Transcript
BRAD SCOTT: It first became apparent to cruising yachties, people sailing in the Tonga area around the 9th of August. Although some of their online discussions suggest they may have actually seen some steam plumes as early as 7th of August.
JENNY MEYER: And it seems like there are quite big masses of this floating pumice, is this something that happens? Is this one of the key indicators of an undersea eruption?
BS: Yeah that's right quite commonly submarine volcanic eruptions produce pumice at the surface. It'll pour out the pumice from the active vent for you know, hours, or usually in the order of hours, maybe even days in extreme circumstances. And they often can be reported or can be seen from the satellite to be covering several square kilometres, to tens of square kilometres. Sometimes the raft might be say between 10 and 50 kilometres long things like that. They grow with time also as the sea currents and wind spreads them out.
JM: So it drifts from its original source, where's the source volcano do you think?
BS: The source volcano is in Tonga. To the north-west of the main island. And it's an area where there was a submarine eruption in 2001 also. So it's an area of known volcanism.
JM: And so for yachties in the area who might come across it, what's the best thing for them to do, go around it?
BS: They tend to take different approaches. The national maritime co-ordination centre has been aware of it and monitoring it. And I imagine they've been putting out notice to mariners. That's a pretty normal procedure that those sorts of people will say, 'hey there's a submarine eruption here that has pumice drifting in this direction'. It's a personal choice for the yachties whether they sail through it or sail round it.
JM: And any indication that this volcano might do more or become more active?
BS: I'm not completely familiar with the 2001 eruption, but from memory that was relatively short lived. This one appears to have basically been only the one eruption. There is an indication there may be a couple of vents active, but that's not clear at the moment. But usually these eruptions are relatively short lived.
JM: And you said that there was some steam seen as well?
BS: That's right. Some of the web page Facebook chatter amongst the cruising yachts suggested that on the 7th they saw clouds of steam on the horizon. Whereas they didn't encounter the pumice I think until the 9th so like two days later.
JM: And that's a normal kind of progression of something like this?
BS: Well it basically depends on how close a cruising yacht is to the area. If it's relatively close to the vent it may see it, where it's a longish way away, they'll see it a day or two before they get to the area.
JM: In the bigger picture this early August eruption near Tonga, do you think it signals anything else in terms of activity in the area, volcanos, earthquakes, that kind of thing?
BS: This would be very much business as usual. Submarine eruptions occur every five to ten years in the Tonga environment. There's other submarine volcanos south of Tonga, back towards the Kermadecs that we know are pretty frequently active. So, it's not out of the ordinary really for that area at all.
JM: What's the usual pattern there with these things as this huge mat of pumice floats around in the ocean?
BS: Just depending on where the eruption is. What the prevailing winds are, the prevailing currents. It'll turn up in other countries. Quite often they'll turn up in Fiji, they'll turn up in Australia. Sometimes they turn up down here. So, it really just depends on the relative position of the eruption to currents and the weather pattern at the time.
JM: Does it go underneath into the ocean as well; does it have like a depth to it?
BS: What you're seeing with the pumice raft is the material floating on the surface, it's normally just a few centermetres to tens of centermetres thick. But some of the pumices will become saturated with time and they will sink to the sea floor. But anything that remains bouyant will just wash up on a beach somewhere.
JM: And they can be quite big these lumps, can't they?
BS: Yeah the pumice sizes can vary from sort of ball bearing, marble size up to basketball size.
