Nested crater morphology, ring-structures and temperature anomalies detected by close-range photogrammetry and thermal remote sensing at Láscar volcano, Chile

Volcanic craters often develop in clusters and enclose smaller, subsidiary vents and ring structures. Details on the ongoing geomorphology and structural evolution, however, are commonly lacking for active volcanic craters due to difficult and hazardous access. Therefore, remote sensing based investigation at active volcanoes is providing unique data allowing entrance to inaccessible summit craters. Here we describe novel drone and satellite data collected at Láscar, the most active volcano in the central Andes. Láscar hosts five partially nested craters, the deepest crater of the eastern three persist active and was the site of numerous violent explosions in the past decades. Using a Pleiades tri-stereo satellite dataset, we constructed a 1-m resolution digital terrain model (DTM) and orthomap that we used to identify subtle structures and morphologies of the eastern three nested craters. However, due to the shadow effect caused by the deep concave shape of the active crater, its geometry remains unclear. We complement this analysis by unoccupied aerial vehicle (UAV) surveys in 2017 and 2020 by employing both an optical and a thermal imaging camera. We systematically mapped the entire crater field and could also fly into the deep active crater to acquire close range images. We applied the Structure-from-Motion (SfM) method that enables us to create centimeter-scale DTMs, optical and thermal orthomosaics. Using this data-set we create an inventory of fumaroles and thermal anomalies. By calculating the difference of the 2017 and 2020 data, we quantify the spatial and volumetric changes that occurred during the observation period. We find changes mostly concentrated at the crater floor, material accumulation, thermal anomalies changing, as well as localized rock falls into the crater. We note that highest temperature anomalies are restricted by the central circular structure at the crater floor, consistent with the location of a thermal anomaly episode that peaked in late 2018, possibly representing the surface expression of the underlying conduit. Thus, by linking the satellite and drone data we derive important morphological, thermal and structural information and discuss the crater morphology and characteristics of episodic unrest phases at Láscar.