Compositional variation through the Surtsey eruption, 1963-1967

Monday, 15 December 2014
James D L White, University of Otago, Dunedin, New Zealand, Sveinn P Jakobsson, Icelandic Institute of Natural History, Reykjavik, Iceland and C Ian Schipper, Victoria University of Wellington, Wellington, New Zealand
The volcanic island of Surtsey is the best-known product of a 3.5-year-long eruption that began in November 1963. During the course of the eruption four volcanic edifices grew from the seafloor along a discontinuous fissure extending 4.5 km, with three of them forming islands. Two of the islands washed quickly away – Syrtlingur lasted 5 months, and Jolnir 11 months. Surtla grew to within meters of the sea surface before its eruption ceased without forming an island. These separate eruptive centers along the Surtsey fissure produced edifices totalling ~0.15 km3 (Syrtlingur and Jolnir ~ 0.07 km3 each; Surtla ~0.01 km3), nearly half the volume of Surtsey below sea level (~0.3 km3). Although Surtsey's explosive activity ceased after 6 months, it was more than 3 years into the eruption when the last pyroclastic activity at Jolnir ceased, and during most of the time that Syrtlingur and Jolnir were erupting, there was no subaerial eruption at Surtsey. Here we present previously unpublished historic data on chemical diversity through the Surtsey eruption, bolstered with new analyses, and address specifically the other centers that were active during the eruption. Whole-rock compositions became progressively more magnesian as the eruption progressed (rising from 7 to 12% MgO), with a subtle concomitant reduction in potassium (0.7% to 0.4%). Glass compositions show considerable variation early in the eruption at Surtsey (~4.5-6.2 % MgO; 46.5-48.2% SiO2), but changed little through the remainder of the eruption (~7% MgO; 48% SiO2) at all sites. Glasses from Jolnir and Syrtlingur have overlapping compositions that form a separate cluster from those of Surtsey (more calcic, less potassic). Surtla, represented by only one sample and erupted at the greatest distance from Surtsey, has slightly more-evolved glass (lower MgO, higher FeOt), but this may reflect post-fragmentation crystal growth. Using this information we present a new assessment of magma-supply dynamics for this classic eruption.