The Fagradalsfjall eruption began on 19 March 2021, when a fissure opened in a valley on Iceland’s Reykjanes Peninsula and started pouring out basaltic lava for the first time in roughly 800 years. It lasted about six months, drew more than 356,000 visitors, and kicked off what many researchers now believe is a new eruptive episode for the peninsula. But the eruption’s real significance goes beyond spectacle: it gave scientists a rare, accessible window into deep-seated magma plumbing, gave civil engineers a live test of lava-diversion barriers, and gave ecologists a front-row seat to how life colonizes barren rock.
How the Eruption Announced Itself
The 2021 eruption did not arrive without warning. On 24 February, a magnitude 5.3 earthquake struck the area, launching an intense swarm of seismicity that migrated along two intersecting fault clusters at depths of roughly two to six kilometers below the surface. One cluster traced the path of a propagating dyke trending northeast-southwest; the other aligned with the mid-Atlantic plate boundary itself. The swarm lasted nearly four weeks, and then, right before the dyke broke the surface, seismicity almost completely faded out.
1Earth and Planetary Science Letters. Swarm seismicity illuminates stress transfer prior to the 2021 Fagradalsfjall eruption in IcelandThe precursory signals actually stretched back further than those four weeks. Analysis of deformation and seismicity for the full year before the eruption identified three distinct cycles of ground uplift and subsidence at the Svartsengi geothermal field, about eight kilometers west of the eventual eruption site. A high-resolution catalogue of roughly 39,500 earthquakes, built using recordings from optical fiber cables, showed that these cycles were driven by repeated intrusions of fluid into a permeable aquifer at about four kilometers depth, with a total estimated volume of around 0.11 cubic kilometers.
2Nature Geoscience. Cyclical geothermal unrest as a precursor to Iceland’s 2021 Fagradalsfjall eruptionOne analysis of the dyke intrusion produced a catalogue of over 80,000 earthquake locations along the 9.5-kilometer, two-segment dyke during the three-week dyking period.
3University of Cambridge Repository. Seismicity associated with magmatic intrusions: insights from the Reykjanes Peninsula, southwest IcelandThat density of data was unusual. The peninsula sits close to Reykjavík and its monitoring infrastructure, which meant the run-up to the eruption was among the best-instrumented precursory sequences ever recorded at a volcanic system waking from a centuries-long dormancy.
A Peninsula Waking Up After Centuries
The Reykjanes Peninsula sits at the landward end of the mid-Atlantic Ridge, where the North American and Eurasian plates pull apart. In the past 4,000 years, three rifting episodes have swept across the peninsula at intervals of about 800 to 1,000 years, activating four of its five volcanic systems: Reykjanes, Svartsengi, Krýsuvík, and Brennisteinsfjöll. The one system that stayed quiet through all of those episodes was Fagradalsfjall. When eruptions occurred there in 2021, 2022, and 2023, it was the first confirmed activity from that specific system in the historical record, and researchers have suggested it could mark the beginning of a new multi-century eruptive episode for the peninsula.
4Earth and Planetary Science Letters. Magma plumbing architectures and timescales of magmatic processes during historical magmatism on the Reykjanes Peninsula, IcelandThat prospect has obvious implications for Reykjavík, just 30 to 40 kilometers away, and for the critical infrastructure on the peninsula itself, including the international airport at Keflavík and Iceland’s largest geothermal power complex. While the 2021 eruption at Geldingadalir was relatively remote and low-risk, the broader reawakening has since produced eruptions that threatened the town of Grindavík and forced evacuations. The 2021 event, in that sense, was prologue.
What Happened at the Surface
Lava broke through at 20:45 UTC on 19 March 2021 along a northeast-southwest trending fissure in the Geldingadalir valley. The eruption lasted until September 2021 and went through five recognized phases, defined by the number of active vents and the style of activity.
