We arrived at Anatahan at
approximately 6.30 am (photo #1). The volcano looked similarly
active compared to our visit by helicopter 2 days previously.
The ship sailed through the ash fall out to the south-west
side of the island, and we continued along the western coast.
The west coast was draped in ash - vegetation
was completely covered giving the island a grey palor (photo
#2). We landed at 8.15am and spend approx. 4 hours ashore.
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Photo 1 |
Photo 2 |
We first worked on the beach area. We made a
trench through the recent deposits exposing a 10 inch section
from the present eruptive phase. There are 3 main layers present:
(1) Lowermost layer—fine-grain ash layer (2 inches) (2) Accretionary
lapilli with some fine ash (6 inches) (3) mixture of coarser
grained ash and angular clasts of scoraceous material (2 inches).
We interpret the layers thus: (1) initial blast; (2) interaction
of magma with water - possibly the pre-existing hydrothermal
system (fumaroles and/or mud pots); (3) continuing magmatic
erution with juvenile? magmatic material.
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| We then continued to the abandoned village where
the second team (led by Patrick Shore) was working on the seismic
station (installed May 6–4 days prior to the eruption). The
area was similarly covered in ash with many buildings having
collapsed roofs (photo #3). We made 2 sections (photo #4) and
made similar observations i.e. initial ash covered by accretionary
lapilli covered by a mixture of ash and scoracious material.
In water collection vessels by the buildings (photo #5), we noticed
(and collected) pumice which was floating on the water. The pumice
was likely erupted as part of layer 3 i.e. continuing magmatic
eruption.We returned to the beach (photo #6) and collected from
a new trench 300m to the south. We also collected older (pre-existng)
lavas from the beach. We returned to the ship at approx 12.30
pm. |
Photo 3 |
Photo 4
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Photo 5
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Photo 6 |
Photo 7 |
From
the ship we set up the COSPEC instrument (photo #7) and starting
a traverse through the plume at approx. 1.30pm.
The telescope was oriented vertically and the ship made a north-to-south
transect through the volcanic plume at a distance of about 1.5
km from shore. |
| We immediately started to recorded the presence
of sulfur dioxide (SO2) in the plume. The transect took 50 minutes
until we no longer recorded SO2. In addition, we sailed through
the ash fallout. During the traverse, the volcano erupted every
5 minutes with a deep resonnating boom (photo #8). The width
of the volcanic plume was approx 6km wide and its direction
was to the south of west. From the COSPEC measurements
we calculated a SO2 concentration in the plume of 429 ppm
meters. To estimate the flux of SO2 from the volcano we need
to know the wind speed. From wind speed data provided by
NOAA for May 21 (10-15 knots for Anatahan) we estimate the
SO2 flux to be 3000 - 4500 tons/day.
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Photo 8 |
Photo 9 |
We deduce:
(1) the eruption is clearly in a magmatic phase i.e. the
SO2 results from degassing of magma. This is consistent with
juvenile material (from the mantle) being brought close to
the surface.
(2) the flux lies with a typical range of erupting volcanoes.
As we sailed away from the island (departing at approx 2.30 pm) there
was a very large eruption with a significantly louder 'boom' than what
we heard previously. The boom was followed by a dark (i.e. ash laded)
billowing plume (photo #9).
The eruption is continuing.
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Summary
The start of the eruption may have been caused by water-magma
interaction (as evidenced by the accretionary lapilli). The
eruption is now in the magmatic phase.
David Hilton (Scripps Institution of Oceanography)
Tobias Fischer (University of New Mexico)
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