Geodiversity Elements of a Young Fissure System as an Immediate Precursory Event of the Youngest Fissure-Fed Eruption within the Arxan UNESCO Global Geopark, Inner Mongolia, NE China

Abstract

The Arxan-Chaihe Volcanic Field (ACVF) is a Pliocene to Recent intracontinental monogenetic volcanic field. Within the ACVF are preserved at least 47 vents in a ~2000 km2 area, forming two major NE-SW trending structural elements. The youngest eruptions took place about 2000 B.P., forming two distinct complex scoriaceous and lava spatter cone systems emitting low viscosity lava that invaded the paleo-Halaha River tributary, forming pahoehoe flow fields. This lava field forms the backbone of the geoheritage values of the Arxan UNESCO Global Geopark. The lava flow fields were believed to be almost exclusively sourced from a single vent complex around the Yanshan -Gaoshan region. However, a recent study has revealed that the flow field is a result of complex eruptions with an early phase from the nearby Dahei Gou vent complex. Here we provide evidence, based on Sentinel satellite imagery, ALOS-PALSAR-derived digital terrain model analysis, and direct field observations, that an even earlier fissure-fed eruption created another complex. This can be seen as a smaller lava flow field on the western side of the main flow field. The Dichi Lake is an iconic geosite of the geopark. It is a maar crater formed by a single explosion through an earlier lava field erupted from a network of fissures ~2.5 km long following a NE-SW trend. The Dichi Lake geosite provides an ideal example demonstrating the effect of fissures opening in water-saturated lowlands resulting in phreatomagmatic eruptions. Moreover, our findings suggest that the youngest eruption in this region had at least three phases, probably not more than a few decades apart, along a 15 km-long fissure network propagated from the S.W. to N.E. We propose Dichi Lake as the centre of a geoheritage precinct, providing a hub of knowledge dissemination highlighting fissure eruptions as a key type of volcanic hazard to be taken seriously in the management of the geopark.