Sarigan Island in the northern Mariana Islands. An undersea eruption appears to be underway south of the island.

Thanks to the watchful eye of Eruptions readers, we had an inkling of this eruption the other day, but now we have confirmation that an undersea eruption is underway in the northern Mariana Islands. The exact volcano erupting is unknown, but it appears to be between Anatahan and Sarigan (see below) - both active volcanoes in the Marianas - at a depth of 300 meters. The eruption has been described by the USGS as hot water vapor and an ash plume that might have reached as high as 12 km (40,000 feet) while observers/residents on Sarigan Island heard a large explosions and minor ash over weekend. The eruption has prompted the leader of the Commonwealth of the Northern Mariana Islands to declare a state of emergency.

Map of the Mariana Islands.

Anatahan had its first historical eruption in 2003 and erupted as recently as 2008. Sarigan has not had any known historical eruptions, but two seismic swarms were centered at the volcano in August 2005. Holocene lava flows on the flanks of the volcano are the only signs of geologically recent activity at Sarigan. Some of you might also remember Sarigan as Mystery Volcano Photo #4.