Magma plumbing and transport dynamics

Oceanic volcanism is characterised by basaltic fissure eruptions and more rarely occurring monogenetic eruptions. In this project (Research Council of Finland -funded FINTIMS project, led by prof. Christoph Beier) our team targeted both types of eruptions in the Northern Rift Zone of Iceland to better understand what geochemical and/or geodynamic processes lead to each type of eruptions. I led the chapter on monogenetic lava shields, detailed below. 

Monogenetic lava shields

Lava shields are gently sloping, conical lava mounds that erupted from a central vent. They include some of the grandest expressions of basaltic magmatism in Iceland, with volumes of single eruptions exceeding 10 cubic kilometers. We haven't directly observed a shield eruption—at least in that scale. The 2021 Fagradalsfjall eruption resembled a lava shield eruption in many ways, prompting more research on understanding this somewhat overlooked but important eruptive mode.

Monogenetic shield eruptions differ both compositionally (less evolved, more variable) and dynamically (lower intensity, longer duration) from the more frequent and more extensively studied fissure eruptions. Moreover, many of Iceland’s lava shields formed in the early Holocene, suggesting a causal link to the increase in magma production rates during deglaciation. The geodynamic conditions and magmatic supply chains that govern lava shield eruptions appear to be fundamentally different from fissure eruptions but remain poorly understood due to the scarcity of geochemically well-characterized eruptions.

We carried out a detailed geochemical case study of the mid-Holocene ~15 cubic km Trölladyngja lava shield, and its nearest neighbouring fissure eruption, Fjallsendahraun (~1362 CE). The data comprise whole-rock, groundmass and glass major, trace element and radiogenic isotope data from the two eruptions. Compared to Fjallsendahraun and other Icelandic fissure eruptions, the Trölladyngja lavas show greater variability in terms of both crystal content, major and trace element compositions as well as and isotopic indicators of mantle source heterogeneity. 

Compiling regional data, we observe that the degree of major element variability appears to increase with eruption volume for Icelandic monogenetic lava shields, whereas their incompatible trace element variability is higher than fissure eruptions for all eruption sizes.

These observations indicate that Trölladyngja and other large-volume lava shield eruptions in Iceland are not fed from a single well-homogenized magma reservoir—a conventional model for fissure eruptions—but rather, involve variable entrainment of a mush pile and/or crystal cumulates and a dynamic plumbing system that experiences syn-eruptive recharge of mantle-derived melts. 

Fissure eruptions

In a separate chapter of the FINTIMS project led by Dr. Adam Abersteiner, we could demonstrate that the 10 ka Sveinar-Randarhólar crater row and associated lavas in the Northern Rift Zone probably originated as a single eruption from the Askja volcano. Strikingly, our results imply that lateral transport of magma took place over a stretch of about 100 km from the magma reservoir to the northernmost crater – the longest such example on Earth from the Holocene.

Work led by Noëmi Löw investigated the origin of the Fjallgarðar Volcanic Ridge (FVR). The FVR is an enigmatic 200 km long segment of subglacially erupted basalts, stretching from central Iceland all the way to the Northern coast, that is off-set to the east from the main plate boundary. We could show that the FVR was formed from at least two different types of sources, of which one matches geochemically with the Kverkfjöll volcano. Again, Kverkfjöll-like lavas erupting in northern FVR imply lateral transport of magma in the upper crust for a staggering 200 km before erupting!

Our combined research shows how fundamentally different geodynamic and geochemical processes lead to two contrasting modes of eruptions in the Icelandic rift zones. Naturally, this paves the way for more research that is needed to work out many remaining questions. Which is the positive feedback loop that keeps research going and us researchers motivated!