A new theory of jökulhlaups published in Nature Communications (https://doi.org/10.1038/s41467-026-70428-w)
Grímsvötn subglacial lake within the caldera of the volcano Grímsvötn in the centre of Vatnajökull ice cap in March 2026 (Photo: Eyjólfur Magnússon). Water accumulates under the ice and is released in semi-periodic subglacial floods. During such flood in 2021 the lake released almost 1 km3 of water.
Volcanically- and geothermally-induced jökulhlaups draining from the subglacial lake Grímsvötn in Vatnajökull ice cap have been the subject of scientific research for almost a century. The first theory describing the dynamics of jökulhlaups, developed by John Nye in 1976, was based on observations and studies of Icelandic jökulhlaups, carried out by e.g., the hydrologist Sigurjón Rist, the geologist Sigurður Þórarinsson and the glaciologist Helgi Björnsson. Nye’s theory assumes jökulhlaups flow subglacially through a conduit extending from the source lake to the glacier margin, which is enlarged by melting of the ice walls. Cylindrically shaped flood conduits at the glacier margin and the development of the jökulhlaup discharge with time supported the theory.
Motion of a GNSS station ca. 17 km south of Grímsvötn during the jökulhlaup in 2021, the largest since the volcanically-induced jökulhlaup in 1996. The first sign of leakage from Grímsvötn was observed on 15 November, a flood wave reached the GNSS station on 24 November and the glacier margin on 27 November. Maximum discharge of ~3600 m3 s–1 was reached in the evening of 4 December. The blue band across the graph indicates the magnitude of the horizontal velocity during the first two weeks of November, before the jökulhlaup.
New radar satellite images, discharge measurements in Gígjukvísl draining from the glacier margin, and GNSS measurements on the glacier surface, both in Grímsvötn and above the subglacial flood path, show more a complex development of jökulhlaups from Grímsvötn than previously assumed. These measurements show that the flood water propagates as a several-km-wide “sheet” beneath the glacier during the start of the jökulhlaup. The passage of the flood front is associated with the lifting the surface of glacier by approximately 1 m. The subglacial flood wave reduces the contact area between the glacier and the bed and results in greatly increased glacier sliding. The horizontal velocity of the glacier was increased closed to 5-fold at a GNSS station located 17 km down-glacier from Grímsvötn (blue curve in the above figure). The floodwaves in 2021 and 2022 took nearly a week to reach the glacier margin ~50 km south of Grímsvötn. A large volume of flood water was temporarily stored at the glacier bed, as depicted in the accompanying schematic picture of the development of the jökulhlaups. The subglacially ponded flood water was then released near the end of the flood.
The study was carried out by scientists at the Institute of Earth Sciences, University of Iceland, Icelandic Meteorological Office, the Natural Science Institute of Iceland, ENVEO IT GmbH in Austria, ICEYE Oy Finland, and the Université de Toulouse in France.
Conceptual model showing the evolution of Grímsvötn jökulhlaups, based on measurements obtained during jökulhlaups in 2021 and 2022. The approximate timing of each panel, relative to subglacial flood-water storage, is shown with the inset graph at the bottom. In 2021, when nearly 1 km3 of water drained from Grímsvötn, around ¼ of that volume was stored beneath Skeiðarárjökull at the peak of the flood. a) Subglacial leakage passing the lake dam (I) starts, which then starts developing and expanding a basal ice conduit. Farther downstream (II) a bottleneck forms in the conduit flow causing flood water to accumulate and spread laterally. b,c) A subglacial flood wave propagates from II to the glacier margin in 4–6 days. d) Ponds that formed in the path of the flood wave, continue to grow, eventually developing into a continuous flooded area that further lifts the ice from the bed due to high water pressure. Ice conduits develop upstream from the glacier margin. e) Immediately following peak water storage in the flood path, the subglacial water pressure drops and consequently uplift stops and horizontal velocity decreases. Subsidence of the glacier surface above the flooded area starts in its uppermost part and propagates down-glacier. f) The flooded area develops into several interconnected ponds. Eventually these ponds disappear and ice motion, as well as discharge in Gígjukvísl, return to normal.
The key data in this study are high-resolution radar images acquired with the ICEYE satellite orbiting 500–600 km above the surface of the Earth. By applying interferometric synthetic aperture radar (InSAR) processing, the motion of glaciers and solid earth can be measured with a few mm accuracy. InSAR is widely used in Earth sciences, including for the monitoring of volcanoes, crustal deformation, and landslides.
Studying jökulhlaups provides insight into the subglacial drainage system, under extreme water pressure and discharge. Understanding the dynamics of jökulhlaups, therefore, has wider implications for glacier dynamics in general as the response of the glacier to subglacial floods shows how glacier dynamics respond to changes in basal hydraulic conditions.