The bedrock and tephra layer topography within the glacier filled Katla caldera, Iceland, deduced from dense RES-survey

Eyjólfur Magnússon; Finnur Pálsson; Alexander Jarosch; Tayo van Boeckel; Hrafnhildur Hannesdóttir; Joaquín M. C. Belart

doi:10.33799/jokull2021.71.039

The bedrock and tephra layer topography within the glacier filled Katla caldera, Iceland, deduced from dense RES-survey

Eyjólfur Magnússon
, Finnur Pálsson
, Alexander Jarosch
, Tayo van Boeckel
, Hrafnhildur Hannesdóttir
, Joaquín M. C. Belart

Science Institute

University of Iceland

Research output: Contribution to journal › Article › peer-review

Abstract

— We present results from recent low frequency radio echo-sounding (RES) campaigns over the ice-covered caldera of Katla central volcano, beneath the Mýrdalsjökull ice cap, southern Iceland. The current RES-survey both partly repeats and enhances the RES-profile grid of a previous survey in 1991 with denser sounding lines and improved instruments. The RES-data, obtained in 2012–2021, include ∼760 km of 2D migrated RES-profiles covering an area of 116 km² . Around 14 km² subsections of this area were surveyed with RES-profiles 20 m apart allowing 3D migration of the RES-data. Our study confirms findings from previously published bedrock mapping, including main topographic features, ice volume stored within the 100 km² caldera (45±2 km³, in autumn 2019) and maximum ice thickness (740±40 m). However, the significantly expanded level of detail and features observed in the new bedrock map reveals further evidence of a complex and eventful formation history of the caldera interior. This bedrock map is unprecedented in terms of detail for an ice-covered volcano. The new RES-data allows for a unique comparison of bedrock maps obtained from RES-data with 2D and 3D migration, demonstrating the limitations of 2D migrated RES-data in areas of high topographic variability. Reflections from the 1918 Katla eruption tephra layer within the ice were detected in a much wider area within the caldera than in the 1991 RES-data. We also observe a second internal layer at 420–580 m depth within the northern part of the caldera, identified here as the tephra from the 1755 Katla eruption. The 1918 tephra layer is typically observed at 200–300 m below the glacier surface. However, the layer depth varies from ∼100 m depth at the western rim of the caldera down to 460 m depth, where geothermal activity beneath ice cauldrons melts ice from below. At the most prominent geothermal areas all ice beneath the 1918 tephra has been melted leaving the tephra at the bed. Furthermore, the obtained tephra layer maps reveal footprints of some previously unidentified geothermal areas.

Original language	Undefined/Unknown
Pages (from-to)	39-70
Number of pages	32
Journal	Jokull
Volume	2021
Issue number	71
DOIs	https://doi.org/10.33799/jokull2021.71.039
Publication status	Published - 8 Dec 2021

