Temperature (T) is a key parameter controlling the rheology of lava flows. Since hazardous behavior of eruptions prevents direct measurements of hot magmatic bodies [1], the temperature is mostly retrieved by measuring the infrared (IR) radiance of the lava flow [2, 3]. The determination of T is however subjected to important errors related to the poor knowledge of one of the most critical parameters, namely spectral emissivity (ε). In this study, we explored the temperature–emissivity relationship for basaltic magmas, mostly from the 2014–2015 Holuhraun eruption. We performed in situ spectral emissivity measurements at relevant magmatic temperatures (from room temperature up to 1800 K) over a wide spectral range (400–8000 cm−1) covering TIR, MIR and SWIR regions, using a non-contact IR emissivity apparatus [4]. To unravel the complex radiative behavior of basalts with temperature evolution, structural, chemical and textural analyses (SEM, EMPA, Raman spectroscopy, DSC, XRD, and TEM) were systematically performed. Our results show that spectral emissivity varies accordingly with temperature, wavenumber, and is greatly affected by micro-scale crystallization, emphasizing the effect of small change in silicate structure on magma radiative properties. Because of the multiphase nature of lava, each constitutive phase (glass, melt, crystal, vesicles) contribute differently to the spectral emissivity. The evaluation and quantification of the impact of these phases on effective thermal radiative properties is a key point to improve the accuracy of lava T determination. These new data will ultimately improve our knowledge of the complex lava flow properties that are crucial in thermo-rheological models for hazard assessment [5]. References: [1] Kolzenburg et al. 2017. Bull. Volc. 79:45. [2] Harris, A. 2013: Cambridge University press. 728. [3] Rogic et al. 2019 Remote Sens., 11, 662 [4] De Sousa Meneses et al. 2015. Infrared Physics & Technology 69. [5] Thompson and Ramsey, 2021, Bulletin of Volcanology, 83:41. Keywords: Spectral emissivity, temperature, IR spectroscopy, rheology, basalt

Lava flows are one of the main hazards related to effusive basaltic volcanism. To minimize their impact during emplacement, we use lava flow potential distance-to-run predicted by propagation models. These models are partly based on infrared (IR) measurements of lava radiative heat fluxes by remote sensing (RS) methods (ground-based or satellite-based detectors) [1]. These results are however subjected to important errors related to the poor knowledge of spectral emissivity (ε), commonly considered constant by these well-established techniques[2, 3]. This oversimplification is an important source of uncertainties in derived temperatures, which restrain our capacity to accurately model active lava flows. In this study, we developed new algorithms that take into account the effect of spectral emissivity for calculating radiative heat fluxes. We describe the temperature-emissivity relationship with equations established at two wavelengths of interest for RS (10.9 μm and 1.6 μm) that are retrieved from in situ measurements of spectral emissivity for basaltic magma from the 2014–2015 Holuhraun eruption. Spectral emissivity data were systematically acquired over a wide spectral range (400–8000 cm−1) covering TIR, MIR and SWIR, and up to 1473 K [4]. Our results show that spectral emissivity varies linearly with temperature in TIR (10.9 μm), and nonlinearly in SWIR (1.6 μm). We confronted our lab-based results to the field IR data retrieved by [5] and found that temperature precision increases compared to data using constant emissivity value. These new insights will ultimately improve the thermo-rheological models of lava flows [6] in order to support hazard assessment in volcanic systems. References: [1] Kolzenburg et al. 2017. Bull. Volc. 79:45. [2] Harris, A. 2013: Cambridge University press. 728. [3] Rogic et al. 2019 Remote Sens., 11, 662 [4] De Sousa Meneses et al. 2015. Infrared Physics & Technology 69. [5] Aufaristama et al. 2018, Remote Sens, 10,151. [6] Thompson and Ramsey, 2021, Bulletin of Volcanology, 83:41. Keywords: Spectral emissivity, temperature, IR spectroscopy, remote sensing, basalt