The 2023–2025 Sundhnúkur eruptions () are a series of volcanic eruptions on the Reykjanes Peninsula, near the town of Grindavík, Iceland. Nine eruptions have occurred since December 2023, with the most recent taking place in August 2025. The seismic and volcanic activity caused significant disruption across the western part of the peninsula, especially for the town of Grindavík. However, the Capital Region, including Reykjavík, has remained physically unaffected.

Volcanic activity began with an intense earthquake swarm in the Eldvörp–Svartsengi volcanic system 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 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. During this event, a fissure opened less than from the nearby town of Grindavík. The eruption breached anti-lava defences and destroyed three homes. Additionally, the eruption formed a new graben, although it was substantially less extensive than the one formed in November 2023. Tragically, just before this eruption, one person was reported missing and presumed to have fallen into a crack caused by seismic activity, resulting in their death. On 8 February 2024, the third eruption caused extensive damage, including the disruption of a hot-water pipeline from the Svartsengi power station. Although the eruption lasted only about two days, it resulted in a loss of hot water supply for several days across the Reykjanes Peninsula. The Capital Region, however, 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 so far, 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. The eighth eruption commenced on 1 April 2025 and concluded approximately seven hours later the same day, marking the shortest and least intense event in the eruptive series to date. In the days following the eruption, a substantial magmatic dike intruded underground without breaching the surface. The ninth eruption of the series commenced on 16 July 2025 and persisted for roughly 20 days. While it posed no threat to infrastructure, the gas pollution spread unusually far during the eruption's early stages and the measured pollution levels in nearby towns and cities were higher than those typically observed during the previous eruptions in the series.

Land uplift has resumed since the last eruption and is ongoing, with the current rate of vertical ground deformation lower than that observed before the previous eruption, although magmatic inflow into the magma chamber remains steady. Since March 2024, each eruption has been preceded by the accumulation of roughly of magma in the reservoir. By the end of November 2025, the interval since the previous eruption had become the longest recorded within the eruptive sequence that began in November 2023, exceeding 115 days since the conclusion of the July–August 2025 eruption. In mid-March 2026, the volume of magma beneath the area reached more than , representing the highest threshold observed in the eruptive series to that point. , the accumulated volume was estimated at between and , more than a year after the latest eruption in the series concluded. The Icelandic Meteorological Office considers a new magmatic intrusion to be more likely than not to occur, which may culminate in a volcanic eruption.

Etymology The name "Sundhnúkur" literally translates to "passage peak". It is the highest point in the old crater row in the area, standing at above sea level. 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. Notably, the passage through the rocks is known by a different name, emphasising that Sundhnúkur specifically denotes the peak itself as a landmark rather than the passage. Sailors of the time also needed to be knowledgeable about various leading marks to safely reach the harbour. 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 aā 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 and 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 400 to 600 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 between 100 and 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 2400–2600 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 seismic swarm was unusually extensive and has been attributed to a roughly magmatic intrusion that originated south-west of Kálffellsheiði , followed the Sundhnúksgígar crater chain, and terminated offshore just beyond the town of Grindavík , reaching a shallowest depth of approximately . The largest earthquakes originated beneath this crater chain, the strongest of which reached magnitude 5.3. This occurred in the Eldvörp–Svartsengi volcanic system, which has experienced 16 magmatic intrusions since January 2020 and 12 of them since October 2023. Nine 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 12 identified volcanic eruptions on the Reykjanes Peninsula.

Earthquakes October 2023 earthquakes An intense earthquake swarm began near Svartsengi, north of Grindavík, on the night of 24 October 2023. By the morning of 25 October, the Icelandic Meteorological Office (IMO) had reported over 1,000 earthquakes, including events of magnitudes 3.9 and 4.5 at around depth. The swarm, interpreted as being triggered by stress changes related to previous intrusive magmatic activity on the Reykjanes Peninsula, continued through 26 and 27 October, with about 5,800 earthquakes recorded since its onset. On 28 October, ground uplift near Svartsengi, likely due to a magmatic intrusion, was detected, and by 29 October, deformation rates were higher than during similar events in 2020 and 2022. Seismic activity had decreased by 29 October, but over 7,000 earthquakes had occurred since the beginning of the swarm. On 30 October, GPS data confirmed that deformation was still ongoing, and modelling suggested that the magmatic intrusion was occurring at about depth, with around 1,300 earthquakes detected on the Reykjanes Peninsula in the previous 24 hours. On 31 October, an intense earthquake swarm beneath the mountain Þorbjörn, with a maximum magnitude of 3.7, indicated continued magma movement. IMO and Civil Defence closely monitored the situation, noting that rapid changes were possible, but that magma movements could also fade without leading to an eruption.

The first clear signs of a new 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, showed accelerated land uplift centred near the Blue Lagoon, approximately north-west of Þorbjörn. It was the fifth occurrence of such uplift in the area since 2020, suggesting increased pressure likely due to magma intrusion. The rapid uplift and associated fracturing raised the likelihood of renewed seismic activity in the area that could potentially allow magma to move closer to the Earth's surface. Geophysical models were used to estimate the depth and size of the intrusion, although experts emphasised that the unrest might still subside without producing a volcanic eruption.

November 2023 earthquakes