Grímsvötn ash layers, a 3D modeling GPR investigation of the englacial debris within the Vatnajökull ice cap

Abstract

In this report, three-dimensional (3D) modeling is used to facilitate Ground Penetrating Radar (GPR) imagery of the Vatnajökull ice cap, finding and mapping the englacial layers of tephra recorded in the stratigraphy above Grímsvötn, Iceland's most active subglacial volcano. GPR measurements together with 3D modeling, and 2D fracture mapping has proven to be an excellent combination for these types of investigations, showing significant results in visualizing how the volcanic layers and fractures are distributed throughout the recrystallized snow and ice held within the caldera. Through the course of the project, a more complete modeling methodology has been developed, based on earlier work of 3D modeling and research data provided from Iceland, enabling a detailed visual investigation of the dynamics between Grímsvötn and Vatnajökull. GPR measurements show that the ash remnants of the 2004 and 2011 eruptions can be observed in most of the stratigraphy at the site of Grímsvötn, and hyperbola mapping reveal a connection between the surface topography and subsurface fractures. From modeling, an estimation of sequence thicknesses and snow/ice accumulation has been made, and general inspection reveals lower ice and snow thickness between the years 2004-2011 compared to years 2011-2019 due to the effects of compaction. On closer inspection, the two ash layers each have a ‘unique topography’ and are found buried at alternating depths throughout the caldera glacier. General mass accumulation appears greatest at the caldera rims and lower at the caldera center, but modeling reveals that there is a differing pattern of volume accumulation and/or compaction seen at the south caldera wall, where the 2011 ash layer is buried beneath 17 to 20 meters of combined ice and recrystallized snow, while the 2004 layer has a slim burial of 5-6 meters from the layer above. This difference is explained by compression gradients caused by Vatnajökull’s southward flowing movements into the caldera, as well as added snow and ice cover provided at the southern caldera wall (where avalanches are more prone and midday sun exposure is low). Thermal heat flux at the caldera edges post eruption may also have quickened the ice melt above the 2004 ash layer to the south, an idea supported by the discovery of fractures and associated surface depressions along the south wall, likely formed as a response to hydrothermal activity from the volcano below. Based on the results provided in this report it is concluded that the differences in ice and snow thickness over the sections within Vatnajökull are largely a result of the dynamic interaction between the Vatnajökull ice cap and the active volcano below it. Furthermore, interaction between the glacier and the caldera meltwater is believed generate the same or more melting than the climatic conditions on the surface.