Everybody do the Tangaroan!

It’s new, it’s exciting, and while it kind of sounds like a dance move, the Tangaroan style is in fact the newly classified third type of eruption style. We’ve got eruptive and effusive and now this.

Where?

It was classified by a team of researchers from the National Oceanography Centre (Southampton, UK) and Victoria University (Wellington, New Zealand) who studied Macauley volcano in the south west Pacific.

When?

The news appeared in this month’s edition of Nature Geoscience. The paper is called ‘Highly vesicular pumice generated by buoyant detachment of magma in subaqueous volcanism‘.

So what’s the deal?

Well, the Tangaroan style is specifically an underwater eruptive style. If it had happened subaerially it would be intermediate – somewhere between effusive and eruptive. And the main defining feature of this style is its foaminess. See, lots of vesicles form in the magma, and as it bubbles up it turns into a kind of foam, which detaches as packets of pumice and rises. Because of the effects of decreasing water pressure, the bubbles continue expanding so you end up with various sized bubbles by the end of it.

Pumice (which is usually a sign of explosive activity in subaerial volcanoes) is quite common in underwater volcanoes, and this new research means that underwater volcanoes currently marked as having explosive eruptions in the past may be reassessed under this new category. Exciting stuff!

Quite interesting:

The style is called Tangaroan after the Maori god of the sea, but it also acts as a homage to the ship used to collect samples, which shares the name. Fun fact: Tangaroa is also part of the Cook Islands’ mythos and has yellow hair, so when Europeans first visited, they were considered the children of Tangaroa.

Pumice rafts off New Zealand

In August of this year, a vast pumice raft was spotted off the coast of New Zealand. This is a rather interesting phenomenon – rocks floating in the ocean! – and it arises from the fact that pumice is lighter than water.

See, pumice has tons of vesicles in it – namely, air holes, gaps in the rock. It is made from very viscous, ‘bubbly’ magma. In other words, it is the froth on the latte of a volcanic eruption.

First noticed by a New Zealand marine aircraft, and reported by science writer Rebecca Priestly, who happened to be sailing close by, this particular pumice rafting event was not caused by what we would think of as a ‘normal’ volcano. No – it was underwater. It was apparently caused by Havre seamount in the Kermadec Islands, the volcanic byproducts of an underwater subduction zone just north of New Zealand.

The weirdest thing for me when studying geology was finding out that pumice rafts can appear as a result of underwater volcanic activity. Undersea volcanoes are usually typified by mafic magmas, whereas pumice is more commonly associated with felsic or andesitic activity – stratovolcanoes such as Mount St Helens, or Vesuvius.  It’s got the same chemical formula as obsidian, which is often associated with a type of felsic magma called rhyolite.

And, to further these associations, when Vesuvius erupted in 72AD the resultant Plinian style eruption column rained out a crap-ton of pumice over the sea in the Bay of Naples.  This made sense to me when I first heard it – Vesuvius is andesitic. So in my mind, pumice equals felsic, andesitic, big Vesuvius-type stuff.

What was new to me was realising that pumice can quite often be mafic, and that underwater volcanic eruptions involving mafic material and pumice are actually quite common. Just take a look at this image from a study published in PLOS earlier this year[1].

[1]The study is actually open access, so you will be able to download and read at your leisure.

So what kind of conditions would lead to an undersea volcano producing pumice rafts? Well, they are very common around subduction zones, where you will find more violent forms of eruptive activity that are more likely to cause the viscous bubbliness necessary for pumice to form. Not that it doesn’t happen in spreading ridge settings, it’s just less common. The major pumice rafts in the news recently have been from the Tonga – Kermadec region.

This particular raft grew to over 20,000 square kilometres. A venerable floating island indeed. It is made all the more interesting by the fact that pumice in recent years has been shown to be a decent substrate for distribution of marine life around the oceans. Take a look at this snapshot of a piece of pumice colonized by various life forms. How cool is that?