Volcanic eruptions in Iceland
The 2023–2024 Sundhnúkur eruptions () were a series of volcanic eruptions on the Reykjanes Peninsula, near the town of Grindavík, Iceland. Between December 2023 and December 2024, there were seven eruptions, following an intense series of earthquakes in November 2023. Although localised, the seismic and volcanic activity have caused significant disruption across the western part of the peninsula, especially for the town of Grindavík. However, the Capital Region, including Reykjavík, has remained unaffected. The eruptions were preceded by an intense earthquake swarm in the Eldvörp–Svartsengi volcanic system that began on 24 October 2023, caused by a magmatic intrusion underneath the area. The frequency and intensity of the earthquakes dramatically increased on 10 November 2023, with around 20,000 tremors recorded by that time, the largest of which exceeded magnitude 5.3. Grindavík was subsequently evacuated due to the creation of large-scale subsidence, including the formation of an extensive graben valley, which caused significant damage. This extensional tectonic activity likely altered magma pathways and triggered subsequent eruptions.
The volcanic eruption series at the Sundhnúksgígar crater chain began on 18 December 2023, with an initial eruption that lasted for three days. This eruption was preceded by land uplift in the Svartsengi area, which subsequently deflated upon eruption, indicating the accumulation of magma at a depth of beneath Svartsengi. This magma source fed the initial eruption as well as all subsequent eruptions in the series. The second eruption occurred on 14 January 2024, lasting approximately two days. This event had a fissure opening less than from a nearby town. The eruption breached anti-lava defences and destroyed three homes. Additionally, the eruption formed a new graben that was considerably less extensive than the one formed in November 2023. Tragically, just before this eruption, one person was reported missing and was presumed to have fallen into a crack caused by seismic activity, resulting in their death. On 8 February 2024, the third eruption, the shortest so far at roughly one day, caused extensive damage, including the disruption of a hot water pipeline from the Svartsengi power station. This led to a loss of hot water supply for several days across the Reykjanes Peninsula. However, the Capital Region remained unaffected. The fourth eruption started on 16 March 2024 and became the longest in the series, spanning 54 days. A magmatic intrusion had occurred earlier in the month but did not reach the surface. The fifth eruption, which began on 29 May 2024, continued for 24 days. This eruption caused damage to power lines and cut off several road sections. On 22 August 2024, the sixth eruption commenced, lasting 14 days. It released of lava, covering an area of and resulting in of land subsidence. Despite being the largest eruption in the series, it did not cause any infrastructure damage. The seventh eruption began on 20 November 2024 and extended over 18 days. It quickly engulfed the parking lot of the Blue Lagoon and threatened protective barriers in the area.
Etymology.
The name "Sundhnúkur" literally translates to "passage peak". It is the highest point in the old crater row in the area, standing at . Historically, the name reflects the navigational challenges faced by sailors due to numerous rocks and skerries near the shore. Sundhnúkur served as a crucial visual marker for sailors navigating to the town of Grindavík, as its peak was visible from beyond the shore. The actual passage through the rocks has a different name, underscoring that Sundhnúkur refers specifically to the peak as a landmark rather than the passage itself. Sailors of the time also needed to be knowledgeable about various leading marks to safely reach the harbor. The term "Sundhnúksgígar" , meaning "passage peak craters", was coined by geologist Jón Jónsson in 1974, highlighting the series of craters associated with this notable landmark.
Background.
The Sundhnúksgígar crater row was formed approximately 2420 ± 100 radiocarbon years before present (BP), as determined by carbon-14 (14C) dating conducted by Uppsala University, using 1950 as the baseline. This ancient lava flow was measured to cover an area of and has an estimated volume of at least . It originated from a crater row extending nearly in length. The crater row includes two named craters: Sundhnúkur and Melhóll , with the latter situated nearly to the south-west of Sundhnúkur. The lava from this period was a mixture of pāhoehoe and ʻaʻā lava, indicative of varying eruption dynamics. Additionally, there are likely traces of an even older lava field beneath the circa 2400-year-old lava, though much of it has been obscured by the more recent eruption. The town of Grindavík is located along the south-western boundary of this ancient lava field. The most recent shield volcanoes to form in this particular area of the Reykjanes region were Sandfellshæð and Þráinsskjöldur , which emerged around 14,000 years ago. This volcanic activity was triggered by the retreat of glaciers at the conclusion of the last glacial period. As the ice melted, the Earth's crust underwent isostatic rebound, a process where the reduction in pressure allowed magma to rise more easily to the surface, initiating volcanic activity. Given these factors, it is considered highly unlikely that a shield volcano is currently forming in the region.
The Reykjanes Peninsula, located in south-western Iceland, has entered a new volcanic cycle after a period of approximately 800 years of relative inactivity. This resurgence was marked by heightened seismic events and volcanic activity near the hyaloclastite mountain Þorbjörn beginning in late 2019. The preceding period of volcanic activity occurred between 780 to 1240 AD, with the final phase, known as the Reykjanes Fires, occurring between 1210 and 1240 AD. This phase resulted in the formation of one of the largest continuous lava fields in Iceland's historical record, Arnarseturshraun and Eldvarpahraun . This activity was concentrated primarily west and north of Svartsengi and Þorbjörn. Volcanic periods on the peninsula typically span 300 to 400 years, during which eruptions occur at six to seven distinct locations, including regions near the Capital Region. These periods are composed of multiple phases, each lasting from several years to several decades, typically interspersed with breaks that can last up to 150 years. Furthermore, volcanic activity may shift to different areas of the peninsula every 30 to 50 years. Based on historical patterns, the current cycle is expected to continue until approximately 2300–2400 CE.
