The 6-month long eruption at Holuhraun (August 2014-February 2015) in the Bárðarbunga-Veiðivötn volcanic system was the largest effusive eruption in Iceland since the 1783-1784 CE Laki eruption. The lava flow field covered~84 km 2 and has an estimated bulk (i.e., including vesicles) volume of~1.44 km 3. The eruption had an average discharge rate of~90 m 3 /s making it the longest effusive eruption in modern times to sustain such high average flux. The first phase of the eruption (August 31, 2014 to mid-October 2014) had a discharge rate of~350 to 100 m 3 /s and was typified by lava transport via open channels and the formation of four lava flows, no. 1-4, which were emplaced side by side. The eruption began on a 1.8 km long fissure, feeding partly incandescent sheets of slabby pāhoehoe up to 500 m wide. By the following day the lava transport got confined to open channels and the dominant lava morphology changed to rubbly pāhoehoe and 'a'ā. The latter became the dominating morphology of lava flows no. 1-8. The second phase of the eruption (Mid-October to end November) had a discharge of~100-50 m 3 /s. During this time the lava transport system changed, via the formation of a b 1 km 2 lava pond~1 km east of the vent. The pond most likely formed in a topographical low created by a the pre-existing Holuhraun and the new Holuhraun lava flow fields. This pond became the main point of lava distribution, controlling the emplacement of subsequent flows (i.e. no. 5-8). Towards the end of this phase inflation plateaus developed in lava flow no. 1. These inflation plateaus were the surface manifestation of a growing lava tube system, which formed as lava ponded in the open lava channels creating sufficient lavastatic pressure in the fluid lava to lift the roof of the lava channels. This allowed new lava into the previously active lava channel lifting the channel roof via inflation. The final (third) phase, lasting from December to end-February 2015 had a mean discharge rate of~50 m 3 /s. In this phase the lava transport was mainly confined to lava tubes within lava flows no. 1-2, which fed breakouts that resurfaced N 19 km 2 of the flow field. The primary lava morphology from this phase was spiny pāhoehoe, which superimposed on the 'a'ā lava flows no. 1-3 and extended the entire length of the flow field (i.e. 17 km). This made the 2014-2015 Holuhraun a paired flow field, where both lava morphologies had similar length. We suggest that the similar length is a consequence of the pāhoehoe is fed from the tube system utilizing the existing 'a'ā lava channels, and thereby are controlled by the initial length of the 'a'ā flows.

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.[1] Sulfur concentrations have been measured in 28 melt inclusions (MIs) in plagioclase, clinopyroxene, and olivine crystals extracted from tephra produced during the explosive eruption of Grímsvötn in May 2011. The results are compared to sulfur concentrations in the groundmass glass in order to estimate the mass of sulfur brought to surface during the eruption. Satellite measurements yield order of magnitude lower sulfur (~0.2 Tg) in the eruption plume than estimated from the difference between MI and the groundmass glass. This sulfur "deficit" is readily explained by sulfur adhering to tephra grains but principally by sulfide globules caused by basalt-sulfide melt exsolution before degassing. A mass balance calculation reveals that approximately~0.8 Tg of SO 2 is present as globules, representing~50% of the total sulfur budget. Most of the sulfide globules likely reside at depth due to their elevated density, for potential later remobilization by new magma or hydrothermal circulation.