Report on Visit to Anatahan - Wednesday 21 May, 2003

David Hilton and Tobias Fischer

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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.


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.

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


Photo 5


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.


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.

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)

Last updated Monday, January 31, 2005