Huge Eruption May Have Been Bigger

Dec. 21, 2006 — One of the largest volcanic eruptions on record just got bigger.

The Taupo Volcanic Zone of New Zealand appears to have had twin eruptions only 20 miles apart within days of each other a quarter-million years ago. Each eruption belched out more than 25 cubic miles (100 cubic kilometers) of rock and volcanic ash.

This is the first evidence of twin supervolcanic eruptions.

"It's possible one of these triggered the other," said geologist Darren Gravley of the University of Auckland, New Zealand. But exactly how the triggering might have worked is uncertain.

What is clear from the explorations of Gravley and his colleagues of the Mamaku and Ohakuri volcanic deposits is that they were created very close in time. That's surprising, since most caldera or "supervolcano" eruptions in any one region tend to be tens of thousands of years apart, or at least that's been the general idea until now.

Among the signs that the rocks from the two eruptions were piled on one another is the conspicuous lack of erosion on the first volcanic deposits — which is striking, considering the rainy climate.

Previous studies that looked only at the radioisotope dates of the volcanic rocks from the eruptions missed the timing details, Gravley said, because they have a margin of error of 10,000 years — way too low a resolution.

"You’ve got to look at the physical evidence," said Gravley. "It's really getting into the nitty-gritty. From the stratigraphy (rock layers) it’s clear two were erupting at the same time. That just blows away any (regional frequency) studies out of the water."

Gravely and his colleagues have published their double eruption discovery in the latest issue of the Bulletin of the Geological Society of America.

The bad news is that double eruption represents a whole new way that supervolcanoes can threaten humanity.

"This is of course a major issue to consider for volcanic risk," said caldera researcher Gerardo Aguirre-Díaz of the Universidad Nacional Autónoma de Mexico in Juriquilla, Mexico.

Caldera eruptions are far less frequent than other volcanoes, but when they do erupt, "the consequences for the surroundings and in general for the world would be enormous, because these explosive eruptions are many orders of magnitude bigger than a more common eruption from a volcano, such as Mount St. Helens or Vesuvius."