Lava flow behavior is predominantly controlled by radiative cooling determined by the ratio of flow rate to flow distance and the thermal diffusivity of the lava. The advection and cooling rates of lava flows (and lakes) and the strength of internal convection will influence flow morphology (Figure 1). For example, an open-channel basaltic flow will lose heat radiatively but over time and distance the surface cools forming a glassy crust that thickens. Typically, the ultimate flow length is limited by eruption rate/volume, cooling efficiency, tube formation, slope, etc. When cooling is limiting as in the case of Figure 1, the propagation length is proportional to extrusion rate and proximity to the vent. Many prior investigations have modeled lava flow propagation, but assumptions of parameters such as emissivity and vesicularity can led to inaccuracies in forecasting complex flows. By determining the fundamental changes in emissivity with the formation and break up of crust, the effect of these parameters on modeled effusion rates will be known and the models improved (as will the capability to forecast future changes). This work investigates how emissivity of lava flow surfaces evolves during propagation and cooling and, more critically, how this influences different thermal parameters of the lava flow including surface temperature and cooling rates. One method to explore this relationship is application of a thermorheological cooling model. The aforementioned relationship also makes it important for hazard management and predictive models to determine effusion rates (as well as model strain rate and apparent viscosity) and hence the potential for flow advance.
Figure 1. Open channel basaltic lava flow at Tolbachik volcano during the 2012-2013 eruption. The different forms of heat loss and gain used in thermorheological flow models are shown, including conduction (Qcnd), convection (Qcnv), radiative (Qrad), crystallization (Qxst) and viscous heating (Qvsc).
A newly developed miniature multispectral thermal infrared camera system has been tested in Hawaii to investigate the changes in emissivity of active lava surfaces at both the lava lake and propagating lava flows (Figure 2). In conjunction, high temperature hyperspectral TIR laboratory experiments are being conducted to investigate these changes in a controlled environment. This work will help define whether a material’s emissivity truly does change with its state and if so, by what magnitude. More critically, is how this change effects the cooling rate and the models used to predict flow length over time. Ultimately, the research will constrain the thermal history, and spatiotemporal and thermorheological evolution of lava surfaces.
Figure 2. Emissivity spectra derived from the first generation of field-based IR cameras. The measurements acquired over Kilauea’s lava lake show the molten surfaces have an 8-10% lower emissivity than the cooler, crust-covered surfaces.
Along with improving the inputs into these models, we are also using them in a reverse sense to examine existing flows in the southwestern Arsia Mons lava flow field on Mars (Figure 3). These are some of the youngest and roughest lava flows seen on the planet. The various flow lengths and textures of flow field provide a wide parameter space from which to test the modeling of effusion rates/volumes. Modeling is done using an assumption of lava with the rheological properties of the 2010 channelized flow at Piton de la Fournaise volcano with modifications to the cooling rate based on planetary specific properties such as gravity, atmospheric temperature and pressure, etc. Iterative runs of the model with slight variation in certain input values such as phenocryst content and crust temperature (and knowing the final length) results in predictions of lava flux at the time of the eruption. Discrepancies in modeled flow length and true flow length give an indication which inputs have the most impact and further constrain the errors.
Figure 3. CTX composite image of the study site in the southwestern Arsia Mons lava flow field. Color coded outlines delineate the flows modeled for this study.
Further laboratory experiments will be conducted on samples collected from the natural lava flows and lake spatter in Hawaii and simulated flows from the Syracuse lava pour facility. The lab data will be compared to the lower spectral resolution data acquired with the field instrument (and the lower spatial resolution airborne- and satellite-based data), as that sample was molten/cooling (Figure 4). Discrepancies are being investigated to determine any impact on the accuracy of the derived radiative temperature, emissivity, and other variations in surface characteristics. In addition, improvements in the methods for extracting the thermal parameters and comparing them with those at lower resolutions will be performed. Further, the sensitivity of established lava flow propagation models will be investigated to determine the magnitude these thermal measurements will make to the final model output. These will be iterated back into the model study of the planetary flows as well.
Model refinement will be conducted to identify and track the changes in the thermally derived parameters of the propagating flows. The multispectral TIR camera will undergo further calibration and be deployed again to Hawaii to investigate specific stages of flow propagation from the initial effusion, to crustal formation, and re-inflation (breakout events). The thermo-rheological history of the flows will be identified and the flux compared to the refined model. Ultimately, the modeling will improve hazard assessment of active and potentially active lava flows and lakes, thus improving the capability to improve the forecasts of lava volumes and distribution. As the model refinement is validated on Earth, these changes will also be applied to the Mars data analysis in order to determine the most accurate estimate of the eruption rate and duration of the SW Arsia flow field.
Figure 4. Image of multispectral TIR data acquisition of the lava flow entering the sea at Kamokuna (Hawaii), on 22 January 22 2017. The instrumentation in the foreground (arrow) is the new multispectral FLIR camera system with the computer/power housed in the weatherproof cases.