| dc.description.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. | sv |
