Downward-propagating eruption following vent unloading implies no direct magmatic trigger for the 2018 lateral collapse of Anak Krakatau

Abstract

The lateral collapse of Anak Krakatau volcano, Indonesia, in December 2018 highlighted the potentially devastating impacts of volcanic edifice instability. Nonetheless, the trigger for the Anak Krakatau collapse remains obscure. The volcano had been erupting for the previous six months, and although failure was followed by intense explosive activity, it is the period immediately prior to collapse that is potentially key in providing identifiable, pre-collapse warning signals. Here, we integrate physical, microtextural and geochemical characterisation of tephra deposits spanning the collapse period. We demonstrate that the first post-collapse eruptive phase (erupting juvenile clasts with a low microlite areal number density and relatively large microlites, reflecting a crystal-growth dominated regime) is best explained by instantaneous unloading of a relatively stagnant upper conduit. This was followed by the second post-collapse phase, on a timescale of hours, which tapped successively hotter and deeper magma batches, reflected in increasing plagioclase anorthite content and more mafic glass compositions, alongside higher calculated ascent velocities and decompression rates. The characteristics of the post-collapse products imply downward propagating destabilisation of the magma storage system as a response to collapse, rather than precollapse magma ascent triggering failure. Importantly, this suggests that the collapse was a consequence of longer-term processes linked to edifice growth and instability, and that no indicative changes in the magmatic system could have signalled the potential for incipient failure. Therefore, monitoring efforts may need to focus on integrating short-and long-term edifice growth and deformation patterns to identify increased susceptibility to lateral collapse. The post-collapse eruptive pattern also suggests a magma pressurisation regime that is highly sensitive to surface-driven perturbations, which led to elevated magma fluxes after the collapse and rapid edifice regrowth. Not only does rapid regrowth potentially obscure evidence of past collapses, but it also emphasises the finely balanced relationship between edifice loading and crustal magma storage.

Magma residence time, ascent rate and eruptive style of the November ash-laden activity during the 2021 Tajogaite eruption (La Palma, Spain)

“…Plagioclase compositions, indeed, are particularly sensitive to changes in temperature, H 2 O pressure (pH 2 O), and H 2 O content [Cutler et al 2022]. As shown in Figure 6, the first clear compositional difference through time is in the higher anorthite content (An 61-68 ) of the November plagioclase compared with that erupted in September (An 53-66 ); the more calcic plagioclases suggest relatively higher pH 2 O and temperatures [Couch 2003;Cutler et al 2022], as well as increasing melt maficity as observed also by Ubide et al [2023]. Other differences observed are lower K 2 O and Na 2 O and higher Al 2 O 3 contents in plagioclases from November compared with September.…”

A Global Database of Giant Landslides on Volcanic Islands

“…In the intervening period, important new research on giant landslides has been published from many parts of the Atlantic Ocean including Cape Verde (Martínez-Moreno et al 2018;Barrett et al 2020;Cornu et al 2021), the Azores (Hildenbrand et al 2018;Marques et al 2020Marques et al , 2021, the Canary Islands (Coello-Bravo et al 2020), and the Lesser Antilles (Solaro et al 2020) as well as the Bismarck Archipelago in the Pacific Ocean (Watt et al 2019). Moreover, the collapse of Anak Krakatau on 22 December 2018 stimulated much research (Williams et al 2019;Grilli et al 2019Grilli et al , 2021Hunt et al 2021;Cutler et al 2022) and it is anticipated that the collapse of Hunga Tonga-Hunga Haʻapai on 15 January 2022 will provide the impetus for many new studies. Consequently, this feels like an auspicious time to update the global giant landslides on volcanic islands database.…”

Interactions of magmatic intrusions with the multiyear flank instability at Anak Krakatau volcano, Indonesia: Insights from InSAR and analogue modeling

“…We also actively induce the destabilisation as we pull on the paper representing the sliding plane, which is different to the natural process. At Anak Krakatau, gravitational instability mixed with the structural architecture of the cone and caldera causing the gradual sliding are considered to be the most likely explanation (Hunt et al, 2021;Cutler et al, 2022). This also prevents any quantitative assessments inferring the stresses, pressures, or strains acting on the faults using the scaled models as the underlying processes are not the same, however, the resulting structural architecture is likely comparable.…”