Anatahan showing no signs of activity

After about eight months of continuous eruptive activity, Anatahan’s volcano rested beginning Saturday and showed no signs of activity, according to the U.S. Geological Survey and the Emergency Management Office.

The agencies said the eruption appeared to have stopped between 7:01pm Saturday and 9:08am Sunday.

The agencies based that finding on remote sensing data, since all seismic stations on Anatahan have been offline.

They said the last visible ash from the volcano was Saturday night. The Washington Volcanic Ash Advisory Center and the Air Force Weather Agency have ceased issuing ash advisories.

Citing a pilot report at around 9:08am Sunday, the agencies said “no activity was occurring at the volcano.”

Later that day, they also said that the VAAC detected no ash through satellite monitoring despite mostly clear skies.

This is the first time that the USGS and the EMO reported no volcanic activity on Anatahan since eruptions began to escalate in January this year. Since then, tremor levels on Anatahan have been fluctuating, with occasional strombolian explosions occurring.

Strombolian eruptions are characterized by the intermittent explosion or fountaining of lava from a single vent or crater. Each episode is caused by the release of volcanic gases, and they typically occur every few minutes or so, sometimes rhythmically and sometimes irregularly.

The volcano’s strongest historical eruption happened on April 6, with the volcano ejecting ash to 50,000 feet. The USGS and the EMO estimated the volume of ash emissions in that eruption at about 50 million cubic meters.

The volcano first erupted on May 10, 2003 after centuries of dormancy, with ash plume rising to an altitude of over 30,000 feet and covering over 1-million-square kilometers of airspace above the Pacific Ocean. That eruption, which ceased by mid-June that year, deposited about 10 million cubic meters of material over Anatahan island and the sea.

The second batch of eruptions began about April 9, 2004, after more than a week of increasing seismicity. The second eruption consisted of passive extrusion during mid-April, which later became strombolian explosions every minute or two on April 24, the agencies said. The strombolian explosions continued every minute or two through mid-July, often sending a thin plume of gas and ash upward a few thousand feet and 100 km downwind.

According to the agencies, the second eruption essentially ended on July 26, although visitors to the island three months later could still see very small amounts of steam and ash rising 100-200 ft above the crater rim and could smell sulfur dioxide near the crater.

Anatahan remains off-limits to the public, except for government and approved scientific missions, pursuant to a continuing emergency declaration by Gov. Juan N. Babauta.