PVMBG reported decreasing activity at Ibu. Both the number of earthquakes and the height of ash plumes began to decline on 15 June. In addition, the distances of ejected incandescent material also shortened to a radius of 1 km. White, gray, and black ash plumes rose 100-3,000 m above the summit and drifted in multiple directions during 19-21 June. Because of the decreasing activity PVMBG lowered the Alert Level to 3 (the second highest level on a four-level scale) at 1032 on 21 June and advised the public to stay 4 km away from the active crater and 5 km away from the N crater wall opening. On 22 June white-and-gray ash plumes rose as high as 3 km and drifted NW, W, and SW, and on 23 June ash plumes rose as high as 5 km and drifted NW, W, and SW. On 24 June white plumes rose 1 km and drifted W and NW and on 25 June white-and-gray ash plumes rose 400 m and drifted NE, E, and W.

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.

Source: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)