5Earth and Planetary Science Letters. Swarm seismicity illuminates stress transfer prior to the 2021 Fagradalsfjall eruption in IcelandOne of the most visually striking phases involved uncommonly periodic lava fountaining: jets of lava roughly 100 to 400 meters high that lasted a few minutes, paused for a comparable interval, then repeated. Researchers traced this rhythmic behavior to gas dynamics in a shallow, magma-filled cavity near the surface. During each pause, gas accumulated at the top of the cavity. When pressure instabilities triggered the collapse of a foam layer into the conduit, the fountaining burst resumed. As the foam collapsed, fresh magma flowed in from depth, further degassing under reduced pressure and extending each fountaining episode beyond its initial burst.
6PubMed Central. Near-surface magma flow instability drives cyclic lava fountaining at Fagradalsfjall, IcelandVolcanic tremor patterns told a complementary story. In May, minute-long tremor bursts matched the pulsating fountains and pointed to degassing in a shallow reservoir connected to the top of the conduit, no deeper than about 100 meters. As the eruption matured, both the conduit and the shallow reservoir progressively enlarged, a process researchers tracked quantitatively. Sudden changes in tremor characteristics corresponded to observed crater collapses. By July and August, the eruption shifted to a different mode: hour-long cycles of episodic lava effusion, driven by magma flowing and cooling in a feeder dyke connected to a deeper sill at roughly five kilometers depth.
7Journal of Geophysical Research: Solid Earth. Dynamics of the 2021 Fagradalsfjall Eruption (Iceland) Revealed by Volcanic Tremor PatternsWhat the Lava Revealed About the Magma Below
The 2021 eruption offered a rare opportunity to study what was happening at the base of Iceland’s crust in something close to real time. The magma supplying the eruption came from bodies sitting near the Moho, the boundary between the crust and the mantle, at about 15 kilometers depth. What made this eruption scientifically valuable was how dramatically the lava’s chemistry shifted as the eruption progressed. During the first 45 days, lavas showed sudden and significant swings in composition, followed by smaller fluctuations for the remaining months. Isotope analysis pointed to contributions from multiple distinct magma bodies, each with its own chemical fingerprint.
8AGU Advances. The Petrology and Geochemistry of the 2021 Fagradalsfjall Eruption, Iceland: An Eruption Sourced From Multiple, Compositionally Diverse, Near‐Moho SillsThe physical properties of the lava itself also shifted. Over the course of the eruption, the melt’s viscosity dropped by about 25 percent, from 40 pascal-seconds to 30 pascal-seconds at a constant temperature of 1,200°C. That trend was not smooth: at least three upward spikes in viscosity were identified, around days 80, 120, and 138.
9ScienceDirect. A lower bound on the rheological evolution of magma in the 2021 Fagradalsfjall FiresThese spikes likely reflect moments when the eruption tapped into slightly different batches of magma, consistent with the multi-source plumbing system identified by the geochemistry. For volcanologists, this kind of detailed record is unusual. Most eruptions either happen in remote locations with limited sampling, or produce such large volumes of lava so quickly that subtle compositional shifts get blurred. Fagradalsfjall’s modest eruption rate and proximity to a major research hub made it possible to track the plumbing system’s behavior in unusual detail.
Gas Emissions and Air Quality
Although the eruption was small by Icelandic standards, it was not without atmospheric consequences. Ground-based UV spectrometers measured sulfur dioxide emissions throughout the six-month eruption, estimating a total release of roughly 970 kilotons of SO₂, give or take a wide uncertainty range. That figure sounds large but amounts to only about six percent of the SO₂ pumped out by the much bigger Holuhraun eruption in 2014–2015. Average emission rates across the whole eruption came in at about 64 kilograms per second, though they varied considerably between phases and even within individual days. On 11 August, for instance, measurements ranged from 17 to 78 kilograms per second.
10Journal of Volcanology and Geothermal Research. SO2 emission rates and incorporation into the air pollution dispersion forecast during the 2021 eruption of Fagradalsfjall, IcelandDespite the eruption’s relatively modest output, fine particulate matter concentrations rose measurably across long distances. In the capital region, 25 to 35 kilometers from the eruption site, peak daily-mean concentrations of the finest particles (PM1) climbed from background levels of 5–6 micrograms per cubic meter to 18–20 micrograms per cubic meter. The share of PM1 within the coarser PM10 fraction increased by about 50 percent, signaling that the eruption was producing particularly fine particles, likely sulfate aerosols rather than coarse ash. Elevated concentrations were detected as far as 300 kilometers away. In areas with otherwise low background pollution, average PM10 and PM2.5 levels rose by roughly half, though in more polluted locations the eruption’s contribution was not detectable above existing sources.
