New Zealand Volcanism

If you live in New Zealand, you are aware of potential natural disasters. Where I live, there will be a major earthquake at some time, but hopefully well in the future. In other places there are volcanoes, and some are huge. Lake Rotorua is part of a caldera from a rhyolite explosion about 220,000 years ago that threw up at least 340 cubic kilometers of rock. By comparison, the Mount St Helens eruption ejected in the order of 1 cubic km of rock.  Taupo is even worse. It started erupting about 300,000 years ago and last erupted about 1800 years ago, when it devastated an area of about 20,000 square km with a pyroclastic surge and its caldera left a large lake (616 square kilometers area). Layers of ash a hundred meters deep covered nearby land. The Oranui event, about 27,000 years ago sent about 1100 cubic km of debris into the air, and was a hundred times more powerful than Krakatoa. Fortunately, these supervolcanoes do not erupt very often, although Taupo is also uncomfortably frequent, having up to 26 smaller eruptions between Oranui and the latest one. However, as far as we know, nobody has died in these explosions, largely because there were no people in New Zealand until well after the last one, the Maoris arriving somewhere like 1350 AD.

The most deadly eruption in New Zealand was Tarawera. Tarawera is a rhyolite dome, but apparently the explosion was basaltic.  Basaltic eruptions, like in Hawaii, while destructive if you are in the way of a flow, are fairly harmless because the lava simply flows out like a very slow moving river. Escape should be possible, but some eruptions, like Tarawera, become explosive too. The rhyolite eruptions like those at Taupo are explosive because molten rhyolite is often very wet, so when the pressure comes off as the magma comes to the surface, the steam simply sends it explosively upwards, but basaltic volcanoes are different. A recent article in Physics World explains why there are different outcomes for essentially the same material.

Basaltic magmas are apparently less viscous, and as the magma comes to the surface, the gases and steam are vented and the magma simply flows out, so what you get are clouds of steam and gas, often with small lumps of molten magma which gives a “fireworks” display, and a gently flowing river of magma. It turns out that the differences actually depend on the flow rate. If the flow rate is slow, or at least how the theory runs, the gases escape and the magma flows away and cools during the flow. If, however, it rises very quickly, say meters per second, it can cool at around 10 – 20 degrees per second. If it cools that quickly, the average basaltic magma forms nano-sized crystals. The theory then is, if it can get about 5% of the magma in this form, the crystals start to lock together, and when that happens the viscosity suddenly increases. Now the steam cannot escape so easily, the pressurised magma from below pushes it up, and at the surface the magma simply explodes with the steam content. That, of course, requires water, which is most likely in a subduction zone, and of course the subduction zone around New Zealand starts under the Pacific, where there is no shortage of water. It was the water content that led to the Tarawera event generate a pyroclastic surge, from which, once it starts, there is no escape, as the citizens of Pompeii would testify to if they were capable of testifying. And these sort of crises are those you cannot do anything about, other than note the warning signs and go elsewhere. The good thing about such volcanoes is that there is usually a few days warning. But if Taupo decided to erupt again, how far away is safe?

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