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.
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?