Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long1. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens2,3,4, or magma flowing vertically into dykes from an underlying source5,6, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Bárðarbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Bárðarbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Bárðarbunga caldera, accompanied by a series of magnitude M > 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.
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Support for this work was received from the European Community’s Seventh Framework Programme Grant No. 308377 (Project FUTUREVOLC), the Icelandic Research Fund (Project Volcano Anatomy), the Research Fund at the University of Iceland, NERC, the Geological Survey of Ireland and the US National Science Foundation (NSF). COSMO-SkyMed data were provided by the Italian Space Agency (ASI) and TerraSAR-X data by the German Space Agency (DLR) through the Icelandic Volcanoes Supersite project supported by the Committee on Earth Observing Satellites (CEOS). RADARSAT-2 data were provided by the Canadian Space Agency and MDA Corporation. Natural Resources Canada Earth Sciences Sector contribution number 20140314. An intermediate TanDEM-X digital elevation model was provided by DLR under project IDEM_GEOL0123. We thank the following key persons for help with instrumentation and data: B. H. Bergsson, Þ. Jónsson, V. H. Kjartansson, S. Steinþórsson, P. Erlendsson, H. Ólafsson, J. Söring and D. Craig. We also acknowledge the many others who have contributed to GPS, seismic and other field work in the study area. For GPS equipment and support, we acknowledge services provided by the UNAVCO Facility with support from the US NSF and National Aeronautics and Space Administration (NASA) under NSF Cooperative Agreements Nos EAR-0735156 and EAR-0711446. S. Jónsson (KAUST, Saudi Arabia) and T. Villemin (EDYTEM, Université de Savoie, France) also provided support to GPS. Seismic equipment: The British Geological Survey donated several of the broadband seismic sensors around Vatnajökull. We thank the SEISUK facility for loans to R.S.W. of seismometers under loan 980. Landsvirkjun contributed GPS instruments and seismic sensors north of Vatnajökull. The surveying aeroplane of Isavia (Icelandic Aviation Operation Services) mapped the subsidence of Bárðarbunga. The Icelandic Coast Guard provided aeroplane and helicopter support for field studies.
The authors declare no competing financial interests.
Extended data figures and tables
Filled uptriangles correspond to continuous GPS stations, open triangles to campaign GPS sites, filled downtriangles correspond to seismic stations, and black ‘stars’ correspond to co-located GPS and seismic stations. Station names for GPS are indicated with four upper-case letters and for seismic stations with three lower-case letters. The different tracks for SAR satellite data are plotted with solid lines for RADARSAT-2, dashed lines for COSMO-SkyMed (CSK) and dotted lines for TerraSAR-X (TSX). Orbit numbers are indicated for CSK and TSX. The red stars correspond to the 2014 eruptive fissures at Holuhraun. Background map shows ice caps (white), central volcanoes (dotted lines), calderas (hatched lines) and fissure swarms (grey shading)46. Names of selected volcanoes are shown, and T indicates Tungnafellsjökull central volcano. Inset, locations of the Eastern Volcanic Zone (EVZ), the Western Volcanic Zone (WVZ), the Northern Volcanic Zone (NVZ) and the South Iceland Seismic Zone (SISZ), with their fissure swarms and central volcanoes; box shows area of the main figure.
a, Location of dyke segments (red lines labelled with numbers) delineated by relatively relocated earthquakes. The triangles show locations of the nearest seismic stations used to locate the events. Green, stations operated by the Icelandic Meteorological Office (IMO); blue, station operated by the University of Cambridge; and red, station operated by University College Dublin. The two stations on the ice are joint IMO and University of Cambridge stations. All stations telemetered data to IMO. Also shown are ice cauldrons formed (circles), outline of lava flow mapped from radar image on 6 September, and boundaries of graben activated in the dyke distal area (hatched lines). b, The dyke segments. Columns show segment number (Nr), latitude and longitude of beginning (Lat1, Lon1) and end points (Lat2, Long2) of each segment, segment length (L), depth range (D), strike, dip, the RMS value of the deviation (in metres) of the earthquakes from the plane they define, and the number of earthquakes used to define each dyke segment plane (#Eq).
