Measuring the viscosity of lava in the field: A review

Abstract : Many scientists who have worked on active lava flows or attempted to model lava flows have recognized the importance of rheology in understanding flow dynamics. Numerous attempts have been made to estimate viscosity using flow velocities in active lava channels. However, this only gives a bulk or mean value, applies to channelized flow, and the need to estimate flow depth leads to a large degree of uncertainty. It is for this reason that some scientists resorted to more direct methods for measuring the lava viscosity in the field. Initial attempts used crude instruments (such as forcing a rod into a flow using the operators body-weight), and even the latest instruments (motor-driven rotational viscometer) are significantly less refined than those one would encounter in a well-equipped laboratory. However, if suitable precautions are taken during instrument design, deployment in the field and post-processing of data, the results form an extremely valuable set of measurements that can be used to model and understand the complex rheological behavior of active lava flows. As far as we are aware, eleven field measurements of lava rheology have been published; the first took place in 1948, and the latest (at the time of writing) in 2016. Two types of instrument have been used: penetrometers and rotational viscometers. Penetrometers are suitable for relatively high viscosity (10^4-10^6 Pa s) lava flows, but care has to be taken to ensure that the sensor is at lava temperature and measurements are not affected by the resistance of outer cooled crust. Rotational viscometers are the most promising instruments at lower viscosities (1-10^4 Pa s) because they can operate over a wider range of strain rates permitting detailed flow curves to be calculated. Field conditions are challenging and measurements are not always possible as direct approach to and contact with active lava is necessary. However it is currently the only way to capture the rheology of lava in its natural state. Such data are fundamental if we are to adequately model and understand the complex behavior of active lava flows.
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Magdalena Oryaëlle Chevrel, Harry Pinkerton, Andrew J.L. Harris. Measuring the viscosity of lava in the field: A review. Earth-Science Reviews, Elsevier, 2019, ⟨10.1016/j.earscirev.2019.04.024⟩. ⟨hal-02150640⟩

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