The Thórólfsfell tuya, South Iceland – A new type of basaltic glaciovolcano

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: Hodgetts, A.G.E., McGarvie, D., Tuffen, H., Simmons, I.C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2021
• Subjects: Tindfjallajökull; Þórólfsfell; Iceland
• Online Access: https://eprints.lancs.ac.uk/id/eprint/154149/; https://eprints.lancs.ac.uk/id/eprint/154149/1/Hodgetts_et_al_2021_accepted_manuscript_compressed.pdf; https://doi.org/10.1016/j.jvolgeores.2021.107175

Basaltic tuyas are glaciovolcanoes that form when substantial focused eruptions take place beneath thick ice. None have been witnessed, so models reconstructing tuya formation are grounded in detailed fieldwork. A key feature of many basaltic tuyas is the presence of volcanic and volcaniclastic rocks that indicate the sustained presence of an encircling meltwater lake during the eruption. Here we provide the first description of Þórólfsfell (Thórólfsfell), a basaltic tuya from Iceland, which is sufficiently distinct from previously described tuyas to be considered a new type of basaltic glaciovolcano. Thórólfsfell is an asymmetric tuya with an area of c.8 km2, base-to-top height of c.450 m, and volume of c.2.2 ± 0.4 km3 that has been emplaced onto the approximately 12o sloping lower southern flanks of Tindfjallajökull central volcano. Thórólfsfell shares only two major morphological characteristics with other basaltic tuyas: (1) a sub-horizontal top comprising subaerial lavas; (2) a clear vertical topographic expression, which reflects preferential upwards edifice growth due to lateral confinement by encircling ice and/or meltwater. There is no evidence for the presence of a large and long-lived syn-eruptive meltwater lake. The Thórólfsfell eruption is effusion-dominated, and there is a gradual reduction in cooling fractures in lavas with elevation. The eruption is divided into three Stages. Stage I forms a c.110 m thick drape onto an irregular but persistently c.12o dipping basement of older basaltic tuffs; Stage I consists of palaeoslope-parallel lava lobes with abundant cooling fractures, accompanied by abundant breccias. Stage II comprises a c.240 m thick stack of c.12o dipping stacked lava lobes with abundant cooling fractures, and occasional autobreccias. Stage III is c.110 m thick, and whilst early lavas have cooling fractures, the final Stage III lavas are sub-horizontal, subaerial pahoehoe lava flows. Our model for the formation of Thórólfsfell has two key features. The first is that the inclined ...