Textural and rheological evolution of basalt flowing down a lava channel

Bénédicte Robert (1) , Andrew J.L. J.L. Harris (1) , Lucia Gurioli (1) , Etienne Médard (1) , Alexander Sehlke (2) , Alan Whittington (3)

1 LMV - Laboratoire Magmas et Volcans
2 Mizzou - University of Missouri [Columbia]
3 IPGP - Institut de Physique du Globe de Paris

The Muliwai a Pele lava channel was emplaced during the final stage of Mauna Ulu’s 1969–1974 eruption (Kilauea, Hawaii). The event was fountain-fed and lasted for around 50 h, during which time a channelized flow system developed, in which a 6-km channel fed a zone of dispersed flow that extended a further 2.6 km. The channel was surrounded by initial rubble levees of ’a’a, capped by overflow units of limited extent. We sampled the uppermost overflow unit every 250 m down the entire channel length, collecting, and analyzing 27 air-quenched samples. Bulk chemistry, density and textural analyses were carried out on the sample interior, and glass chemistry and microlite crystallization analyses were completed on the quenched crust. Thermal and rheological parameters (cooling, crystallization rate, viscosity, and yield strength) were also calculated. Results show that all parameters experience a change around 4.5 km from the vent. At this point, there is a lava surface transition from pahoehoe to ’a’a. Lava density, microlite content, viscosity, and yield strength all increase down channel, but vesicle content and lava temperature decrease. Cooling rates were 6.7 °C/km, with crystallization rates increasing from 0.03 Фc/km proximally, to 0.14 Фc/km distally. Modeling of the channel was carried out using the FLOWGO thermo-rheological model and allowed fits for temperature, microlite content, and channel width when run using a three-phase viscosity model based on a temperature-dependent viscosity relation derived for this lava. The down flow velocity profile suggests an initial velocity of 27 m/s, declining to 1 m/s at the end of the channel. Down-channel, lava underwent cooling that induced crystallization, causing both the lava viscosity and yield strength to increase. Moreover, lava underwent degassing and a subsequent vesicularity decrease. This aided in increasing viscosity, with the subsequent increase in shearing promoting a transition to ’a’a.

Mots clés: Lava channel; Vesicles; Cooling; Crystallization; Rheology; Pahoehoe; ’a’a

Domaines: Volcanologie; Géophysique [physics.geo-ph]

Bénédicte Robert, Andrew J.L. J.L. Harris, Lucia Gurioli, Etienne Médard, Alexander Sehlke, et al.. Textural and rheological evolution of basalt flowing down a lava channel. Bulletin of Volcanology, 2014, 76, pp.824. ⟨10.1007/s00445-014-0824-8⟩. ⟨hal-01133620⟩