Based on analyses of satellite imagery and information from PVMBG, the Darwin VAAC reported that during 19-20 and 23 May ash plumes from Ibu rose 1.5-1.8 km (5,000-6,000 ft) a.s.l. and drifted NE, E, and S.

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: Darwin Volcanic Ash Advisory Centre (VAAC)