Thermal anomalies detected by satellites (MODVOLC thermal alerts) through June 2009 suggested continued growth of a lava dome in the crater (BGVN 34:05). The Center of Volcanology and Geological Hazard Mitigation (CVGHM) reported that prior to 11 July 2009, white and gray plumes from Ibu rose ~ 600 m above the crater rim. After 11 July, the plumes were gray and rose only ~ 400 m above the crater rim. Ash from the gray plumes fell on areas within a 3-km radius of Ibu.

Observers noted an increase of eruptive activity after mid-July. During the period 27 July to 3 August, the total number of eruptive events showed a tendency to increase. Each eruptive earthquake was then followed by the expulsion of lava that reached the upper slopes. Plumes seen during 15 July to 4 August 2009 were grayish-white and reached a height of ~ 300-400 m above the crater rim. The lava extrusions accompanied rather strong rumbling noises on five occasions. The incandescent material was seen coming from the summit on 2 August 2009, and lava flows were seen. Later that day, a thunderous sound was followed by incandescence at the summit.

On 3 August, incandescent material was ejected as high as 20 m above the crater. The total of explosion earthquakes increased from the 20-49 events of mid-July to 50-80 events during 27 July to 4 August. Villagers in Desa Duono, Going, and Sanghaji noted strong rumbling sounds. No volcanic earthquakes were recorded during that time frame. On 4 August 2009, 82 volcanic earthquakes were recorded. Each eruptive earthquake was followed by the expulsion of lava which reached the upper slopes.

The observation post in the village of Duono, 5 km NW of Ibu, reported that the lava dome continued to grow. As a result, local residents were advised to prepare for times when they needed to wear masks that cover both the nose and the mouth. Visitors and tourists were asked to remain at least 2 km from the crater.

Geological Summary. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, has contained several small crater lakes. The 1.2-km-wide outer crater is breached on the N, creating a steep-walled valley. A large cone grew ENE of the summit, and a smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. The first observed and recorded eruption was a small explosion from the summit crater in 1911. Eruptive activity began again in December 1998, producing a lava dome that eventually covered much of the floor of the inner summit crater along with ongoing explosive ash emissions.

Information Contacts: Center of Volcanology and Geological Hazard Mitigation (CVGHM), Saut Simatupang, 57, Bandung 40122, Indonesia (URL: http://vsi.esdm.go.id/); Hawai'i Institute of Geophysics and Planetology (HIGP) Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).