Iceland frequently experiences earthquakes due to its position on the Mid-Atlantic Ridge, where the Eurasian and North American tectonic plates meet. However, the 2023 swarm was unusually extensive and is linked to a magmatic intrusion approximately long, extending south-west from Kálffellsheiði and aligning with the Sundhnúksgígar crater chain, at a depth of at the shallowest point. The largest earthquakes originated beneath this crater chain, then spread south-westward beneath the town of Grindavík and offshore. This occurred in the Eldvörp–Svartsengi volcanic system, which has experienced 13 magmatic intrusions since January 2020 and nine of them since October 2023. Seven of these intrusions have resulted in volcanic eruptions, whereas the others did not reach the surface. Previous volcanic unrest during this period had been predominantly associated with the Fagradalsfjall system, where three out of four magmatic intrusions led to eruptions. Since 2021, there have been ten identified volcanic eruptions on the Reykjanes Peninsula.
Earthquakes.
October 2023 earthquakes.
An intense earthquake swarm began near Svartsengi, north of Grindavík, on 25 October 2023, with the Icelandic Meteorological Office (IMO) reporting over 1,000 earthquakes, including magnitudes 3.9 and 4.5 at around depth. The swarm, attributed to stress changes from previous magmatic activity, continued through 26 and 27 October, with over 5,800 earthquakes recorded since the start of the swarm. On 28 October, signs of ground uplift near Svartsengi, likely due to magmatic intrusion, were observed, and by 29 October, deformation rates were higher than past events. Seismic activity had decreased by 29 October, but over 7,000 earthquakes had been recorded by that date since the beginning of the swarm. On 30 October, GPS data confirmed ongoing deformation at around depth, with 1,300 earthquakes detected in the previous 24 hours. On 31 October, a magnitude 3.7 earthquake swarm at the mountain Þorbjörn indicated continued magma movement. IMO and Civil Defence were closely monitoring the situation, with the possibility of rapid changes, though magma movements may have subsided without eruption.
The first signs of a magma intrusion near the Svartsengi area, located north-west of Þorbjörn on the Reykjanes Peninsula, were detected in late October 2023. This phenomenon, identified through the latest GPS data and InSAR images derived from satellite data, indicated an accelerated rate of land rise, centred near the Blue Lagoon, approximately north-west of Þorbjörn. It was the fifth occurrence of such land uplift in the area since 2020, suggesting increased pressure likely due to magma intrusion. The rapid uplift raised the likelihood of more frequent seismic activity in the area that could potentially allow magma to move closer to the Earth's surface. Experts conducted models to estimate the depth and size of the intrusion, amid expectations of a potential volcanic eruption within the next year.
November 2023 earthquakes.
In November 2023, seismic activity around the mountain Þorbjörn remained intense, with over 800 earthquakes recorded on 1 November, including a magnitude 3.7 quake. GPS data confirmed ongoing uplift due to magma intrusion at a depth of . On 2 November, approximately 400 earthquakes were recorded, with the largest being magnitude 2.8, continuing the seismic trend. By 3 November, a magnitude 4.3 earthquake occurred, and the area continued to experience significant seismic activity, though there were no signs of magma moving toward the surface. Seismicity dropped on 4 November, but uplift continued, with rockfall risks from larger earthquakes. On 6 November, 1,300 earthquakes were detected, with uplift reaching since the start of the inflation, and magma inflow into a sill-type intrusion estimated at . Seismic activity remained episodic, with another 900 earthquakes on 7 November. On 8 November, approximately 1,200 earthquakes were recorded, and magma accumulation persisted at depth with no signs of surface eruption. On 9 November, seismic activity spiked, with 1,400 earthquakes and seven above magnitude 4.0, including a magnitude 4.8. Nearly 800 earthquakes occurred on 10 November, continuing the pattern of magma accumulation and seismic release, but with no indications of an imminent eruption.
Around 15:00 UTC on 10 November 2023, a significant wave of seismic activity began approximately north-east of Grindavík. As a result, the civil protection phase was raised from Uncertainty Phase to Alert Phase less than three hours later. By 23:00 UTC, the Icelandic Meteorological Office (IMO) observed that the earthquake swarm had extended beneath the town. A state of emergency was declared shortly thereafter, and the evacuation of Grindavík commenced. The largest of the earthquakes to date reached magnitude 5.3 on 10 November 2023. By this time, over 22,000 earthquakes had been recorded since the beginning of the swarm in October 2023. The Icelandic Meteorological Office (IMO) predicted that an eruption was likely, stating that "it will take several days (rather than hours) for magma to reach the surface". The greatest extent of the magma intrusion was inferred to be around the Sundhnúksgígar crater chain, approximately north of Grindavík. Instruments detected the presence of sulfur dioxide (SO2) in the atmosphere on 14 November, indicating that magma was only a few hundred metres (a couple of thousand feet) under the surface. Although the number of earthquakes decreased somewhat since 10 November, the IMO was still recording between 700 and 1,000 earthquakes daily by 14 November.
Ground deformation sensors at Festarfjall and the Svartsengi area recorded that the ground had moved apart by . Satellite measurements recorded the subsidence depth of about over a swathe of land approximately 5 km long and 2 km wide (), extending from the Sundhnúksgígar craters to the western side of Grindavík. The creation of this graben-like formation on 10 November has enabled scientists to estimate the volume of the magmatic intrusion, which formed in a remarkably short period of six hours, as approximately . Further analysis revealed that about flowed into this magma intrusion, providing valuable insights into the dynamics of volcanic processes and the scale of subsurface magma movement. It was estimated that the subsidence continued at a daily rate of about in the week following the disaster. A large crack opened up through the town, which old maps indicate is a reactivation of an existing fault. Scientists at the University of Iceland believe