Volcanoes - Ministry of Civil Defence and Emergency Management

Crater Lake at Ruapehu viewed from its north side in February 2004.
The lake is nearing its pre-1995 eruption levels as the 2004 winter
begins and in future years will rise even further behind a barrier of
loose debris that covers the outlet area. Photo: Tanya OʼNeill.
date have focused only on the easily recognisable and
datable large-scale events at each volcano. We know
that these events represent just the “tip of the iceberg”
of volcanic history; the hidden record of minor eruptions
is probably the greatest unknown factor in volcanic
studies in New Zealand. For instance, the eruption
of 1995/96 at Ruapehu caused over $130 million
in damage, but was only just a minor “blip” in the
geological record and easily overlooked. Filling in the
blanks of our geologic records will allow the application
of statistical eruption models that have been developed
on other volcanoes around the world, such as in Fiji,
to derive new probabilistic forecasts of the likelihood
of volcanic activity of various types in years to come.
These probabilistic models will be developed for each
volcano, not only for forecasting the chances of eruption
onset over any given time-frame, but also for estimating
the chances of any particular area being affected by
volcanism in future years.
As well as filling in gaps in our knowledge
of recent events at Taranaki/Egmont, Ruapehu and
Tongariro volcanoes, we also want to know how these
volcanoes began “life” and how they have developed
since then. For example the present visible mountain
of Taranaki is only around 10,000 years old, although
volcanism at this location has been going on for at least
130,000 years. Hence, we know very little about this
volcano for up to 90% of its lifetime. At what stage of
life is it now, adolescent, mature, or geriatric? These
distinctions are more than trivial, we want to know
if our volcanoes are in a process of decline, or major
growth, or if they go through long-term cycles of both
throughout their lifespan. Hence, investigating longterm
changes at these volcanoes will help us predict
Fig 1: Lahar hazard map for Ruapehu and its surrounds, based on
mapping and dating deposits of lahars around the volcano.
what they are capable of. Deposits in coastal exposures
in North and South Taranaki and river valleys in the
central and southern North Island will hold the keys for
us to build an understanding of the long term life-cycles
of our mountain volcanoes. Related to this long-term
history will be determining the role of climate change in
modifying volcanic processes. For example, eruptions
on the ice and snow-covered volcanoes during colder
“ice-age” times in the geologic past will have led to
different types of eruption processes to those of today.
ANALYSING THE RISK
On the other side of the coin – where is this new
geological research leading us? How will it help us live
and prosper in the shadow of volcanic hazard? The Civil
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TEPHRA
June 2004