PVMBG reported that the eruption at Ibu continued to intensify during 11-21 May, with ash plumes rising higher and having denser ash content. Seismicity was also high and increasing. Beginning on 11 May the ash plumes began rising 4-5 km above the summit; the plumes were dense and gray and drifted N and NW, and incandescent ejecta was visible. Eruptive events were recorded on 12 and 14 May, though weather conditions prevented visual observations. During 13-16 May gray-to-black ash plumes rose as high as 5 km and drifted multiple directions. Roaring and banging noises were heard in areas as far away as the Ibu observation post (9 km W). At 1500 on 16 May the Alert Level was raised to 4 (the highest level on a four-level scale) and the public was advised to stay 4 km away from the active crater and 7 km away from the N crater wall opening. BNPB reported that 263 residents evacuated from three villages, Gam Ici, Goin, and Sangaji Nyeku.

White-and-gray ash plumes with variable densities rose 4-5 km above the summit and drifted multiple directions during 17-18 and 20-21 May. Photos from just after 2000 on 18 May showed lightning in the dense ash plumes. Only white steam-and-gas plumes were visible on 19 May, rising 200-300 m above the summit and drifting N, NE, and E. According to a news report the total number of evacuees rose to more than 400 by 19 May; the residents were from seven villages in the West Halmahera District.

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.

Sources: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM), Badan Nacional Penanggulangan Bencana (BNPB), Antara News