geology pale in comparison..
      Well not quite….

      • The Cascades has a sneaky monster of a volcano… I just can’t remember where it’s at. I think it went off sometime before 22 myr ago. (48 comes to mind) . a bit later the Columbia Flood basalts occured and buried most of its ejecta. I think the Painted Canyons(hills?) are related?

        • Isn’t that related to the history of the Yellowstone Hotspot? If you follow the trail of the calderas, you end up several hundred miles to the south but if you follow the Snake River Plain, it takes you right up to the CFB vents. The joining of the two seems to have occured ~11 MY ago.

          Now, how do you explain the cut through the Rockies that is Snake River Plain which runs from the CFB vents to the caldera trail?

        • Good morin’ Lurk, Oliver. That volcano is a bit of a mystery “Roadside Geology of Oregon” is my favorite reference on that. seems it may or may not be related to the Yellowstone hot spot.
          The problem lies in the disposition of the old seafloor bed that dove under the N.American plate
          causing the Cascade eruptions and prior to the E.Oregon volcanoes to start. According to the
          book the south end of the Cascades was quite something multiple volcanoes going off for nearly 10million years mainly formed the Klamath and Siskyou ranges lots and lots of rhyolite ash added
          to the John Day formation. Then, thanks to quite probably, a chunk of the old Farallon plate breaking off and sinking under the N.American plate that stopped.and the activity moved father
          east into E. Oregon. As that subsided, this created the spreading and weakening of the plate
          and created the Basalt flows of the Columbia River plateau. Also this may be the weakening
          of the crust that created the Yellowstone hot spot. It proceeded down the Snake River Valley
          to its present location today.
          BTW I have a theory that the Olympic Wallowa Lineament (OWL) may be that chunk of
          plate..

    • 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.

      • oh, nice and juicy stuff Mr. Mollusc! Chemistry is where I am weak but what you are describing is pretty much how I envision it happening. I hope the discussion keeps going in this direction.

    • This says nothing: It was a group of school children.

      But I wouldn’t have liked to be the teacher in this case. When you look close, there is even a small pyroclastic / debris flow coming down in direction of the group – luckily it was a very small flow and behind another hillside, also sometimes just zoomed in.

    • Hi Inge, no, I didn’t. Here’s my email address:
      myfullname AT t-online.de

      No dots in the name. Look forward to it!

    • Hi Inge.
      As i said yesterday, the only one checking the official VC address is Carl and he has not been answering for, i think 7 or 8 weeks now. I will mail Bruce address to you.
      To everyone else: Whoever wants to contact VC for one or the other reason, please use MY email linked above.

  15. Re. Iceland: Is something going on in Vatnajökull? It is a bit difficult to see due to storm, but these stations show something a bit unusual: http://hraun.vedur.is/ja/oroi/fag.gif (this is a station in the south of Vatnajökull, at Öraefajökull), and at Grímsfjall: http://hraun.vedur.is/ja/oroi/grf.gif The last one could also be a glitch (we once had something similar at Askja and somewhere else), but it is arriving at the same time as the movement at FAG. Station JOK shows it also (starting around midmight), though the “warming up” there could be weather related: http://hraun.vedur.is/ja/oroi/jok.gif . Regrettably VOT seems to be out of order.

    There are wind gusts over 30 m/sec., but they arrive a bit later, and would just influence the high frequencies, I think.

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