Bibliographical note

Funding Information: This work was funded by the Icelandic Research fund of Rannís within the project Katla Kalda (project nr. 163391) with further support from The Icelandic Road Administration’s Research Fund (project nr. 1800-403). Pléiades images used to produce surface DEMs were acquired at a subsidised cost thanks to the CNES ISIS program. The images used to produce surface DEM in 2019 were further supported through CEOS (Committee on Earth Observation Satellites) contribution to the Iceland Volcanoes Supersite. We thank Hugh Tuffen and an anonymous reviewer for very constructive and detailed reviews, which substantially improved this paper. Magnús T. Guðmundsson, Þórdís Högnadóttir, Guðfinna Aðal-geirsdóttir, Helgi Björnsson, Páll Einarsson, Bryn-dís Brandsdóttir and Robin E. Bell are thanked for valuable discussions on various aspects of this paper. Sveinbjörn Steinþórsson, Ágúst Þór Gunnlaugs-son, Bergur Einarsson, Ásgeir Arnór Stefánsson and Ingibjörg Eiríksdóttir as well as JÖRFÍ volunteers are thanked for their work during field trips. Funding Information: The jökulhlaups in 1955, 1999 and 2011 were sudden and unforeseen. Following the 2011 jökul-hlaup, attempts were made to monitor the cauldrons by surveying RES-profiles with low frequency radar (5 MHz) to look for signs of water accumulation. This was done simultaneously with the ground DGNSS surface elevation profiling. RES-profiling across the Mýrdalsjökull cauldrons has continued, following the same track as accurately as possible, once or twice every year since May 2012 looking for signs of significant water accumulation as temporally elevated reflec- tions beneath the cauldrons (Magnússon et al., 2017). Similar RES-monitoring has also been successfully adopted for the Eastern Skaftá cauldron in Western Vatnajökull since 2014 (Magnússon et al., 2021). In 2016, the monitoring on Mýrdalsjökull initiated with a support from the Icelandic Road Administration Research Fund, developed into the research project Katla Kalda, supported by Rannís via the Icelandic Research Fund. The basic objective of the Katla Kalda project is to improve understanding of the collection and drainage of water from subglacial geothermal areas. To achieve this, the repeated RES-survey, initiated in 2012, was continued and expanded. High resolution (sub-meter) optical satellite images (Pléi-ades) have been repeatedly acquired for the purpose of producing surface DEMs and detecting surface elevation changes within the cauldrons. Surface changes of Mýrdalsjökull in the past decades have also been studied (Belart et al., 2020). Continuously recording GNSS instruments were deployed in various ice cauldrons to obtain records of jökulhlaup timing, duration and surface subsidence. Conductivity variation in the rivers draining these jökulhlaups was also studied (Einarsson, 2019). An automatic weather station (to estimate the surface energy balance) has been operated during summers and information on surface melting as well as motion has been obtained by deploying mass balance stakes in and around ice cauldrons in spring and revisiting them in the autumn. Finally, high Figure 2. a) The location of RES-data used in the pre-existing bedrock DEM (b) of our study area (Björns-son et al., 2000, with further unpublished improvements, by the IES-glaciology group, for limited area based RES-profile and point survey in 2000–2003). c) The location of RES-data used to create the new bedrock DEM (d) presented here. The white dotted line in d) indicates which part of the bedrock DEM is revised here. The background shaded relief images in a and c show the glacier surface in 2010 (Jóhannesson et al., 2013) and the red hachured polygon indicates the rim of the Katla caldera (Björnsson et al., 2000). The cyan lines in a–d show glacier margin and nunataks in 2010. e) An example of a 2D migrated RES-profile, measured in 2016, from A to B (locations shown in c). Red lines indicate traced bedrock, yellow dotted lines the traced 1918 tephra layer. – a) Lega íssjármælilína (að mestu mældar 1991) sem eldra botnkort (b) byggir á. c) Lega mælilína til grundvallar þess korts sem hér er birt (d). Svæðið utan hvítu punktalínunnar er óbreytt frá eldra korti. Myndir a og c sýna yfirborð jökulssins (skuggamynd og hæðarlínur) og útmörk (blágrænar línur) sumarið 2010. Rauð hökuð lína á a–d, sýnir brún Kötluöskjunnar. e) Dæmi um íssjársnið með tvívíðri staðsetningarleiðréttingu endurkastsflata (e. migration), mælt vorið 2016 frá A til B (staðsetning sýnd á c). Rauðar línur sýna rakin botnendurköst en gula punktalínan endurköst frá gjóskulaginu sem féll á jökulinn í gosinu 1918. Funding Information: This work was funded by the Icelandic Research fund of Rannís within the project Katla Kalda (project nr. 163391) with further support from The Icelandic Road Administration’s Research Fund (project nr. 1800-403). Pléiades images used to produce surface DEMs were acquired at a subsidised cost thanks to the CNES ISIS program. The images used to produce surface DEM in 2019 were further supported through CEOS (Committee on Earth Observa-tion Satellites) contribution to the Iceland Volcanoes Supersite. We thank Hugh Tuffen and an anonymous reviewer for very constructive and detailed reviews, which substantially improved this paper. Magnús T. Guðmundsson, Þórdís Högnadóttir, Guðfinna Aðal-geirsdóttir, Helgi Björnsson, Páll Einarsson, Bryn-dís Brandsdóttir and Robin E. Bell are thanked for valuable discussions on various aspects of this pa-per. Sveinbjörn Steinþórsson, Ágúst Þór Gunnlaugs-son, Bergur Einarsson, Ásgeir Arnór Stefánsson and Ingibjörg Eiríksdóttir as well as JÖRFÍ volunteers are thanked for their work during field trips. Publisher Copyright: © 2021, Iceland Glaciological Society. All rights reserved.

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10.33799/jokull2021.71.039

Cite this

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title = "The bedrock and tephra layer topography within the glacier filled Katla caldera, Iceland, deduced from dense RES-survey",

abstract = "— We present results from recent low frequency radio echo-sounding (RES) campaigns over the ice-covered caldera of Katla central volcano, beneath the M{\'y}rdalsj{\"o}kull ice cap, southern Iceland. The current RES-survey both partly repeats and enhances the RES-profile grid of a previous survey in 1991 with denser sounding lines and improved instruments. The RES-data, obtained in 2012–2021, include ∼760 km of 2D migrated RES-profiles covering an area of 116 km2 . Around 14 km2 subsections of this area were surveyed with RES-profiles 20 m apart allowing 3D migration of the RES-data. Our study confirms findings from previously published bedrock mapping, including main topographic features, ice volume stored within the 100 km2 caldera (45±2 km3, in autumn 2019) and maximum ice thickness (740±40 m). However, the significantly expanded level of detail and features observed in the new bedrock map reveals further evidence of a complex and eventful formation history of the caldera interior. This bedrock map is unprecedented in terms of detail for an ice-covered volcano. The new RES-data allows for a unique comparison of bedrock maps obtained from RES-data with 2D and 3D migration, demonstrating the limitations of 2D migrated RES-data in areas of high topographic variability. Reflections from the 1918 Katla eruption tephra layer within the ice were detected in a much wider area within the caldera than in the 1991 RES-data. We also observe a second internal layer at 420–580 m depth within the northern part of the caldera, identified here as the tephra from the 1755 Katla eruption. The 1918 tephra layer is typically observed at 200–300 m below the glacier surface. However, the layer depth varies from ∼100 m depth at the western rim of the caldera down to 460 m depth, where geothermal activity beneath ice cauldrons melts ice from below. At the most prominent geothermal areas all ice beneath the 1918 tephra has been melted leaving the tephra at the bed. Furthermore, the obtained tephra layer maps reveal footprints of some previously unidentified geothermal areas.",

