Interactions between eruptive activity and sector collapse at Anak Krakatau, Indonesia

Lead Research Organisation: NERC British Geological Survey
Department Name: Earth Hazards & Observatories


Anak Krakatau volcano, Indonesia, collapsed catastrophically on 22nd December 2018, forming a landslide-generated tsunami that caused over 400 deaths on surrounding coastlines. Very few volcanic landslides of this size and type, known as sector collapses, have been studied in detail. Because of this, our understanding of the factors that lead to sector collapse, and therefore our capacity to forecast such events and their associated hazards, remains relatively limited. Although there have been few historical examples of large volcanic landslides, they are common on longer, geological timescales and occur across all volcanic settings. The collapse of Anak Krakatau thus provides an important opportunity to improve our knowledge of this fundamental volcanic process.

Anak Krakatau is the volcanic island that formed after the devastating eruption of Krakatau (also known as Krakatoa) in 1883, first emerging above sea level in 1929. Approximately half of the subaerial island of Anak Krakatau was removed by its recent sector collapse. The collapse occurred during an ongoing, relatively low-intensity eruption, similar to the type of activity that had characterised previous decades. However, satellite observations suggest that this style of activity changed around the time of the collapse, to a much more powerful explosive eruption. The precise timing of this change, and its potential role in the collapse, is something we will explore in detail in this research. Following the collapse, explosive activity continued and may have changed yet again, as seawater interacted with shallow erupting magma. This later stage of activity erupted large volumes of new material, rapidly filling the landslide scar and extending the island coastline in the days after the collapse.

Our research will determine the specific role of eruptive activity in the sector collapse of Anak Krakatau. We will address whether changes in eruption behaviour, involving the ascent of fresh magma, preceded the collapse and thus acted as a trigger; or whether it was the collapse itself which led to the powerful explosive eruption, by suddenly depressurising the shallow magma stored beneath the volcano. We will also define the nature of eruptive activity that took place immediately after the collapse. In this phase of the eruption, material appears to have been ejected at a very high rate, and we will test the hypothesis that the collapse destabilised the underlying magma system, leading to a change in eruption behaviour. Such processes may be common at volcanoes affected by large sector collapses, forming part of a cycle of destruction and regrowth, but are currently poorly understood.

Our work will draw upon detailed field sampling of eruption deposits spanning the collapse period. Field datasets will be interpreted alongside satellite imagery and other remote observations, numerical models that simulate eruption processes, and analyses of the chemical and textural record of magmatic processes preserved in our eruption-deposit samples. Together, our results will allow us to identify changes in the storage conditions, ascent rate and eruptive behaviour of magmas involved in different stages of activity. Our results will allow us to explore controls on the timing of the sector collapse, the role of eruptive activity in the collapse, and the impact the collapse itself had on the underlying magma system. By producing a comprehensive record of the Anak Krakatau collapse and eruptions, we will advance our understanding of volcanic sector collapses in general. We will also develop a much clearer picture of eruption processes and instabilities at Anak Krakatau, which will inform hazard mitigation plans for potential future landslides as the volcano regrows.

Planned Impact

This research will provide a comprehensive understanding of the volcanic processes involved in the sector collapse of Anak Krakatau, including the role of eruptive activity in the build up to collapse, the nature of explosive activity that accompanied the collapse, the impact of collapse on the underlying magma system, and the nature of the volcanism that led to rapid post-collapse regrowth of the island. As well as providing significant insights into eruption-associated sector collapses in general, these results have a broad importance to non-academic stakeholders including Indonesian hazard monitoring agencies and those working in the field of tsunami modelling and tsunami hazard mitigation.

