NEW TECHNIQUE OPENS WINDOW ON VOLCANIC AREA
New Zealand and Japanese scientists are using revolutionary electromagnetic technology to gain a better understanding of
what drives the volcanoes and geothermal systems in the central North Island.
The subsurface processes that characterise the volcanic plateau, caldera volcanism in particular, have been a challenge
for scientists to understand. The volcanic plateau is recognised internationally as one of the largest and most
frequently active volcanic systems on earth.
For decades scientists have speculated about the size and location of magma bodies (huge volumes of molten rock) that
lie under what they call the Taupo Volcanic Zone (TVZ). The TVZ is a broad belt of volcanism and geothermal activity
that extends northeast from Mt Ruapehu to White Island. Finding a way to successfully identify and image the magma has
eluded scientists.
There are problems in using conventional seismic reflection techniques for imaging the earth’s interior in volcanic
areas. Seismic signals, so useful for finding oil and gas in non-volcanic areas, are difficult to interpret because of
"noise" and reverberation in the TVZ.
So scientists from the Institute of Geological & Nuclear Sciences Limited (GNS) have turned to electromagnetic techniques that measure the conductivity of structures
within the earth’s crust and upper mantle. One of these techniques is magnetotellurics, which uses low frequency
electromagnetic waves to measure electrical conductivity of rocks within the earth.
By making measurements at a number of places, scientists can build up a picture of rock formations. Geothermal fluid,
clay, and magma are good conductors of electricity and are relatively easy to identify using this technique.
Using state-of-the-art equipment supplied by the Geological Survey of Japan, GNS and Japanese scientists recently took
magnetotelluric measurements at a number of sites northeast of Taupo.
The data has shown several features in the crust and upper mantle. Most intriguing is an electical conductor 10 to 12km
beneath the TVZ which scientists believe may be magma. Further measurements will map the extent of this feature and
confirm its identity.
GNS geophysicist and project leader Hugh Bibby emphasises that the research is still in its early stages, but he
believes the potential of this technique is considerable. When magma can be identified with confidence, it will open the
door to research that has not been possible until now.
" Identifying magma with this technique will help define new research targets that will lead to a greatly improved
understanding of how these magmatic systems work," Dr Bibby says.
" One of the outcomes will be improved monitoring of volcanic areas in the central North Island."
When it last erupted in 181AD, Taupo threw out 40 cubic kilometres of volcanic material which covered parts of the
central North Island in several metres of ash and pumice. It produced a violent blast of ground-hugging molten material
that destroyed almost everything in its path to a distance of about 90km from Taupo. It also erupted a 50km-high ash
column that affected visibility in both hemispheres.
" Prior to such big eruptions, a huge volume of magma needs to accumulate at shallow depth. By identifying the magma we
will be able to determine if there are any changes in the rate at which it is accumulating. Ultimately we want to find
out what makes it suddenly become unstable and produce an eruption."
Answers to these question should not only help forecast eruptions, but will also help researchers understand the heat
transfer mechanisms occurring in the TVZ – one of the largest and youngest accumulations of acidic volcanic rocks in the
world.
Dr Bibby’s work is funded by the Foundation for Research Science and Technology with the Japanese Government
contributing additional funds because of the relevance of the research to Japan and other volcanic areas worldwide.
Dr Bibby has won international acclaim for his trailblazing work in the use of electrical techniques to understand the
subterranean workings of geothermal and volcanic areas. His success at obtaining meaningful results from large-scale
field projects has seen magnetotellurics develop into a valuable method for improving the knowledge of New Zealand’s
unique caldera volcanism.
Caldera volcanoes usually erupt so violently that they leave a depression in the ground.
For more information contact:
Hugh Bibby
Institute of Geological & Nuclear Sciences Ltd,
Ph: 04-570-4803 (reception), 04-570-4804 (direct)