author = "Eyj{\'o}lfur Magn{\'u}sson and Finnur P{\'a}lsson and Alexander Jarosch and Boeckel, \{Tayo van\} and Hrafnhildur Hannesd{\'o}ttir and Belart, \{Joaqu{\'i}n M. C.\}",

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T1 - The bedrock and tephra layer topography within the glacier filled Katla caldera, Iceland, deduced from dense RES-survey

AU - Magnússon, Eyjólfur

AU - Pálsson, Finnur

AU - Jarosch, Alexander

AU - Boeckel, Tayo van

AU - Hannesdóttir, Hrafnhildur

AU - Belart, Joaquín M. C.

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N2 - — We present results from recent low frequency radio echo-sounding (RES) campaigns over the ice-covered caldera of Katla central volcano, beneath the Mýrdalsjökull ice cap, southern Iceland. The current RES-survey both partly repeats and enhances the RES-profile grid of a previous survey in 1991 with denser sounding lines and improved instruments. The RES-data, obtained in 2012–2021, include ∼760 km of 2D migrated RES-profiles covering an area of 116 km2 . Around 14 km2 subsections of this area were surveyed with RES-profiles 20 m apart allowing 3D migration of the RES-data. Our study confirms findings from previously published bedrock mapping, including main topographic features, ice volume stored within the 100 km2 caldera (45±2 km3, in autumn 2019) and maximum ice thickness (740±40 m). However, the significantly expanded level of detail and features observed in the new bedrock map reveals further evidence of a complex and eventful formation history of the caldera interior. This bedrock map is unprecedented in terms of detail for an ice-covered volcano. The new RES-data allows for a unique comparison of bedrock maps obtained from RES-data with 2D and 3D migration, demonstrating the limitations of 2D migrated RES-data in areas of high topographic variability. Reflections from the 1918 Katla eruption tephra layer within the ice were detected in a much wider area within the caldera than in the 1991 RES-data. We also observe a second internal layer at 420–580 m depth within the northern part of the caldera, identified here as the tephra from the 1755 Katla eruption. The 1918 tephra layer is typically observed at 200–300 m below the glacier surface. However, the layer depth varies from ∼100 m depth at the western rim of the caldera down to 460 m depth, where geothermal activity beneath ice cauldrons melts ice from below. At the most prominent geothermal areas all ice beneath the 1918 tephra has been melted leaving the tephra at the bed. Furthermore, the obtained tephra layer maps reveal footprints of some previously unidentified geothermal areas.

AB - — We present results from recent low frequency radio echo-sounding (RES) campaigns over the ice-covered caldera of Katla central volcano, beneath the Mýrdalsjökull ice cap, southern Iceland. The current RES-survey both partly repeats and enhances the RES-profile grid of a previous survey in 1991 with denser sounding lines and improved instruments. The RES-data, obtained in 2012–2021, include ∼760 km of 2D migrated RES-profiles covering an area of 116 km2 . Around 14 km2 subsections of this area were surveyed with RES-profiles 20 m apart allowing 3D migration of the RES-data. Our study confirms findings from previously published bedrock mapping, including main topographic features, ice volume stored within the 100 km2 caldera (45±2 km3, in autumn 2019) and maximum ice thickness (740±40 m). However, the significantly expanded level of detail and features observed in the new bedrock map reveals further evidence of a complex and eventful formation history of the caldera interior. This bedrock map is unprecedented in terms of detail for an ice-covered volcano. The new RES-data allows for a unique comparison of bedrock maps obtained from RES-data with 2D and 3D migration, demonstrating the limitations of 2D migrated RES-data in areas of high topographic variability. Reflections from the 1918 Katla eruption tephra layer within the ice were detected in a much wider area within the caldera than in the 1991 RES-data. We also observe a second internal layer at 420–580 m depth within the northern part of the caldera, identified here as the tephra from the 1755 Katla eruption. The 1918 tephra layer is typically observed at 200–300 m below the glacier surface. However, the layer depth varies from ∼100 m depth at the western rim of the caldera down to 460 m depth, where geothermal activity beneath ice cauldrons melts ice from below. At the most prominent geothermal areas all ice beneath the 1918 tephra has been melted leaving the tephra at the bed. Furthermore, the obtained tephra layer maps reveal footprints of some previously unidentified geothermal areas.

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DO - 10.33799/jokull2021.71.039

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