Viscosity is one of the most important physical properties controlling lava flow dynamics. Usually, viscosity is measured in the laboratory where key parameters can be controlled but can never reproduce the natural environment and original state of the lava in terms of crystal and bubble contents, dissolved volatiles, and oxygen fugacity. The most promising approach for quantifying the rheology of molten lava in its natural state is therefore to carry out direct field measurements by inserting a viscometer into the lava while it is flowing. Such in-situ syn-eruptive viscosity measurements are notoriously difficult to perform due to the lack of appropriate instrumentation and the difficulty of working on or near an active lava flow. In the field, rotational viscometer measurements are of particular value as they have the potential to measure the properties of the flow interior rather than an integration of the viscosity of the viscoelastic crust + flow interior. To our knowledge only one field rotational viscometer is available, but logistical constraints have meant that it has not been used for 20 years. Here, we describe new viscosity measurements made using the refurbished version of this custom-built rotational viscometer, as performed on active pāhoehoe lobes from the 61G lava flow of Kilauea's Pu'u 'Ō'ō eruption in 2016. We successfully measured a viscosity of ~380 Pa s at strain-rates between 1.6 and 5 s-1 and at 1144 °C. Additionally, synchronous lava sampling allowed us to provide detailed textural and chemical characterization of quenched samples. Application of current physico-chemical models based on this characterization (16±4 vol.% crystals; 50±6 vol.% vesicles), gave viscosity estimates that were approximately compatible with the measured values, highlighting the sensitivity of model-based viscosity estimates on the effect of deformable bubbles. Our measurements also agree on the range of viscosities in comparison to previous field experiments on Hawaiian lavas. Conversely, direct comparison with sub-liquidus rheological laboratory measurements on natural lavas was unsuccessful because recreating field conditions (in particular volatile and bubble content) is so far inaccessible in the laboratory. Our work shows the value of field rotational viscometry fullyintegrated with sample characterization to quantify three-phase lava viscosity. Finally, this work suggests the need for the development of a more versatile instrument capable of recording precise measurements at low torque and low strain rate, and with synchronous temperature measurements.

To examine whether there was any physical or thermal interaction between trees and lava when a lava flow inundates a forest we studied the Kīlauea's July 1974 lava flow. We mapped the location of ∼600 lava-trees and the lava type (pāhoehoe versus 'a'ā), and sampled an additional ten lava-trees for chemical and textural analysis to infer flow viscosity and dynamics. The emplacement event lasted 3.5 hours and markers on the outer surface of the lava-trees allowed us to define initial high effusion rate and velocity (∼400 m 3 /s and 5-10 m/s) that then declined to 9 m 3 /s and 4 m/s during a waning phase. We find that lava passing through the forest underwent an initial cooling rate of 4 °C/km which increased to 10 °C/km late in the eruption. This is no different to cooling rates recorded at Kīlauea for tree-free cases. There thus appears to be no effect on cooling for this case. The lava-trees did, though, form a network of vertical cylinders obstacles and evidence for local diversion of flow lines are noticed. However, this varies with lava type, as almost no lava-trees form in ʻaʻā. We find a relation between the percentage of ʻaʻā and the number of lava-trees per hectare. The pāhoehoe-ʻaʻā transition for this flow occurs at a viscosity of 10 3 Pa s and this appears to be a threshold below which lava-trees can form so as to behave as a network of obstacles, and above which they cannot.