Hazard monitoring agencies: Results, including technical summaries, will be shared fully with the appropriate Indonesian agencies responsible for volcanic and tsunami hazards: the Centre for Volcanology and Geological Hazard Mitigation (CVGHM), and the BMKG (the Meteorology, Climatology and Geophysical Agency, responsible for tsunami warning). We have already been in contact with relevant stakeholders to discuss our research plans, and these relationships will be developed further through pre-fieldwork meetings with these agencies. The unforeseen nature of the Anak Krakatau collapse highlights the unpredictability of sector collapses and the challenges involved in mitigating their effects. The controls on sector collapse remain poorly understood, but our detailed analysis of the event will identify the role of eruptive activity in the collapse, and will also provide a clearer understanding of post-collapse regrowth at Anak Krakatau, enabling future activity and growth to be monitored in the context of recent events. Our evaluation of a variety of remote sensing data will also help directly inform future developments of hazard monitoring at Krakatau. Collectively, our results will provide a much improved basis for forecasting future instabilities, and for developing landslide-tsunami monitoring efforts and mitigation plans on that basis.

Tsunami modellers: More generally, our results will form important additions to a comprehensive landslide-source and tsunami dataset for the Anak Krakatau event, making it the best constrained example of a large scale volcanic-island tsunami. This example can be used as a benchmark dataset against which tsunami models can be tested and improved, and then used for future landslide-tsunami hazard modelling scenarios. This is important to tsunami modellers in both academic and non-academic contexts, including those involved in coastal flood risk management and tsunami preparation in all volcanic settings, and the reinsurance sector. Results will be shared with international tsunami modelling groups (via active collaborations with S. Grilli, U. Rhode Island) and by providing a volcanological summary of the event to relevant stakeholders (e.g. to the quarterly technical meeting of the Intergovernmental Oceanographic Commission).

Wider public engagement: Our work is also likely to be of wide public interest, particularly given the effects of the Anak Krakatau collapse and its international media coverage, as well as broader awareness of Krakatau as a source of hazardous volcanic eruptions. One benefit of this type of research is increased awareness, education and engagement with scientific topics. We will distribute results through public talks, community events, museum exhibits and will also aim to publicise our research through local and national media.
Description The main outcomes of this research are being finalised, and this will be updated further in due course. The overall objectives of the project have been met, with the initial aim being to establish whether a shift in magmatic activity drove the collapse of Anak Krakatau, or if the reverse was true. This question is important in helping address whether monitoring data could identify precursory signals of volcanic lateral collapses. Our analysis of samples spanning the collapse demonstrates no distinctive change in the magmatic system around the event, and that the eruptive behaviour is consistent with being a response to unloading by the landslide. This is significant in terms of planning and monitoring for future events, as it implies that longer-timescale deformation processes may hold a better prospect than short term monitoring of magmatic processes in recognising incipient collapse. Ongoing research is following this up by assessment of gas outputs spanning the event, the longer-term construction of the volcano, and further collaboration to build on this investigation of collapse and tsunami hazards at island volcanoes.
Exploitation Route The outcomes will be relevant to volcano and tsunami hazard agencies globally in terms of improving the understanding of the triggers of volcanic lateral collapses, and giving insights into what signals may precede such an event. From this and other research, improved monitoring strategies can ultimately be developed. The results will also feed into improvements in landslide tsunami modelling - the outputs of this reserach have been used to develop updated constraints on the event parameters (landslide source) that have been tested against current numerical tsunami models (in both a published research study and an updated study that is has been submitted for publication). Results will be shared fully via Indonesian partners in the project to feed into regional hazard management strategies.
Sectors Environment,Leisure Activities, including Sports, Recreation and Tourism,Government, Democracy and Justice

