SERNAGEOMIN reported that during 25 November-15 December gas-and-steam plumes with variable amounts of ash rose from Chaitén to altitudes of 2.6-3.1 km (8,500-10,200 ft) a.s.l. and drifted E. Intense gas emissions came from the S flank of the first new lava dome (Dome 1), and from the NE part of the second new dome (Dome 2). On 4 December ash ejections originated from the WNW area of the dome complex. Ash plumes rose from Dome 2 to an altitude of 3.6 km (11,800 ft) a.s.l. and drifted W.

An overflight on 6 December revealed that the old lava dome was almost completely covered by Dome 1 (reddish to brown in color). Most of the eruptive activity was concentrated at the site of Dome 2, NE of Dome 1. Dome 2 was grayish in color and exhibited pinnacles and a very uneven top. Constant rockfalls originated from the slopes. Gravitational collapses of the spines produced block-and-ash flows that traveled N, NW, and S, and towards the contact area of the two domes. Domes 1 and 2 both exceeded the height of the caldera rim; Dome 1 was about 250 m above the N rim of the caldera, and Dome 2 was about 350 m above the rim. During 9-15 December, Dome 2 continued to grow rapidly and generate block-and-ash flows. The Alert Level remained at Red.

Based on observations of satellite imagery and web camera views, the Buenos Aires VAAC reported that during 10-11 and 13-14 December ash and steam plumes continuously rose to altitudes 1.8-2.4 km (6,000-8,000 ft) a.s.l. and drifted E, NE, N, and NW. Thermal anomalies were detected on satellite imagery on 10 and 14 December.

Geological Summary. Chaitén is a small caldera (~3 km in diameter) located 10 km NE of the town of Chaitén on the Gulf of Corcovado. Multiple explosive eruptions throughout the Holocene have been identified. A rhyolitic obsidian lava dome occupies much of the caldera floor. Obsidian cobbles from this dome found in the Blanco River are the source of artifacts from archaeological sites along the Pacific coast as far as 400 km from the volcano to the N and S. The caldera is breached on the SW side by a river that drains to the bay of Chaitén. The first recorded eruption, beginning in 2008, produced major rhyolitic explosive activity and building a new dome and tephra cone on the older rhyolite dome.

Sources: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Buenos Aires Volcanic Ash Advisory Center (VAAC)