The Taupo Volcanic Zone – Part I

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.

The Tectonic Setting

Not only does New Zealand have a bird which can’t even fly as its national symbol, it’s highest mountain is also prone to falling down.
http://tpo.tepapa.govt.nz/ViewTopicExhibitDetail.asp?ExhibitionID=0x000a4de2&ExhibitID=0x000a4ede

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. <http://en.wikipedia.org/wiki/Grauwacke> 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.

No, i am not a lizard!
Credit Wikipedia

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.

Volcanism in the TVZ

ars.els-cdn.com/content/image/1-s2.0-S0377027309003291-gr1.jpg

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

References:

for this first bit your first port of call has to be In Search of Ancient New Zealand, Hamish Campbell and Gerard Hutching, Penguin and GNS, a brilliant and very accessible book for anyone new to geology, whether you’re interested in New Zealand or not. The great part of the book is how it manages to so effortlessly tie in all the various aspects of geology into one seamless whole. Highly recommended.

Online, there is great introductory material put together here at Te Ara: http://www.teara.govt.nz/en/geology

and if you poke around the geonet site you will also find some really juicy material.

For the best introduction to the TVZ, try the Taupo field trips:
http://cdn.onlinehosting.co.nz/~gsnz/file_downloads/fieldtrip/MP117B_FT1.pdf


Sheepy Dalek Part
Thank you all, for the well wishes for VolcanoCafe. Glad you like the blog so much.
Bruce provided the educative part and here comes the fun part. Today there are 3 riddles to be solved. Alan will do his own Dinging and Kilgharrah will do the other 2 riddles. I sent her the answers.

Alan´s evil riddle # 25
This might sound fishy, but if Britain had a Jurassic Park, you’d find me near the top and the bottom! I’ll bet you couldn’t turn me into wood either!

What am I?
What am I found in?
Who are my bed-mates?
AlanC

Name That Lava !

Happy hunting!

Questions:
1: What are we made of (type of lava)?
2: When and where were we made (exact vent name and month required!)?
3: What are we called (you might cook in us!)?
4: What died near us and why?

Name that volcano! Suzies riddle

You should be wary of me in Iceland
So lets get together in the Kenya
Then you can try me for size in Philippines
Ask me to marry you in Indonesia
And make our love affair legal in Finland!

The bar is open, let the birthday celebrations continue!
Spica

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220 thoughts on “The Taupo Volcanic Zone – Part I

  1. Back to Alan: Amber, or something being preserved in amber?
    The Jurassic Park story started with cloning animals created with dinosaur DNA from a mosquito preserved in amber.
    Amber is no wood but fossilized tree resin.
    Amber fish is a fish of the southern Atlantic coast ( Seriola Carolinensis).

  2. Vale of White Horse. River Ock, name derived from Celtic for Salmon – The River Ock is a small English river which is a tributary of the River Thames. It has as its catchment area the Vale of White Horse, a low-lying and wide valley in South Oxfordshire and flows into the River Thames, at Abingdon on the reach above Culham Lock.

  3. Corallian Limestone – ridge of Corallian Limestone rises above the Vale of Avon and the Thames Valley in its Oxfordshire stretch. The Oxfordshire Corallian ridge is an escarpment holding back the hanging valley that is the Vale of White Horse and its hardness forced the River Thames to take a wide northern detour, to cut through the low ridge at Oxford.

  4. Aother take on Alan’s riddle could be that Jurassic Park was created on an island.
    I suggest Isle of Skye, “Scotlands Dinosaur Isle” as a British match.

  5. Another one with dead fish: Three fissures above Santa Catalina opened on October 18, 1730. An avalanche started on November 27, 1730, “rushing at unbelievable speed towards the sea. It arrived at the shore on December 1 and formed a small island in the water where dead fish were found.”

  6. Well done to Sherine France who identified LAKI as the answer to the Name that Volcano riddle!

    Laki means ‘law’ in Finnish – ‘to meet’ in Swahili – ‘size’ in Tagalog – ‘husband’ in Indonesian – and I think we all agree that one should be ‘wary’ of it as it is truly lethal!

    http://en.wiktionary.org/wiki/laki

  7. More dead fish due to June1731 eruptions near Timanfaya: “Different openings soon joined into one and the river of lava flowed down to the sea. A new cone appeared among the ruins of Maretas, Santa Catalina and Timanfaya. A crater opened on the side of a mountain near Maso spewing out white fumes which had never been seen before.Towards the end of June, 1731, all the western beaches and shores were covered with an incredible number of dead fish of all species — some with shapes which islanders had never known before.”
    Also http://www.lanzarote-guide.com/en/volcanoes

    • One more eruption: “On Christmas Day, 1731, the whole island shook with tremors, more violent than ever before. And on December 28, a stream of lava came pouring out of a newly risen cone in the direction of Jaritas. It burned the village and destroyed San Juan Bautistas chapel near Yaiza” (same source).

  8. Is Alan’s riddle something to do with the Isle of Portland / Purbeck? The top stone in the Jurassic layer is called Roach stone. Portland stone is oolitic limestone – oolitic means “roe stone” because is looks like fish eggs. It comes from the top Tithonian layer of the Jurassic period.

