JAKARTA: The volcanic island Anak Krakatau, which erupted and collapsed a week ago triggering a deadly tsunami, is now only less than one-third of its pre-eruption size, scientists say.
The Indonesian Center for Volcanology and Geological Disaster Mitigation says Anak Krakatau now has a volume of 40-70 million cubic metres compared with about 180 million before the Dec 22 eruption and tsunami.
The analysis shows the scale of the island’s collapse, shedding light on the power of the tsunami that affected more than 300 kilometres of coastline in Sumatra and Java. More than 420 people died.
The centre says the crater’s peak was 110 metres high as of Friday compared with 338 metres in September.
The shrinking of the volcano in height and size reduces the risk of another landslide leading to a tsunami, Antonius Ratdomopurbo, secretary the energy ministry’s geology agency, told reporters in Jakarta on Saturday.
The current eruptions from the Mount Anak Krakatau are described as Surtseyan, when lava flowing from the crater interacts with sea water, he said. The word derives from Surtsey, a volcanic island that formed in the sea off Iceland in 1963.
“With Anak Krakatau’s small body volume at present, it will be unlikely for us to see a big landslide that can trigger a tsunami,” Ratdomopurbo said. “A tsunami could only happen if there’s a fault reactivation in the Sunda Strait.”
Indonesian authorities have widened a no-go zone around the volcano to five kilometres and directed airlines to avoid the airspace above the Mount Anak Krakatau after ash clouds were spotted as high as 24,000 feet above sea levels this week.
The deadly tsunami in the Sunda Strait on Dec 22 killed at least 426 people and injured more than 7,000 residents and holiday makers in Lampung and Banten provinces.
The killer wave was likely triggered by a flank collapse — where a section of the volcano gave way — on the south and southwest sides of the Mount Anak Krakatau, according to Indonesian geologists. The volcano will grow as the crater still contains magma that will cause volcanic activity, Ratdomopurbo said.
“I would expect the shape of the island to change a lot in the next few days, weeks and perhaps even months,” Thomas Giachetti, a volcanologist at the University of Oregon, said in an email. “It will enter a phase of construction and destruction until the cone is high enough again to prevent significant mixing of the magma with external water and the eruptions to return to a more Strombolian type.
“It is possible that the new shape of the volcano also allows for a slow movement of volcanic material into the sea and into the caldera formed by the 1883 eruption.”
Indonesia’s 17,000 islands are prone to earthquakes because the country straddles the Pacific Ring of Fire — an arc of fault lines and volcanoes that causes frequent seismic upheavals. Two of the largest eruptions in the past 200 years occurred in Indonesia at Mount Tambora in 1815 and Krakatau in 1883. In a 1919 eruption of Mount Kelud, more than 5,000 people were killed.
In September this year more than 2,000 people were killed and about 80,000 displaced in Central Sulawesi after a devastating earthquake and tsunami struck the island. That was preceded by a series of deadly earthquakes early in 2018 that rattled the popular tourist destination of Lombok, east of Bali.