11Atmospheric Chemistry and Physics. Fine-scale fluctuations of PM1, PM2.5, PM10 and SO2 concentrations caused by a prolonged volcanic eruption (Fagradalsfjall 2021, Iceland)The findings highlight a point that is easy to overlook: an eruption does not need to be explosive or ash-rich to degrade air quality. The fine sulfate particles produced by quiet, effusive eruptions can travel far and settle into population centers at concentrations high enough to matter for respiratory health, especially over weeks and months of sustained emission.
Building Walls Against Lava
The 2021 eruption gave Iceland a live proving ground for lava diversion, something that had been discussed and occasionally attempted but never systematically documented in real time at a single event. During the eruption, engineers built three earthen dams from material available on-site, along with a 300-meter diversion barrier and a shorter 35-meter barrier. The barriers were designed to delay, contain, or redirect lava flows to buy time for protecting infrastructure and tourist areas downstream.
12Bulletin of Volcanology. Experience in diverting and containing lava flow by barriers constructed from in situ material during the 2021 Geldingardalir volcanic eruptionThe experience produced practical lessons about how different lava types interact with barriers. ʻAʻā lava, the rough, clinkery kind, thickened considerably behind a barrier as fresh lava intruded beneath its chilled outer crust, piling up against the structure. Pāhoehoe lava, the smooth, ropy kind, behaved differently: thin sheets accumulated and eventually crept over or around the barrier. In most cases pāhoehoe did not erode or damage the barriers as it crept over the top, but it could cause failures on downstream slopes if the dam crest was made of loose material. One dam succeeded in delaying the lava flow for up to 16 days, which in an emergency near inhabited areas could be the difference between an orderly evacuation and a chaotic one. This experience proved directly relevant when later eruptions on the peninsula threatened the town of Grindavík, and large-scale protective barriers were rapidly constructed.
13Bulletin of Volcanology. Experience in diverting and containing lava flow by barriers constructed from in situ material during the 2021 Geldingardalir volcanic eruptionVolcano Tourism on a Massive Scale
More than 356,000 tourists visited the eruption site while it was still active, an extraordinary number for an ongoing volcanic event.
14Natural Hazards. The eruption in Fagradalsfjall (2021, Iceland): how the operational monitoring and the volcanic hazard assessment contributed to its safe accessThe site’s accessibility played a major role: it was less than an hour’s drive from Reykjavík, and the lava initially flowed into an uninhabited valley. Maintaining safe access required continuous real-time monitoring, including gas transport forecasts, lava flow modeling, and regular reassessment of exclusion zones as new fissures opened. The monitoring team worked long hours throughout the eruption to identify low-risk access areas and keep the public informed.
Managing the site was not without tension. A study of stakeholder perceptions found that the collaborative process among municipal authorities, landowners, tour operators, government ministries, civil protection, and search-and-rescue teams was initially viewed positively. But gaps emerged around longer-term sustainable management, particularly regarding authority, responsibility, funding, and access rights. These are not abstract governance questions: with the peninsula continuing to erupt in subsequent years, the precedents set in 2021 shaped how later events were managed.
15PubMed Central. Stakeholder perceptions of managing an emergent tourist destination: the Fagradalsfjall volcanic eruption in IcelandReal-time photogrammetric monitoring, using both satellite and airborne imagery processed within three to six hours of acquisition, provided unprecedented data on lava volume, thickness, and effusion rates. By late September, 33 such surveys had been completed. This rapid-turnaround approach allowed authorities to assess how quickly the lava field was growing and whether it was advancing toward tourist areas or infrastructure.
