Geochemical changes in obsidian outcrops with elevation at Hatis volcano (Armenia) and corresponding Lower Palaeolithic artifacts from Nor Geghi 1

Most descriptions of obsidian-bearing rhyolitic lava flows and domes are largely based on relatively simple cases of tectonic plate subduction in North America, but Armenian geologists proposed since the 1960s that these models are less suitable for describing rhyolitic volcanism in their research area. Obsidian-producing volcanoes that lie in the Armenian Highlands, they argued, are more complex in form and stratification. Hatis volcano in central Armenia is one such example. As we document, Hatis is highly unusual, perhaps unique, in that its obsidian changes in composition with elevation. Prior studies of Hatis obsidian recognized the existence of two different chemical types. Here, though, we report a series of four obsidian chemical types and their spatial distributions across the slopes. Our findings were enabled by the use of portable XRF during our field surveys of Hatis. Additionally, we recognized each of these four chemical types of Hatis obsidian at the Lower Palaeolithic site of Nor Geghi 1, where thousands of obsidian artifacts reflect Pleistocene hominin behaviors from Marine Isotope Stage (MIS) 11 (~424–374 ka) to 9 (~337–300 ka). Thus, all four types of Hatis obsidian are archaeologically significant despite the fact that their outcrops span more than 500 m (from <1600 to greater than 2100 m asl) in elevation on the volcanic slopes, thereby enabling future studies on links between altitude and hominin toolstone acquisition behaviors over hundreds of millennia.