Indonesian Volcano Shrinks After Collapse Triggers Tsunami

Viriya Singgih
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Indonesian Volcano Shrinks After Collapse Triggers Tsunami

(Bloomberg) -- The Mount Anak Krakatau volcano in Indonesia has shrunk to a third after a series of eruptions and a flank that collapsed triggered a tsunami a week ago that killed more than 400 people and injured several thousands.

The height of the more than 90-year-old volcano has been reduced to about 110 meters from 338 meters after eruptions intensified in the past week, according to the Center for Volcanology and Geological Hazard Mitigation. The eruptions also eroded the mass volume of the volcano to about 40 to 70 million cubic meters from 180 million earlier, it said.

The shrinking of the volcano in height and size reduces the risk of another landslide leading to a tsunami, Antonius Ratdomopurbo, secretary of Indonesian energy ministry’s geology agency, told reporters in Jakarta on Saturday. The current eruptions from the Mount Anak Krakatau are called Surtseyan, when the lava flowing from the crater interacts with sea water, he said.

“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 kilometers 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 the 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 Mount Anak Krakatau 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.”

Southeast Asia’s largest economy has been hit by earthquakes and tsunamis this year, posing a challenge for President Joko Widodo as he bids for re-election in voting scheduled for April. The disasters have also hurt the all-important tourism industry and weighed on the nation’s currency, among the worst performers in Asia in 2018.

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 island, near Bali.

(Updates with comment from volcanologist in seventh paragraph.)

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