Blue contours show the elevation of the ice surface in the Bárðarbunga caldera on 5 September 2014, about three weeks after the onset of unrest. The data were obtained using aircraft flown 100–150 m above glacier surface, using radar altimetry and submetre differential GPS Omnistar. The system provides 2 m absolute accuracy of surface elevation along the survey profiles47, shown as black dotted lines. The subsidence relative to the pre-unrest ice surface is indicated with coloured shading (key at right). It is greatest in the central part of the caldera, where it had a maximum of 16 m.
Extended Data Figure 4 Geodetic model with a contracting point pressure source, a two-segment-dyke and no a priori constraints.
The dyke segments are rectangular with uniform opening37 and the model shown is that with maximum probability. Free parameters are: position and volume change of the point pressure source39, and position, strike, dip and opening of the dyke. The panels show from left to right: data (a, d, g), model (b, e, h) and residuals (c, f, i). GPS data in all panels span 15 August to 4 September 2014. The top row shows an interferogram spanning 6 July 2012 to 4 September 2014; the middle row shows an interferogram spanning 2 August to 3 September 2014; the bottom row shows the data from Extended Data Fig. 3 from aeroplane radar profiling, but these data were not used to constrain the model.
Extended Data Figure 5 Geodetic model with a contracting point pressure source, two caldera faults, and a four-segment dyke.
The dyke is divided into multiple rectangular patches37 and the model shown is that with maximum probability. Lateral dyke position is fixed from relocated seismicity. Position of the point source and faults, volume change of the point source and opening and strike-slip of the dyke are free parameters. Left column (a, d, g), data; middle column (b, e, h), model; right column (c, f, i), residuals. The data are detailed in Extended Data Fig. 4 and all of the data were used to constrain the model.
Extended Data Figure 6 Geodetic model with a contracting flat-topped chamber, two caldera faults, and a four-segment dyke.
A deflating penny-shaped crack38 is used to represent the top of a flat-topped chamber48, and the dyke is divided into multiple rectangular patches37. The model shown is that with maximum probability. Lateral dyke position is fixed from relocated seismicity. Position of the crack and faults, volume change and radius of the crack and opening and strike-slip of the dyke are free parameters. Left column (a, d, g), data; middle column (b, e, h), model; right column (c, f, i), residuals. The data are detailed in Extended Data Fig. 4 and all of the data were used to constrain the model.
a–h, Energy profiles for dyke segments 1, 2, 3, 4, 5, 6, 7a and 8a, as described in Fig. 3b. Blue lines indicate the strain energy potential change as a function of the strike, and the red lines the gravitational potential change. Green is the total potential energy change. Energy is shown in terajoules (1012 J). The lowest point on each energy curve is defined as 0 TJ. Error bars, 1 s.d. of the error in the numerical integration.
a, Bedrock (brown line) and ice topography (light blue line) along the dyke path. b, Depth of earthquake hypocentres (grey dots) below sea level projected on the dyke segments and lines (red) of constant lithostatic pressure, assuming constant crustal density of 2,800 kg m−3 and ice density of 920 kg m−3. Line spacing corresponds to 25 MPa. c, Lithostatic pressure at sea level calculated along dyke segments 1, 2, 3, 4, 5, 6, 7a, 8a and 8b (blue line). The calculations take into account both the subglacial bedrock topography and ice thickness. Light blue triangles indicate the beginning of a segment and red triangles the end of a segment. It is assumed that between segments the dyke propagates along a straight path. Dyke propagation was halted for the longest time at the end of segment 4 (see Figs 2 and 4), before an increase in lithostatic pressure.
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Sigmundsson, F., Hooper, A., Hreinsdóttir, S. et al. Segmented lateral dyke growth in a rifting event at Bárðarbunga volcanic system, Iceland. Nature 517, 191–195 (2015). https://doi.org/10.1038/nature14111
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