16Geophysical Research Letters. Volume, Effusion Rate, and Lava Transport During the 2021 Fagradalsfjall Eruption: Results From Near Real‐Time Photogrammetric MonitoringThe Eruptions That Followed
Fagradalsfjall did not erupt just once. A second eruption occurred at Meradalir in August 2022, and a third at Litli-Hrútur in July 2023, both in the same volcanic system. The trio of eruptions gave researchers an unusual chance to track how the magma feeding the system evolved over a span of years. Analysis of olivine crystals from all three eruptions showed that the 2022 and 2023 lavas crystallized from more chemically evolved magma than the 2021 lavas, meaning the magma had undergone more fractional crystallization in its storage zone between eruptions. The shift in chemistry implied that crystal-liquid separation in these sub-alkaline magma reservoirs operates on timescales of just a few years, a fundamental finding for understanding how common basaltic magma systems evolve.
17Czech Polar Reports. The 2023 Litli-Hrútur eruption of the Fagradalsfjall Fires, SW-Iceland: Insights from trace element compositions of olivineThe 2023 eruption at Litli-Hrútur also brought environmental consequences that the 2021 event had largely avoided. Lava flows ignited the largest moss wildfires since modern record-keeping began in Iceland.
18PubMed. Hybrid particulate matter generated by lava-ignited wildfires at the Litli-Hrútur 2023 eruption, IcelandMoss is one of the dominant ground covers in Iceland’s lava fields, and thick moss carpets can burn once ignited by advancing lava. These fires added a secondary source of particulate matter on top of volcanic emissions, creating a hybrid plume whose health effects are still being studied.
Life Colonizing Fresh Rock
Having three eruptions from the same system within three years created an accidental experiment in primary succession, the process by which life establishes itself on completely sterile new surfaces. Researchers sampled the fresh basaltic lava from all three eruptions and found that microbial communities colonized the rock rapidly and followed a consistent, predictable trajectory. Cell counts increased as a function of lava age, climbing at a rate of roughly 182 cells per gram of lava per day over the first two years. The youngest samples, collected just 35 days after emplacement, already harbored about 6,100 cells per gram. The oldest samples measured by microscopy, at 827 days, contained roughly 220,000 cells per gram.
19PubMed Central. Three eruptions at the Fagradalsfjall Volcano in Iceland show rapid and predictable microbial community establishmentThe colonization followed a two-stage pattern: an initial phase of rapid but variable microbial establishment, followed by stabilization after the first winter. A machine-learning model trained on microbial community data from the 2021 lava accurately predicted the successional stage of communities on the 2022 and 2023 lavas, suggesting the process is reproducible and not just a product of chance dispersal.
20PubMed Central. Three eruptions at the Fagradalsfjall Volcano in Iceland show rapid and predictable microbial community establishmentInterestingly, the lava from the 2022 Meradalir eruption, which was emplaced against a slope and consisted of friable, porous spatter, reached comparable cell counts in one year that took two years on the denser 2021 flows. Texture and porosity seem to matter as much as time when it comes to how quickly microbes gain a foothold on volcanic rock. These early microbial pioneers break down minerals and begin building the thin layer of organic material that eventually allows mosses, lichens, and eventually higher plants to establish, a process that across Iceland’s older lava fields has played out over centuries.
Seismic Properties Around the Dyke
The swarm of earthquakes before and during the eruption was not just useful for forecasting; it also illuminated the physical state of the rock. Analysis of the ratio of compressional to shear seismic wave speeds (Vp/Vs) across the dyke revealed a significant change right at the eruption site: the ratio jumped from an average of 1.66 on one side to 1.71 on the other. That boundary coincides with the eruption location and indicates different rock properties on either side, likely reflecting the presence of melt, altered rock, or fluids injected during the intrusion.
21Geophysical Journal International. In-Situ Vp/Vs ratio variations in seismic swarms as indicator of magmatic processes: Fagradalsfjall volcanic activity, SW IcelandThis kind of measurement matters because it offers a potential tool for detecting where magma is present underground even when it has not yet reached the surface. If characteristic Vp/Vs signatures can be reliably linked to the presence of melt, future monitoring efforts on the Reykjanes Peninsula could use real-time seismic wave analysis to track intrusions as they develop. Given that the peninsula shows no signs of settling down, refining these tools has practical urgency.