When will the eruption stop? Experts weigh in

The ash clould from the ongoing eruption of Chilean volcano Cordón Caulle has grounded many New Zealand flights.

When will it stop?

In short, experts don't know.

Although they can say a similar eruption at the same volcano during the 1960s lasted two months.

Dr Tom Wilson, University of Canterbury and Dr Graham Leonard, volcanologist at GNS Science (both co-leaders of the international Volcanic Ash Impacts Working Group) provided the following update:

"The explosive eruption of the past several days continues.

"SERNAGEOMIN [the Chilean agency monitoring the eruption] notes that the volcanic earthquake signals are a little more steady and not quite as pulsating, and today the eruption cloud is a little lower than the maximum has been.

"However, forecasting what will happen in the future in explosive volcanic eruptions like this is very hard. Monitoring agencies cannot easily say if over coming days an eruption like this is particularly likely to increase or decrease in size and ash cloud height.

"The height of the eruption cloud at the volcano is important, because as it nears the 10km mark it is much more easily entrained into the jetstream that is bringing the ash all the way to New Zealand.

"The explosive eruption cloud started out reaching up to 15km elevation, and it has recently reached as high as 9 or 10 km.

"We have some indication from colleagues in Argentina that the ash started out as rhyo-dacitic (fragments of pumice) but may be including more andesitic (fragments of scoria) particles recently.

"This difference does not have a major implication in New Zealand as the ash is only an issue for airplanes here and by the time ash reaches us just the extremely fine flour-sized grains of volcanic glass are left. Whether these grains are from pumice or scoria doesn't make a lot of difference to jet engines - both are damaging.

"The 1960 eruption appears to have lasted about two months, but with varied eruption styles - there were periods of more explosive and alternately more lava-producing eruption.

"In short, we know this volcanic complex can produce eruptive episodes at least months long, and that these can be episodic. There are no strong indicators being reported from SERNAGEOMIN that the eruption is about to stop, but we can't necessarily expect a full two month duration either.

"This is an excellent opportunity to look at the impacts from local volcanic ash to agriculture, infrastructure and people's homes and work. While a Chilean eruption is only a risk to aviation in New Zealand, local eruptions in New Zealand will probably occur at stages in our lives. Eruptions from Ruapehu, Tongariro and perhaps Taranaki could give us mm to cm of ash to deal with, much like the San Carlos de Bariloche area in Argentina is coping with right now."