A NEW PORTABLE PENETROMETER FOR MEASURING THE VISCOSITY OF ACTIVE LAVA

Viscosity is a fundamental physical property of lava that dictates the style and rate of effusive transport. The viscosity of lava has been predominantly measured in the laboratory on re-melted rocks. While laboratory viscosity measurements of lava are well-constrained with temperature, shear rate, and oxygen fugacity, the measurements are restricted to one or two phases, either pure liquid (melt) or liquid + solids (melt + crystals). However, in nature, lava always erupts with gas (i.e., volatiles) contained within a three-phase lava mixture. To date, the only method capable of measuring the viscosity of three-phase, low silica (e.g., basaltic) lava is to do so in situ during an eruption. Here we present a new self-contained prototype capable of obtaining a material’s viscosity based on Stokes’ Law of viscous drag. This penetrometer device uses a stainless steel rod with a semi-spherical tip fixed to a load cell and equipped with a thermocouple mounted at the measurement location. As the rod tip is pushed into a material, the load cell records compressional force over time. Additionally, a smaller free-moving rod that rides on the target surface is pushed backward along the penetration rod, and its displacement (i.e., penetration depth) is recorded with a laser distance sensor. The prototype is approximately 1.5 meters long and powered entirely by a single-board computer (i.e., Raspberry Pi). The penetrometer was tested and calibrated in the lab using analog materials such as Silly Putty^TM and Tryptone with viscosities ranging from 10⁴-10⁵ Pa s. Lastly, we present preliminary results from field measurements conducted during the recent 2023 Litli-Hrútur eruption in Iceland. Here, the penetrometer recorded the in situ rheology of ‘A‘ā lavas (~10⁵ Pa s) and pahoehoe lavas (~10⁴ Pa s) showcasing it as an efficient method of measuring natural lava viscosity.