Appendices 5-13 - Nautilus Cares - Nautilus Minerals

'leaky' and recent lava flows have been observed on the seabed in the area. In addition,
there is a volcanically active area, the Southern Rifts, associated with, but displaced from the
eastern part of the BSSL.
Being a spreading axis volcanism is probably common along the length of the BSSL. At the
Solwara 1 site there are fresh glassy lavas with little or no sediment cover (M.J. White pers.
Com.) these are considered of recent origin (a few to less than a couple of hundred years
old). As mentioned above several lava fields appear to be associated with the Willaumez
Transform, but are somewhat displaced from it (C.McKee, pers. Com.), detailed outcrop data
is unavailable for other areas.
Decker and Decker (1981) estimated that on the Mid Atlantic Ridge there is 1 eruption every
ten years for each 300km length of the spreading axis. The spreading rate of the Mid Atlantic
Ridge is ~1-2 cm a year, the BSSL is spreading at a rate of 11 -14 cm a year. From the
Decker's work and the observed spreading rates it can be estimated that for each 300km of
the BSSL there may be at least one eruption every year or roughly three eruptions a year
somewhere along its ~1000km length.
Despite decades of seismic monitoring of the Bismarck Volcanic Arc, only four episodes of
suspected BSSL eruptive activity have been recorded instrumentally. Due to the lack of other
evidence it is impossible to confirm these were eruptions, but volcanic activity is the best
candidate to fit the seismic signals. In the early 1960's seismic events where detected NW of
Rabaul, these were attributed to possible activity on the Eastern Rifts (J. Letter, pers. Com.).
In 1972 seismic and acoustic evidence pointed to activity near the termination of the WT-ETZ
(Johnson et al. 1981). In 1994 a swarm of earthquakes 50km to the north of Rabaul was
thought to be associated with eruptive activity on the eastern end of the Eastern Rifts (C.
McKee, pers. Com.), and in 2002 low frequency seismic activity NW of Rabaul may have
been due to activity on the Eastern Rifts (I. Itikarai, pers.com.). The seismic arrays
mentioned are designed to detect volcanic activity at nearby volcanoes on North Coast New
Britain and the Islands off Madang and are not designed for monitoring of the BSSL. As such
many BSSL eruptions have probably gone unrecorded.
Despite the strong likelihood of there being frequent deep submarine eruptions along the
BSSL human populations on the shores of the Bismarck Sea have never reported any effects,
which could be attributed to them, or expressed any awareness of such activity taking place.
2.2 Why do BSSL Eruptions go Unnoticed?
It is possible to calculate the volume of gas in magmas when erupted at sea level pressure
and temperature. At sea level 1m 3 of gas weighs about 1.37kg. So it can be calculated that
what had been 1m 3 of basalt with 60kg of gas dissolved in it will expand to ~83m 3 of
fragmental magma/rock and gas. This is if the gas is at surface temperature, if however,
magmatic temperatures are taken into account - at 1200°C (typical basalt temperature) this
gas volume will be ~722 m 3 (i.e. gas volume doubles for every 273°C rise in temperature). At
the earth's surface this gas can escape easily, as basalts are very fluid, and this massive
increase in gas volume usually only leads to gentle to moderately violent eruptions (i.e.
'Hawaiin' type).
To get an idea of gas volumes involved in submarine eruptions we can work downwards from
sea level using Boyle's Law. Boyle's Law states that gas volume will half with each 10m
increase in water depth. Using this simple formula it is found that between 190-200m water
depth 1m 3 of basalt magma at 1200°C will only contain about 1mm 3 of bubbles. At the depth
of the Solwara1 extraction area (1500-1700m) the volume of bubbles in any liquid magma
would only be 5.06x10 -43 to 1.51x10 -50 m 3 , an infinitesimal amount of expansion, making
violent explosions/eruptions impossible.
Because of the fact that high hydrostatic pressures severely restrict bubble growth, as
described above, natural eruptions at the BSSL go unnoticed by local populations living on