How shear helps lava to flow

Abstract

Understanding the thermo-rheological regime and physical character of lava while it is flowing is crucial if we are to adequately model lava flow emplacement dynamics. We present measurements from simultaneous sampling and thermal imaging across the full width of an active channel at Piton de la Fournaise (La Réunion, France). Our data set involves measurements of flow dynamics at three sites down-channel from the vent. Quantification of flow velocities, cooling rates, sample texture, and rheology allows all thermo-rheological parameters to be linked, and down- as well as cross-channel variations to be examined. Within 150 m from the vent, we recorded an unexpected velocity increase (from 0.07 to 0.1 m/s), in spite of cooling rates of 0.19–0.29 °C/m and constant slope. This change requires a switch from a Newtonian-dominated regime to a Bingham plug–dominated regime. Sample analysis revealed that the plug consists of foam-like lava, and the shear zones involve vesicle-poor (low-viscosity) lava. With distance from the vent, shear zones develop, carrying the vesicular plug between them. This causes flow to initially accelerate, helped by bubble shearing in narrow lateral shear zones, until cooling takes over as the main driver for viscosity increase and, hence, velocity decrease.

Magma fragmentation and particle size distributions in low intensity mafic explosions: the July/August 2015 Piton de la Fournaise eruption

“…A total of 124 images of explosions, 307 images of the lava flow from Cone A and two lava flow videos were taken with this camera. Measurements of lava temperature, lava flow velocity and lava channel dimensions were obtained from the FLIR camera images 67 . Temperatures were corrected for emissivity and atmospheric effects using the FLIR systems on-board correction facility setting an emissivity for basalt of 0.98 68 .…”

Basaltic dyke eruptions at Piton de La Fournaise: characterization of the eruptive products with implications for reservoir conditions, conduit processes and eruptive dynamics

“…However, two lava fragments sampled within the channel, around 100 m from the emission point record extreme density values (82 vol% for the vesicle-rich lava sample and 27 vol% for the dense one). The dense fragment was collected from the channel margin, while the vesicle-rich one was collected within the channel plug, therefore not representative any more of the vent conditions (Harris et al 2019).…”

“…The lavas sampled down the flow (not shown inFig. 5), namely the dense one from the lava flow edges (shear-zone) and the vesicle-rich one from the lava flow center (plug), have extreme values of porosities and the dense lava contains a larger amount of microlites than the vesicle-rich lava(Harris et al 2019).…”

Crystal Size Distribution (CSD) Analysis of Volcanic Samples: Advances and Challenges

“…The past few decades have also seen a dramatic increase in both analysis of groundmass textures and experimental studies of crystallization relevant to syn-eruptive volcanic processes. Measurements of groundmass crystal populations can be linked to volcanic processes where sample times are well constrained (e.g., Cashman et al, 1999;Hammer et al, 1999Hammer et al, , 2000Cashman and McConnell, 2005;Wright et al, 2012;Preece et al, 2016;Harris et al, 2020) but textural interpretations must rely on experimental constraints when temporal data are not available. Experimental studies, in turn, underline the sensitivity of groundmass crystal populations to initial conditions (superor sub-liquidus) and melt composition as well as cooling and decompression paths (e.g., Brugger and Hammer, 2010a,b;Martel, 2012;Riker et al, 2015a;Befus and Andrews, 2018).…”