Volcanoes - Ministry of Civil Defence and Emergency Management

Vertical deformation in the caldera
volcanoes is monitored using a lake-levelling
technique, in which the whole lake acts as
a level. Nearly 30 years of record from Lake
Tarawera and Lake Taupo have shown regional
deformation of several millimetres per year,
analogous to deep sighing of these volcanoes.
CHEMICAL MEASUREMENTS
Volcanic regions emit gases both
during and between eruptions, and changes in
emission rates and chemistry of volcanic gases
can help us to predict changes in volcanic
activity. Volcanic gases emit through the
main volcanic conduits to form fumaroles and
volcanic plumes, and also diffuse through soils.
Some components of the gas stream react with
groundwater and are essentially scrubbed
when passing through crater lakes or shallow
aquifers. Thus, when monitoring the volcano
chemistry, it is important to consider the
different emission pathways, and monitor both
the gas and fluid phases.
The two most abundant gases emitting
from volcanoes following water vapour are
carbon dioxide (CO 2
) and sulphur dioxide
(SO 2
). These two volcanic gases behave
differently in magmas, and thus, each gas
provides information about activity at different
depths. Carbon dioxide, for instance, has a
relatively low solubility in magma compared
to sulphur dioxide and water vapour. Thus,
as magma starts to move from deep (~35
km) in the crust toward the surface, CO 2
will
become progressively supersaturated in the
magma and be released before other gases.
When monitored periodically using airborne
or ground-based techniques, increases in CO 2
emissions will provide the first indication
that there is magma movement at depth. For
example, in 2000 at Usu Volcano in Japan, a
group of scientists observed a tenfold increase
over typical background levels in diffuse soil
emissions prior to an eruption. The techniques for
measuring CO 2
through soils and in volcanic plumes
have improved dramatically over the last 10 years, and
are being utilised to provide more insight about the
behaviour of CO 2
emissions preceding volcanic eruptions
in New Zealand.
Sulphur dioxide emissions are easy to measure
using remote techniques (COSPEC) from airborne
platforms. SO 2
is released from magmas at shallower
depths and can also be used to detect magma movement
toward the surface (within a few km of the surface). For
Measuring temperatures at the Mt. Ruapehu Crater Lake. Photo: GNS.
instance, preceding the 1995 eruption of Ruapehu there
was an 18% increase in the concentration of sulphate
ions in the crater lake over a period of 5-6 weeks,
suggesting an increase of at least one thousand tonnes/
day of SO 2
over this time period.
Changes in the ratio of gas abundances can also
be used to detect changes within the volcanic system.
Sampling and chemical analysis of gases from individual
fumaroles as well as crater lakes can provide detailed
information about the trace abundance of gases emitting
from the volcano. For example, for the six months prior
to the eruption at Ruapehu in 1995, scientists observed
15
TEPHRA
June 2004