Description The results of this project have been communicated widely to a public audience in both the UK and Indonesia, as well as to academic communities via journal publications and presentations, as noted in more detail above. In Indonesia, a series of talks given in Bandung (2019) were livestreamed to a public audience and led to a number of questions in a public discussion. The aim of the talks was to explain the aims and initial results of our research, but also to discuss the nature of lateral collapse hazards on volcanic islands, following the recent devastating impact of the Anak Krakatau collapse and tsunami. We have also reached public audiences directly in the UK through a series of outreach events (museum exhibitions, public and school group talks and informal discussions), and via media coverage of this project and broader NERC-funded reserach investigating Krakatau (e.g. media coverage on multiple news websites following the AGU conference). Further seminars were given by several members of the research team in a series of online seminars (2020 and 2021) hosted by ITB, Indonesia, and reaching a wide university audience. This has strengthened ongoing collaboration with ITB, and we will continue to contribute to seminars and work with postgraduate students at ITB, as well as developing longer-term collaborations. Results are being fully shared with Indonesian collaborators and we are continuing to work together on this research. Through this route, our findings will be disseminated to hazard management agencies in Indonesia, adding to the understanding of this event and the potential for lateral-collapse tsunami hazards at Anak Krakatau and other Indonesian volcanoes, and thereby informing future monitoring strategies.
Sector Environment,Leisure Activities, including Sports, Recreation and Tourism
Impact Types Policy & public services

Description Collaboration with Institut Teknologi Bandung 
Organisation Bandung Institute of Technology
Country Indonesia 
Sector Academic/University 
PI Contribution As part of the grant, we were invited to ITB to give presentations on ongoing work, and to provide expertise in methods not currently used, for examples application of numerical modelling for volcanic hazard processes. While in the field, we shared knowledge and field expertise. As an outcome of the project, I now co-supervise a student with Dr Mirzam Abdurrachman at ITB, Indonesia. Over the past two years, I have given multiple seminars to ITB regarding BGS research on volcanic activity.
Collaborator Contribution Through the urgency grant, we have begun working with colleagues from ITB who are experts in Indonesia volcanology and Krakatau volcanology. The partners joined us in the field, and contributed personnel to conduct drone surveys. Our colleagues have secured funding from their institution to visit the UK in summer 2020 and further this collaboration.
Impact The collaboration has resulted in presentation at numerous national and international conferences, and in a publication (Alessandro et al. 2020 - see publications). Multiple papers are currently in prep.
Start Year 2019
Description Collaboration with Resilience Development Initiative (RDI) 
Organisation Resilience Development Initiative
Country Indonesia 
Sector Academic/University 
PI Contribution We have conducted monthly meetings with RDI to incountry needs, and organised a stakeholder workshop involving participants from key institutions such as CVGHM, the local volcano observatory.
Collaborator Contribution Partners co-organised the meeting, aided identification of key participants, organised invitations, and led the meeting on the day
Impact Document describing outcomes from the workshop in progress
Start Year 2021
Description Joined the Editorial Board for the Bulletin of Geology, ITB 
Organisation Bandung Institute of Technology
Country Indonesia 
Sector Academic/University 
PI Contribution - co-supervised masters students based at ITB - presented as a guest lecturer at the ITB international science program
Collaborator Contribution - collaborators have contributed to science discussions and scientific papers, and have provided samples for analysis
Impact In addition to the papers listed for the year 2019-2020, we currently have two papers in review/ press on which we are co-authors
Start Year 2020
Description Guest lecture for ITB international science seminar 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Gave an invited presentation to the International science seminar organised by ITB. Presentation attended by many universities across Indonesia through zoom, and was also broadcast on YouTube. Presentation focused on recent eruptive activity of Anak Krakatau, with the following discussion focusing on the physical process of explosive eruptions.
Year(s) Of Engagement Activity 2020
Description Seminar on ash and aviation with a focus on volcanic activity in Indonesia 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Undergraduate students
Results and Impact Provided one hour presentation, with additional one hour question and answer session with regards to volcanic hazards and impacts of volcanic ash on aviation in Indonesia. Seminar presented via zoom with a live feed on youtube. Students and faculty from universities across Indonesia invited.
Year(s) Of Engagement Activity 2021
Description Workshop with Indonesian colleagues to identify areas for future work 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Policymakers/politicians
Results and Impact co-organised and hosted workshop with colleagues from RDI. Presentations on volcanic, landslide, earthquake and tsunami hazard, with discussion sessions for each topic to identify incountry needs, and potential for future collaborations
Year(s) Of Engagement Activity 2021