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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The basalts of the 2021 Fagradalsfjall eruption were the first erupted on the Reykjanes Peninsula in 781 years and offer a unique opportunity to determine the composition of the mantle underlying Iceland, in particular its oxygen isotope composition (δ18O values). The basalts show compositional variations in Zr/Y, Nb/Zr and Nb/Y values that span roughly half of the previously described range for Icelandic basaltic magmas and signal involvement of Icelandic plume (OIB) and Enriched Mid-Ocean Ridge Basalt (EMORB) in magma genesis. Here we show that Fagradalsfjall δ18O values are invariable (mean δ18O = 5.4 ± 0.3‰ 2 SD, N = 47) and indistinguishable from “normal” upper mantle, in contrast to significantly lower δ18O values reported for erupted materials elsewhere in Iceland (e.g., the 2014–2015 eruption at Holuhraun, Central Iceland). Thus, despite differing trace element characteristics, the melts that supplied the Fagradalsfjall eruption show no evidence for 18O-depleted mantle or interaction with low-δ18O crust and may therefore represent a useful mantle reference value in this part of the Icelandic plume system.

The 10 th century Eldgjá fissure eruption is the largest in Iceland in historical time. It erupted 21.0 km 3 of magma, with 1.3 km 3 as tephra in at least 16 explosive episodes from subaerial and subglacial vents, producing magmatic and phreatomagmatic deposits respectively. Grain-size distributions for these end-members show distinct differences at comparable distances from source: the former are coarser and unimodal; the latter are finer and bimodal. These distributions appear to record different primary fragmentation histories. In contrast, the vesicle-size distributions of pyroclasts from each type of deposit show the magma was fully expanded and underwent similar vesicle nucleation and growth prior to fragmentation. This indicates that the role of glacial water was comparatively latestage, re-fragmenting an already disrupting magma by quench granulation. The presence of microlite-rich domains within clasts reveals a history of complex conduit evolution, during the transition from a continuous dyke to focused, discrete vents. ÚtdrátturBasaltgosið á Eldgjársprungunni á 10. öld er staersta gosið á Íslandi á sögulegum tíma. Í gosinu kom upp um 21,0 km 3 af kviku, þar af 1,3 km 3 sem gjóska í það minnsta 16 goshrinum, sem voru magmatísk gos á sprungureinum sem lágu utan Mýrdalsjökuls og freatómagmatísk gos á reinum undir jöklinum. Kornastaerðardreifing gjóskunar frá þessum gosgerðum, framkvaemd á sýnum sem tekin voru í sömu fjarlaegð frá upptökum, er mög frábrugðin hvor annarri: magmatíska gjóskan er grófkorna og eintoppa, en freatómagmatíksa gjóskan er fínni í korninu og tvítoppa. Þessi mismunur í kornastaerðardreifingu endurspeglar mismunandi sundrunarferli kvikunnar í gosi. Aftur á móti, þá sýnir blöðrustaerðardreifing, eins og hún er maeld í vikurkornum frá hvorri gjóskugerðinni um sig, að kvikan var full-blásin áður en að hún sundraðist og að blöðrumyndun og -vöxtur í báðum tilfellum var mjög svipaður. Þetta bendir til þess að þennsla kvikugasa hafði komið af stað sundrun á kvikunni áður en að hún komst í snertingu við utanaðkomandi vatn efst í eða beint ofan við gosrásina. Snertingin við utanaðkomandi vatn einfaldlega leiddi til frekari sundrunar á vikurmolunum vegna hraðkaelingar. Örkristalla innlyksur í gjóskukornunum endurspegla flókna þróun á kvikunni efst í gosrásinni á þeim tíma sem virknin afmarkast við einstök gígop.

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