Whether a catastrophic eruption buries the North Island in ash or leaves most it relatively unscathed depends on the weather, new research has found.
Three of the world's 10 biggest eruptions happened at Taupō. The last time it blew, 26,500 years ago, it "left a giant hole in the Earth's crust - now partly filled by Lake Taupō" says Simon Barker of Victoria University, who led the new study.
- Little warning for Aucklanders if eruption imminent
- Auckland's volcanic eruptions surprisingly recent, study reveals
- Volcanic eruption: How would Auckland cope if one blew?
But what if it erupted today? Dr Barker's team used a "complicated calculation" mixing volcanology, geology and meteorology to find out.
"There's already been a huge amount of study on this volcano, so we had a good idea of the size and type of eruption that could happen in the future.
"We created scenarios based on different eruption sizes, spanning several orders of magnitude in volume, added in the weather data, and ran each scenario 1000 times to see where the ash would fall, and how thick the resulting deposits will be."
Let's hope it's a windy day next time Taupō decides to unleash its fury. Nice, still weather would be the worst-case scenario, he says, because that would result in an ash cloud like the one below - bad news for almost everyone north of Wairarapa.
In the scenario above, Auckland would get coated in a centimetre of ash. The lower Waikato and central North Island could get between three and 10cm.
In contrast, strong winds will blow much of it out to sea.
Last time Taupō erupted, it ejected an estimated 1170 square kilometres of material - around 800 times more than the 1886 Tarawera eruption, and more than 10,000 times more than Ruapehu managed in 1945 - when its ash coated cars as far south as Wellington.
"Another factor is the shape of the volcanic ash cloud - the ranges are from a weaker 'bent' cloud like from Ruapehu in 1995 to the massive 'umbrella' cloud that can blast more than 20km up into the stratosphere. This means that we need to include wind at different altitudes in the calculation."
The data they're collecting also sheds some light on how long each centre will have to evacuate, if necessary, before "tiny particles of glass and crystals" would start raining down.
"This kind of information can help emergency managers work out which areas need immediate evacuation. For instance in a large Taupō explosion, in the most common westerly weather pattern, it would take about three hours for the ash to reach Napier."
EQC, which funded the research, said it would be a big help for emergency planning and risk reduction.