Supervolcanoes

I spent a few days staying at Kuratau on my recent road trip, on the southern shores of Lake Taupo. Here’s an early morning shot of the lake looking north, with Mt Tauhara in the far distance.

I wonder, when people gaze at this beautiful lake, how many realise they’re looking at one of the world’s true supervolcanoes? For example, an eruption around 26,000 years ago at a place called Oruanui is said to be the world’s largest known eruption in the past 70,000 years.

Years ago I attended a lecture by volcanologist Ian Nairn which made a big impression upon me. He talked about the little volcanoes in the central North Island, and the big ones. The little ones include Ngaruahoe (Mount Doom in the “Lord of the Rings”), about 2399 m high. Here’s a picture from a couple of weeks ago, of Mt Ngaruahoe in a dramatic mood:

The big ones are not so obvious. Lake Taupo is one, while others lie to the north and east. He explained that much of the magma in this region is rhyolite, very different from the stuff you see erupting in Hawaii or Iceland. It’s very stiff and so, as the pressure builds up over thousands of years, it bulges upwards to form a large dome. Here is a very innocent-looking example of a small one:

When a rhyolite dome does eventually blow, the high dissolved gas concentration in the magma means that it goes off like a shaken champagne bottle. The column of molten rock and superheated gas can go up 15 km or so. When it eventually falls down, it spills out across the countryside at high speed, solidifying as it cools down to form a kind of rock called ignimbrite. Of course, because of the huge scale of these eruptions (1000 times bigger than Mt St Helens for example), then the whole world changes!

I used to keep a core sample on my desk of genuine, certified, 200 km/hour ignimbrite. (Geologists can estimate its velocity by studying the bottom layer of the deposit. The faster the flow, the higher the hills it can scale.)

Just south of Rotorua, on the road to Atiamuri, there’s a tiny village called Horohoro. It sits on land which has dropped a few hundred metres, exposing the rock underneath. The fault line runs along the cliffs behind this church:

The cliffs are higher than they look, up to 300m in some places. They are impressive, even more so when one realises that a hundred metres or more were laid down overnight in a flow that went for hundreds of kilometers at high speed. This is actually part of the Mamaku Plateau.

15 km away, at Waikite Valley, you can see more of this same flow (these cliffs face north and make up the other side of the fault valley):

The supervolcanoes in the Taupo Volcanic Zone typically blow out so much magma that the ground collapses to form a large “caldera”. These then fill up with water (hence Lake Taupo, although I understand this is also a rift valley). Over time the ground bulges upwards again, leaving a ring of smaller lakes around the rim. My host in Rotorua explained that the centre of one of the supervolcanoes, Okataina, is rising at the rate of a centimeter a year (if I remember correctly).  So some time in the next 100,000 years or so we can expect another large bang!

For a good description of the Okataina Volcanic Centre, check out Ian Nairn’s paper. Wikipedia has some good stuff on supervolcanoes here (two of the top three in its list of the really big ones were in New Zealand!)

I had a great time on my road trip revisiting the geothermal areas created by all this activity, as you could see from that earlier post on mud pools.  More images from the trip soon!