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Explosive phases of the 937-40CE Eldgjá flood lava eruption, Iceland and the variability in magma composition

Thor Thordarson1; Diana Brum D Silveira G Alvarez1; William M. Moreland1,2; Ármann Höskuldsson1, Ingibjörg Jónsdóttir1.

  • Affiliations:  1Faculty of Earth Sciences, University of Iceland, IS101 Reykjavík, Iceland; 2Iceland Meterological Office, IS105 Reykjavík, Iceland 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 102

  • Programme No: 3.16.29

  • Theme 3 > Session 16

Abstract

The 937-40 AD Eldgjá flood lava eruption in Iceland (21 km3) took place on a ~70-km-long vent system trending northeast from the Katla central volcano. It features four distinct vent segments: Eldgjá South, Central, Chasm, and North. The South segment is partly beneath the Mýrdalsjökull glacier, but the reminder of the vent system was subaerial. The eruption produced >16 eruption episodes each featuring phreatomagmatic or magmatic explosive activity of sub-Plinian to Plinian intensities. The tephra fall from these explosive phases covers >20,000 km2 and has a cumulative volume of >6 km3 (1.3 km3 DRE). The lava volume is 19.6 km3. The eruption started on the South segment, ~10-15 km NE of the Katla caldera and for the first ~1.5 yrs the eruption shifted between vents just outside and beneath the glacier producing stratified phreatomagmatic tephra sequence interspersed with sporadic magmatic tephra units. This part represents the Wet Phase of the eruption. In the final 1.5 yrs the activity propagated towards the northeast, away from the Katla volcano, producing magmatic tephra (and lava) representing the Dry Phase. Eldgjá magma is rather evolved (MgO: 4.9-5.7 wt.%), mildly alkalic FeTi-basalt. Analyses of samples representing all eruption episodes show that the most primitive magma (MgO: 5.5-5.7 wt.%; Zr: 220-235 ppm) erupted during the initial Wet Phase. The erupted magma became more evolved (MgO: 4.9-5.5 wt.%; Zr: 240-280 ppm) during the Dry Phase, indicating extraction of magma from shallower levels within the crustal storage zone during the latter half of the eruption.