How Long Will Krakatau Keep Spewing Volcanic Ash? Here's What the Experts Say

The explosive eruption of Mount Anak Krakatau on Friday (4/9) at 23.07 WIB spewed an extremely tall ash column. According to analysis by Erma Yulihastin, a Research Professor at BRIN’s Centre for Climate and Atmospheric Research, the eruption column is estimated to have reached 13 to 14 kilometres.

That height is believed to have nearly penetrated the tropopause, the upper boundary of the troposphere, allowing volcanic material to spread widely into higher layers of the atmosphere.

Residents around Banten, West Java, Lampung, Bengkulu and Jakarta reported volcanic ash spreading into their neighbourhoods. Many shared photos and videos of cars and house terraces covered in volcanic dust.

According to Erma, the volcanic ash produced by the main eruption of Mount Anak Krakatau will generally release sulphur material into the atmosphere for five days, though several conditions need to be considered.

“In general, if there is no rain at all, around five days. The direction of dispersal depends on the wind direction at each level of the atmosphere,” Erma wrote on her personal X account, quoted on Monday (7/9/2026).

Meanwhile, Daryono, a member of the Indonesian Disaster Experts Association (IABI), said ash fall cannot be determined by a date on the calendar. He stressed that the eruption will only truly stop once the supply of fresh magma from the Earth’s depths is exhausted and the pressure within the magma chamber has calmed down again.

“As long as the magma pocket beneath the volcano continues to receive new magma deliveries from deep within the Earth, follow-on eruptions carrying ash will continue to occur in an up-and-down pattern (sometimes calm, sometimes intensifying),” Daryono wrote in a post on his personal X account.

Daryono went on to say that the supply of new magma will stop once the natural thrust from within the Earth loses its energy. Without a strong push, the molten magma inside the volcanic conduit will cool, harden, and eventually become blocked by the surrounding rock.

“That is when the volcano returns to a stable condition and is no longer able to spew material to the surface,” he said.

Determining Factors for the Direction of Volcanic Ash Dispersal

Many have wondered why the eruption material is not moving towards Sumatra, given that surface winds are generally observed moving west or north-west.

Addressing this, Erma explained that the movement of ash is heavily influenced by differences in wind characteristics at each atmospheric level. There are variations in wind direction at upper levels, where some winds move towards the east and the south.

“In essence, if the ash column is tall, the fallout can be distributed in all directions,” Erma said.

Echoing this, Daryono cited two factors that determine the direction and distance of ash dispersal. First, how powerful the volcanic eruption is. Second, which way the wind blows.

“When a volcano erupts strongly, fine ash is thrown very high up to the upper air layers. There, strong winds carry these light dust particles floating for tens or even hundreds of kilometres before finally falling to the ground,” he explained.

Volcanic Ash Dispersal on 7 September 2026 According to the BMKG

Separately, the BMKG detected two clusters of volcanic ash dispersal, based on information from the PVMBG, VAAC Darwin, and analysis of Himawari-9 RGB imagery on Monday (7/9/2026) at 07.00 WIB.

The first cluster, at an altitude of up to 15,000 feet or 4.5 kilometres, is moving in a south-east to north-west direction, covering Banten, West Java, Lampung, Bengkulu, the Indian Ocean west of Bengkulu, and the waters off Banten.

Meanwhile, the second cluster has soared to an extreme altitude of 50,000 feet (15.2 kilometres) and is moving in a south-westerly to westerly direction, away from Indonesian territory over the Indian Ocean.

Given the extent of the dispersal of follow-on material such as sulphur and volcanic ash, members of the public, particularly those active in western Java and Lampung, are urged to always wear masks to protect their respiratory health.