The dramatic collapse of Indonesian volcano Anak Krakatau in December 2018 is the result of long-term destabilizing processes and was not driven by any marked changes in the igneous system that could be detected by current monitoring techniques, new research has shown.
The volcano erupted about six months before it collapsed, when more than two-thirds of its height slid into the sea as the island halved. The event caused a devastating tsunami, which flooded the coasts of Java and Sumatra and killed more than 400 people.
Team led University of Birmingham they examined volcanic material from nearby islands to determine if a strong, explosive eruption observed after the collapse itself caused a landslide and tsunami. Their results were published in Letters on Earth and Planetary Science.
Work with researchers at Bandung Institute of Technology, Oxford University and the British Geological Survey, the team looked at the physical, chemical and microtextual characteristics of the material that erupted. They concluded that the large explosive eruption associated with the collapse was probably caused by the destabilization of the underlying magmatic system as the landslide began.
This means that the catastrophe is less likely to have been caused by magma that forced its way to the surface and caused a landslide. Current volcano tracking methods record seismic activity and other signals caused by magma rising through the volcano, but since this event was not triggered from within, it would not have been detected using these techniques.
Dr. Sebastian Watt, from the Faculty of Geography, Earth Sciences and Environment at the University of Birmingham, is a senior author of the paper. He said: ‘This type of volcanic hazard is rare, extremely difficult to predict and often devastating. Our findings show that although there was a dramatic, explosive eruption after the collapse of Anak Krakatau, it was triggered by a landslide that released pressure on the magma system – like the bursting of a champagne cork. ‘
The results pose a challenge to predict future hazards on volcanic islands. Dr. Mirzam Abdurrachman, of the Bandung Institute of Technology, explains: ‘If major volcanic landslides occur as a result of long-term instability, and can occur without any marked changes in volcanic magmatic activity, it means they can occur suddenly and without any clear warning.
‘This discovery is important for people living in regions surrounded by active volcanoes and volcanic islands in places such as Indonesia, the Philippines and Japan.’
Lead author, Kyra Cutler, of Oxford University, said: ‘Assessing long-term patterns of volcanic growth and deformation will help to better understand the likelihood of failure – this will be especially important for Anak Krakatau as he recovers. The identification of sensitive areas, together with efforts to develop non-seismic tsunami detection, will improve overall hazard management strategies for communities at risk. ‘
Professor David Tappin, (British Geological Survey, University College, London) led marine research that mapped deposits caused by the collapse of the Anak Krakatau eruption in 2018 (Hunt et al. 2021). He said: ‘It is rare that we have the opportunity to study such an eruption and tsunami, with the latest event, Ritter Island, more than 100 years ago. The results in the paper reveal that the propulsion mechanism was the result of long-term destabilization, not an immediate explosive event. This is a big surprise discovery and will lead to a reassessment of how to mitigate the danger of volcano failure and related tsunamis. ‘
Reference: “The eruption that spreads downward after emptying the hole does not imply a direct magmatic trigger for lateral collapse Anak Krakataua 2018.” Kyra S. Cutler, Sebastian FL Watt, Mike Cassidy, Amber L. Madden-Nadeau, Samantha L. Engwell, Mirzam Abdurachman, Muhammad EM Nurshal, David R. Tappin, Steven N. Carey, Alessandro Novellino, Catherine Hayer, James E. Hunt, Simon J. Day, Stephan T. Grilli, Idham A. Kurniawan and Nugraha Kartadinata, 14 January 2022, Letters on Earth and Planetary Science.
DOI: 10.1016 / j.epsl.2021.117332