Dr Darren Gravley explains his research into the twin caldera volcanoes nicknamed ‘double trouble’. Located in the central North Island, these volcanoes erupted violently and possibly just weeks apart.
My PhD was: Here’s a deposit, can you work out where it came from? We know that it’s a volcanic deposit. We know from the style and the look of the deposit that it’s probably come from a caldera eruption. Find out where that caldera was.
So what I did was I mapped the distribution of that deposit, and by mapping the distribution, it was focused around a particular central area. You are really trying to look for clues in the deposits to work out where those deposits came from. I was able to work out that the distribution was around this particular centre, which we named the Ōhakuri caldera.
Looking at the deposits, we saw that it was in close contact with deposits associated with the volcanic eruption that formed the Rotorua caldera, which has now been infilled by the lake. These two eruptions were very closely spaced in time. We had to be really careful about proving or looking for appropriate evidence to suggest that indeed two calderas erupted at the same time, and we are looking at volcanic deposits for any evidence in between or in that contact area for a lapse of time. And the crux of our evidence is that these two eruptions were spaced probably in the order of weeks to months apart.
Let’s put it in perspective globally. One of these caldera-forming eruptions takes place – it’s on average about 1 every 50,000 years. Why did they erupt so closely spaced in time? All of a sudden you start to think maybe not a coincidence, and so then you start to look at whether these two plumbing systems beneath these calderas are hearing each other, are seeing each other. There is a magma reservoir beneath one caldera, and 30 kilometres to the south there is another magma reservoir there. Those magma reservoirs have to empty out to form a caldera, which collapses into the space that is left behind from that magma reservoir.
Did those two magmas know they existed in that sense?