Uncapping subglacial eruption dynamics and glacier response

Volcanoes at high latitudes or at high elevations at temperate latitudes are commonly capped by glaciers. Understanding the dynamics of volcano-ice interaction is important because meltwater production can influence eruption style and lead to sudden outburst floods or explosive lava-water interactions. In addition, glaciovolcanic deposits can be useful terrestrial indicators of past environments. Glaciated volcanic systems are complex because meltwater availability and eruption style are dependent on a combination of magmatic, glacial and edifice characteristics. The relative influence of these characteristics are open fields of research.

SURGE aims to improve our understanding of glaciovolcanic interactions by studying ice-confined deposits at Katla volcano, Iceland. Katla is a highly active and hazardous subglacial volcano from which all historic eruptions have occurred beneath an ice cap and have resulted in outburst floods. While the Holocene eruptive record at Katla is well-known, the older parts of the edifice have received little attention. A second aim of SURGE, therefore, is to improve our understanding of the pre-Holocene eruption history and edifice building at Katla. New geological mapping will be carried out to determine the eruption and emplacement processes of pre-Holocene formations. We will then use thermal modelling to determine the volume of ice that was melted during these eruptions in order to reconstruct the ancient glacier surface and extent. Finally, radiometric dating of the formations will be used to build an age-constrained record of glacier dimensions and eruptive activity at Katla. Our multidisciplinary approach will contribute to understanding of processes that occur beneath an ice cap or are too hazardous to observe. In addition, our estimates of ice volumes melted during these past eruptions will inform future hazard planning. The global prevalence of ice-capped volcanoes means that this study will have widespread relevance.

Taking a closer look at ice-capped volcanoes

Katla is a giant subglacial volcano in southern Iceland. Hidden beneath the ice cap of Mýrdalsjökull glacier, it is highly active and hazardous. All eruptions have occurred beneath the ice cap and resulted in outburst floods. The EU-funded SURGE project will study the ice-confined deposits at Katla in order to gain a better understanding of glaciovolcanic interactions. This is important because meltwater production can influence eruption style and lead to sudden outburst floods or explosive lava-water interactions. Glaciovolcanic deposits can also be useful terrestrial indicators of past environments. The project will take a multidisciplinary approach to investigate what processes occur beneath an ice cap. The findings will be useful for future hazard planning.

The Katla volcanic system is one of the most productive in Iceland. Frequent basaltic and occasional silicic phreatomagmatic eruptions through the ice cap Mýrdalsjökull have provided a rich Holocene tephra record. Understanding of pre-Holocene eruptions and the thickness and extent of ice cover during glacial periods is much more limited.

We present eruption and emplacement models for three formations exposed on the flanks of the Katla volcano. Two are rhyolitic nunataks and one is an alkali basaltic sequence. These formations rise above the surrounding ice and topography, respectively, and show evidence for ice-confined emplacement, indicating their formation at a time when ice cover was thicker and more extensive.

Our models of each formation are based on field study, a photogrammetry survey, and major element geochemical analyses. The basaltic formation of Morinsheiði is an intercalated sequence of volcaniclastic rocks, pillow lavas and pillow breccias, entablature-jointed and lobate lavas, and more massive pahoehoe lava sheets, intruded by several dykes. The top of the sequence is a glacially eroded surface and it is bounded on all sides by deep valleys. The Enta nunatak is a kinked ridge or possibly two en-echelon ridges. A silicic volcaniclastic unit is intercalated with and intruded by fluidal and heavily jointed rhyolite lobes, spines and sheets. This formation is capped by a segment of crater wall composed of scoria. The Kötlujökull nunatak is tabular in shape, has a clastic base and is capped by jointed lava with lobate margins and breakout lobes descending the steep slopes.

Each formation exhibits evidence of multiple eruption styles in varying hydrological conditions, and at least for Morinsheiði a fluctuating water level. These are the preliminary results from the project “SURGE: Uncapping subglacial eruption dynamics and glacier response”, which aims to better understand the relative influences of magma chemistry, eruption style and glacial conditions on meltwater production and retention, glacial response, and the feedback effects for continued eruptions. These models, combined with new 40Ar-39Ar dating of the lavas, will also provide greater insight into the form of Katla and the glacial conditions that prevailed during the late Pleistocene.