EM – Massive volcanic explosion didn’t cause Indonesian island’s 2018 collapse

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January 15, 2022

from the University of Birmingham

The dramatic collapse of Indonesia’s Anak Krakatau volcano in December 2018 resulted from long-term destabilizing processes and was not triggered by significant changes in the magmatic system that could have been detected by current monitoring techniques, new research has found.

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The volcano had erupted about six months before collapsing, which slid more than two-thirds of its height into the sea as the island halved in area. The event triggered a devastating tsunami that inundated the coasts of Java and Sumatra, killing more than 400 people.

A team led by the University of Birmingham examined volcanic material from nearby islands for clues to determine whether the powerful, explosive eruption observed after the collapse had itself triggered the landslide and tsunami. Their findings are published in Earth and Planetary Science Letters.

In collaboration with researchers from the Bandung Institute of Technology, the University of Oxford and the British Geological Survey, the team examined the physical, chemical and microtextural properties of the excavated material. They concluded that the large explosive eruption associated with the collapse was likely caused by the destabilization of the underlying magmatic system when the landslide began.

This means the disaster was less likely caused by magma that rose to the surface and triggered the landslide. Current volcano monitoring methods record seismic activity and other signals caused by magma rising through the volcano, but since this event was not internally triggered, it would not have been detected using these techniques.

Dr. Sebastian Watt from the University of Birmingham’s School of Geography, Earth and Environmental Sciences is the senior author of the article. He said: “This type of volcanic hazard is rare, extremely difficult to predict and often devastating. Our results show that although there was a dramatic, explosive eruption after the collapse of Anak Krakatau, it was triggered by the landslide that released pressure on the magma system – like a champagne cork popping.’

The results pose a challenge for predicting future hazards on volcanic islands. Mirzam Abdurrachman of the Bandung Institute of Technology explains: “If large volcanic landslides occur as a result of long-term instability and can occur without a significant change in magmatic activity at the volcano, this means that they can occur suddenly and without clear warning.

“These Finding is important for people living in regions surrounded by active volcanoes and volcanic islands, such as Indonesia, the Philippines and Japan.”

Lead author Kyra Cutler of the University of Oxford said: “The assessment Longer-term patterns of growth and deformation of volcanoes will help better understand the likelihood of failure – this will be particularly relevant for Anak Krakatau as it rebuilds. The identification of vulnerable areas along with efforts to develop non-seismic tsunami detection will improve overall hazard management strategies for vulnerable communities.’

Professor David Tappin (British Geological Survey, University College, London) led the marine surveys that the mapped deposits resulting from the onset of the 2018 Anak Krakatau eruption (Hunt et al. 2021). He said: “It is rare that we have the opportunity to study such an eruption and tsunami, with the last event, the Isle of Knights, being over 100 years ago. The results in the paper indicate that the propulsion mechanism resulted from a long-term destabilization rather than a sudden explosive event. This is a major surprise discovery and will lead to a reassessment of how to mitigate the risk of volcanic outbursts and the associated tsunamis.’

END

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