Taupo super-eruption secrets revealed

OLIVIA WANNAN

Last updated 21/04/2012

Lloyd Homer/GNS Science

CALM EXTERIOR: Research into the Lake Taupo eruption has thrown up new theories on the tectonic forces involved.

A computer-generated graphic of the Lake Taupo eruption.

One of the most intriguing unsolved cases for New Zealand geologists is the ancient Taupo super-eruption.

Victoria University PhD student Aidan Allan has found new evidence that explains how and why the volcano blew.

While the general public is fascinated by the magnitude – the event buried the North Island in debris, with the ash cloud all the way to the Chathams – geologists' interest lies elsewhere.

They are intrigued because the eruption's cause isn't open-and-shut – while most super-volcanoes simply explode, with Taupo there was a short hiatus just as things got underway.

"There were breaks of weeks to months [in the early stages] and then all hell breaks loose," Mr Allan said.

As geologists worldwide have to make the life-or-death call as to when an eruption has ended, it's crucial to know why this super-volcano acted the way it did.

The event began in standard fashion, at least in super-volcano terms, with a massive 530sq km pool of magma building up below the surface, under more and more pressure.

But deposits show that after the very first lava flowed from the caldera pool, the eruption paused for several months.

There are the markings between lava deposits left by rainfall and insect life, evidence that can't be ignored.

Explaining why that happened is the ultimate cold case – happening 27,000 years ago, rocks and soil are all that's left to tell the tale.

But of all the world's super-eruptions, the one that made Lake Taupo is the youngest, and so the evidence is the best preserved – and what was found might change the way we think about the beginning and ending of eruptions.

Being on the Pacific Ring of Fire meant the impending super-volcano was not alone.

Fifteen kilometres north of where the town of Taupo sits today were separate, much smaller subsurface magma pools, named the Northeast Dome System.

These northeast pools were very different to the massive caldera at Taupo. They weren't under a lot of pressure and lava regularly seeped out volcanic domes.

Even the magma was different, it had some chemicals and trace elements that the Taupo caldera magma did not.

Mr Allan describes the two types of magma as having their own chemical fingerprint – this was the basis for the break in the case.

Looking at the layers of volcanic pumice that were laid down in the early stages of the super-eruption, he saw the fingerprint of the northeast magma over and over again.

"We know that the magma grew beneath [the northeast] system, but we can see the magma's actually erupted from what is now Lake Taupo."

As the two pools of magma were physically separated by 15km of rock, Mr Allan suspected a major tectonic force had shifted the magma from the northeast site to the vents of the Taupo super-volcano.

With layer upon layer of chemical evidence confirming the presence of two types of magma, it seems he'd found a potential suspect for the mysterious halting of the super-eruption.

Caught at the scene of the crime, tectonic forces were now the prime suspect.

Between the two magma pools lay the Kaiapo fault, where the land on both sides was pulling away from each other.

It was up this rifting fault the northeast-type magma travelled, below the surface, on a collision course with the caldera, before erupting at the Taupo vent, Mr Allan said.

A recipient of a Top Achiever doctoral scholarship, he could see from the low levels of mixing of the two magmas in the deposits, the rifting event had to have happened right as the eruption began.

His conclusion is this event both caused the eruption, and put it on hold.

"If you imagine you shake up a Coke bottle and take the lid off, it all happens then and there. [With the Taupo super-eruption] it's like opening the cap a little bit and then tightening it back up."

Enough magma was released so the pressure dipped back below the explosion threshold, and, it being an active tectonic region, this happened twice.

To the observer, seeing small-scale eruptions, followed by lulls lasting months, it might have seemed the volcanic activity was not particularly dangerous. In reality, less than 0.1 per cent of the caldera magma had erupted. It was just a matter of time.

Today, with both systems believed to be dormant, the forces that caused such problems are no longer suspect, it's the profile of the event that's the most relevant.

The investigation, part of research on super-volcanoes led by Victoria University professor Colin Wilson, has implications for hazard monitoring, when there are similar tectonic forces at play, Mr Allan says.

"It throws up a real challenge to know when a large eruption has truly stopped."

SUPER-VOLCANOES

Super-eruptions are caused by the pooling of magma under the surface with no available domes or vents to release tension.

Pressure builds to a saturation point, triggering explosive eruptions thousands of times more powerful than standard eruptions.

Super-volcanoes collapse into calderas, where the ground falls into the space left by the erupted magma. Lake Taupo and Lake Rotorua are both calderas.