Laxton, Kate A; (2022) Natrocarbonatite volcanism at Ol Doinyo Lengai volcano, Tanzania: A multidisciplinary investigation of effusive activity following the 2007-2008 explosive eruption. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Ol Doinyo Lengai is one of the most active and frequently visited volcanoes in Africa. Its geochemically bimodal activity is extraordinary, characterised by low-temperature, low-viscosity natrocarbonatite lava effusions, interspersed with explosive silicate eruptions. As yet, no centralised monitoring agency has been established. Advances in our understanding of the processes sustaining this unique style of volcanism are limited by the remote location and challenging terrain of Ol Doinyo Lengai, resulting in sparsely populated datasets from which few fundamental conclusions can be drawn. My aim in writing this thesis is to support local institutions in their efforts to find cost-effective and sustainable monitoring solutions for Ol Doinyo Lengai. Using a combination of conventional and novel sampling techniques, I present the first morphological, gas, petrological andthermal data obtainedfrom the active crater since the last explosive eruption in 2007-2008. By analysing this multidisciplinary dataset, I show that i) between2008-2019, after effusively erupting ~900,000 m3 of lava, the crater floor has risen by ~60 m, ii) lava samples collected in July 2019 are gregoryite-rich natrocarbonatite of pre2007 ‘type’ composition, suggesting the mechanisms by which natrocarbonatite is generated have remained stable since 1960 despite intermittent explosive activity, iii) gas emissions from typically anhydrous natrocarbonatite lava can be dominated by water vapour, and iv) mean lava pond surface temperatures (~550 °C) measured using a low-cost Raspberry Pi-based thermal camera are consistent with those reported since 1988. To conclude, I discuss these data in the context of conceptual models presented in the literature and summarise my understanding of the processes driving Ol Doinyo Lengai’s most recent activity. Continuations principally of gas, thermal and morphological datasets through sustainable monitoring practices would provide a means for interpretingpre-explosive eruptive signals, contributing to reducing the risk to life and livelihoodsfor local communities and visitors.

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