        • Roach stone is a very shelly type of limestone and is one of three types of Portland stone. The other beds are Whit Bed which is a finer type and Base Bed which has the finest grain of all.

          • There are the Lulworth beds, Portland beds and Corallian beds – all of which are upper Jurassic.

            The Lulworth beds contain fossil soils, the remains of a fossile forrest, fossil crocodiles, dinosaur remains and turtles.

  9. The top layer of the Jurassic era is Tithonian:

    “In the geologic timescale the Tithonian is the latest age of the Late Jurassic epoch or the uppermost stage of the Upper Jurassic series. It spans the time between 150.8 ± 4 Ma and 145.5 ± 4 Ma (million years ago). It is preceded by the Kimmeridgian and followed by the Berriasian stage (part of the Cretaceous).” Source: Wiki.

    & also from Wiki: “The Tithonian was introduced in scientific literature by German stratigrapher Albert Oppel in 1865. The name Tithonian is unusual in geological stage names because it is derived from Greek mythology. Tithonus was the son of Laomedon of Troy. He fell in love with Eos, the Greek goddess of dawn and finds his place in the stratigraphy because this stage, the Tithonian, finds itself hand in hand with the dawn of the Cretaceous.

    The base of the Tithonian stage is at the base of the ammonite biozone of Hybonoticeras hybonotum.

    The top of the Tithonian stage (the base of the Berriasian stage and the Cretaceous system) is at the first appearance of fossils of ammonite species Berriasella jacobi in the stratigraphic record.”

    http://en.wikipedia.org/wiki/Tithonian

  10. Back to our evil friend!
    The giant ammonite: Titanites is an extinct cephalopod genus belonging to the subclass Cephalopoda and family Dorsoplanitidae, that lived during the upper Tithonian stage of the Late Jurassic, 150-145 Mya. http://en.wikipedia.org/wiki/Titanites
    Shrimp Bed of the Portland Freestone near Blackers Hole in the Isle of Purbeck

    • Titanites anguiformis is the characteristic giant ammonite of the Portland Freestone Formation.
      A bedmate: The ammonite Paracraspidites oppressus in the Shrimp Bed at the very top of the Portland Stone Formation.

  11. At the junction between the Frome Clay and the overlying Forest Marble is the Boueti Bed, so called because of the large numbers of the brachiopod, Goniorhynchia boueti found there. It is best viewed on the Herbury peninsula south of Langton Herring.[26] The limestone known as Forest Marble is not a true metamorphic marble but it takes a high polish, and has been used as a building material and marble substitute for many years – http://en.wikipedia.org/wiki/Geology_of_Dorset

  12. Found some ancient cockle beds:

    “ancient “cockle beds” with evaporites are in the lagoonal Lower Purbeck Formation (Upper Jurassic-Lower Cretaceous) of southern England (Arkell, 1947). The “cockles” are bivalves of the species Protocardia purbeckensis (Figure. 4). They occur in members known as the “Hard Cockle Beds” and the overlying “Soft Cockle Beds” (Bristow and Forbes in Damon, 1884; Clements, 1969; Ali, 1981). In the Soft Cockle Member there is secondary gypsum that has replaced anhydrite, which in turn is a replacement of primary gypsum (West, 1964). It contains well-developed nodules and enterolithic veins (West, 1965). Calcitized gypsum occurs in the Hard Cockle Member. The usual association of the small cockle Protocardia purbeckensis with evaporites suggests that the species was tolerant of hypersaline conditions. Comparison with modern analogues suggests that it might have been able to live in brine of up to about 60 parts per thousand salinity.” Source: http://www.southampton.ac.uk/~imw/Purbeck-evaporites.htm

    • You can find gypsum there and
      1. Pseudomorphs of calcite, chalcedony or quartz (or moulds or casts) after gypsum, after anhydrite or after halite.
      2. Length-slow chalcedony (quartzine).
      3. Spherulites of the lutecite variety of chalcedony.
      4. Euhedral crystals of authigenic quartz.
      5. Celestite, sometimes with calciostrontianite (occasionally barytes or barite).
      6. Net-texture, a small-scale relic of gypsum with displaced impurities, now in secondary, sparry limestone, resulting from calcitisation.
      7. Chicken-wire, nodular structure or spherical vugs in limestone or dolomite.
      8. Coarsely crystalline (sparry) limestone, often porous, and without skeletal debris (possibly calcitised evaporites). May be massive, laminated or contorted.
      9. Small contortions that are not obviously of sub-aqueous slumping or other non-evaporitic origin.
      10. Minute rectangular relics of anhydrite in quartz or other minerals such as calcite (obvious in quartz because of contrasting moderate birefringence and the rectangular cleavage – but very small).
      11. Oligomict limestone breccia with a carbonate matrix (possible calcitised evaporite breccia).
      12. (an addition) A crumbly, brown, porous bed of carbonate and clay (like evaporitic cargneule or rauhwacke common in the Trias of the Alps and Pyrenees).

      All found under the same link as above: http://www.southampton.ac.uk/~imw/Purbeck-evaporites.htm

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