2021 | Welding of porous pyroclasts

Aims: Explosive volcanic eruptions eject fragments of magma and country rock into the atmosphere. Depending on entrainment conditions, part of the pyroclasts is deposited as fall out units. In somewhat proximal settings, a combination of magma temperature at fragmentation and mass eruption rate allow for welding taking place between neighbouring clasts, allowing these deposits to remobilize on inclined surfaces and eventually generate partially to completely welded deposits. These processes can create additional hazards such as rheomorphic lavas flows (as recently observed during the ongoing eruptive cycle of Etna volcano with recurrent lava fountains) and secondary pyroclastic density currents (as observed on Stromboli volcano during the 3 July 2019 paroxysm). In this lab-based project, we aim to constrain the timescales required for lapilli welding as a function of 1) bulk rock chemistry (silicic pumice and basaltic scoria), 2) experimental temperature, 3) experiment duration above the glass transition temperature, and 4) loading.

Cooperation: This is a collaboration between Munich, Heidelberg and Mainz with close collaboration with Daniele Andronico from INGV-OE in Catania, Italy. We additionally plan to collaborate with Adrian Hornby (Cornell, USA) without expenses. We aim to use three different starting materials, sourced from recent eruptions: 1) 2011 Cordon Caulle (Chile), 2021 Etna (Italy) and 2015 Calbuco (Chile), representing rhyolite, basalt and andesite, respectively. The unwelded deposits of Cordon Caulle and Calbuco have been sampled by Castro and Kueppers, respectively. We include those samples in this study to investigate bulk rock composition variability. The Etna starting material (both loose scoriaceous lapilli as well as pieces of partially welded deposit will be collected by colleagues in the frame of TeMaS-funded field work during July 2021 (project “Halogens as tracers of magma evolution from the mantle source to the Atmosphere – First case studies at Mt Etna” with Nicole Bobrowski as PI). In Munich and Mainz, the natural samples will be characterized petrophysically (crystallinity, porosity etc.), morphologically (shape and size) and chemically. In Mainz, sintering experiments will be performed by both PIs and graduate students from both universities using up to 1kg of natural lapilli. Clasts will be pre-heated in a large furnace and then added in a welding brick to guarantee grain-supported filling and thermal insulation. Upon cooling, the samples will be cut and polished and investigated for textural changes using a variety of optical and electron beam techniques in Munich and Mainz.

Work plan: We are planning to involve 2 MSc students, one from LMU and one from JGU in this study. Female students will be encouraged to apply. Months 1-3: Sample prep and characterisation (LMU) Month 4: First experimental campaign - Welding experiments I (JGU, LMU, IUP) Months 5&6: Textural investigation of the first experimental campaign (LMU, JGU) Month 7: Second experimental campaign - Welding experiments II (JGU, LMU, IUP) Months 8&9: Textural investigation of the second experimental campaign (LMU, JGU) Months 10-12: Results discussion and preparation of manuscript (JGU, LMU, IUP)

Expected outcome: The analysis of primary volcanic deposits contains abundant information regarding eruption and transport processes, manifested in clast textures, deposit sorting and dispersal area. Additionally, welding of primary deposits is proof that the bulk sediment temperature (a result of single clast temperature and sedimentation rate) was above the glass transition temperature. The experiments planned in this collaborative project aim at constraining the influence of four independent boundary conditions on the welding efficiency. This empirical knowledge will help to decipher syn-eruptive conditions, in particular a better constraining of mass eruption rate. This parameter is of key importance for eruption plume behaviour and accordingly ash dispersal modelling.