50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

4 Institut de Recherche pour le Développement,
Noumea, New Caledonia
(r.j.crimp*massey.ac.nz)
Fluoride is a common constituent of volcanic ash
and has been implicated in human and animal
morbidity and mortality, for example the Laki
Fissure eruptions in Iceland (1783-1784, 1947, and
1970). The 1995-1996 Ruapehu eruptions also
resulted in thousands of sheep deaths which
similarly were attributed to acute fluoride
poisoning.
The island volcano Ambrym has several open
vents, including Marum and Benbow, within a
centrally-located caldera. These have been semicontinuously
active for at least two to three hundred
years. The predominant southeast trade winds
direct volcanic plumes toward the west where the
majority of the population of over 9000 resides.
Wind variability, particularly during the cyclone
season, ensures that no area of Ambrym is entirely
immune from volcanic emissions.
In January 2005, the Ambrym plume was emitting
SO2 at 14 000 - 20 000 tonnes/day, with a fluoride
output estimated to be up to 1 100 tonnes/day. The
continuous degassing from Ambrym’s vents leads
to acid rain and ash fall causing damage to food
crops and vegetation. In terms of acute human
health impacts, respiratory and gastric symptoms
are commonly reported following particularly
intense periods of activity. This study focuses on
the chronic human health impacts that prolonged
exposure to volcanogenic fluoride may have on the
local population.
The plume fluoride appears to be very efficiently
scavenged by rainfall, which the island residents
collect for drinking and cooking purposes.
Drinking water samples collected in January 2005
typically contained between 3-10 ppm F, with
streams, shallow ground wells and coconuts having
similarly high F contents. To put this in
perspective, the World Health Organisation
recommends < 1.0 ppm F in the drinking water of
populations living in tropical environments.
In humans, ingested fluoride is deposited primarily
in bones and teeth. Dental fluorosis – evidenced by
white or brown discolouration of teeth - is the first
visible sign of over-exposure to fluoride and occurs
where levels of fluoride in drinking water are 1-2
ppm. The Dean’s Index is a visual scale for the
classification of fluorosis and was used to assess
over 250 children in West Ambrym. Of these
children, 53% and 8% respectively, fell into the two
highest categories of Moderate and Severe. The
dental results and water analyses suggest that
skeletal fluorosis, a condition involving weakening
of bones, which in its severest form is crippling, is
a potential health issue. As fluorosis has been
linked with Ruapehu’s eruptions, this study is of
significance for researchers investigating the risks
associated with possible prolonged degassing of
any of New Zealand’s Central North Island
volcanoes.
ORAL
WHERE ARE THE GIANT TUFF CONE AND
IGNIMBRITES OF AMBRYM? A MORE
CONVENTIONAL STORY OF MAFIC
VOLCANISM AT AMRBYM VOLCANO,
VANUATU.
Shane J. Cronin & Károly Németh
Massey University, Institute of Natural Resources,
PO Box 11 222, Palmerston North, NZ
(S.J.Cronin*massey.ac.nz)
Ambrym is located in the central part of the
Vanuatu volcanic arc and forms a triangular shaped
island around 60 km long on its NE-SW axis. The
NW-SE oriented rift zone accommodates most of
the historic eruptions from this arc-basaltic volcano
and a series of older overlapping edifices make up
the northern portion. A 12 km-wide caldera
dominates the centre of the island, containing two
active tuff/scoria cones (Benbow, Marum) and
many smaller open vents. Semi-continuous ash falls
and periodic lava flows inundate the caldera and
choke stream networks down-slope with
remobilised pyroclastics. Since the mid 90’s
Ambrym has been regarded as a type-locality for a
“giant tuff cone”, that formed by enormous
phreatomagmatic eruptions, which accompanied
disruption of an edifice to form the large caldera.
This theory, however, is entirely built on field
observations of a layer-cake stratigraphy
constructed from a mosaic of sections. Current field
studies revisiting these sites and others, show that
this mosaic is false, and many of the sections
cannot possibly be related to one another either
genetically or chronologically. Our studies indicate
that the island is a composite structure, formed of
many generations of coalescing monogenetic
volcanic fields. The type localities of the “giant tuff
cone” (dacitic) pycroclastic flow deposits in the
northern shoreline of the island we interpret to be
either (1) mafic phreatomagmatic fall and surge
sequences (in places hydrothermally altered) or in
other cases (2) stacks of lahar and fluvial sediment
derived from the caldera outflow and forming
valley-fills. These successions are typical of many
new sites described around the volcano which
imply several generations of a frequently active
mafic volcano, with coalescing fields of lava, basesurge
deposits and falls (often with accretionary
lapilli), along with abundant deposits of fluvially
reworked pyroclastics. There is no evidence for a
climactic (or for that matter dacitic) explosive
caldera-forming event, which concurs with
50 th Kaikoura05 -21- Kaikoura 2005