The Icelandic Meteorological Office (IMO) reported that a new eruption at the Sundhnúkur crater row began in a location between Stóra-Skógfell and Sýlingarfell, within the Reykjanes volcanic system, after a brief period of precursory seismicity. The eruption was the tenth to have occurred on the Reykjanes Peninsula since March 2021 and the seventh in 2024. A small earthquake swarm started at 2230 on 20 November and was followed by pressure changes in boreholes at 2237, both indicative of a dike intrusion. At 2300 IMO raised the Aviation Color Code to Orange (the second highest level on a four-color scale). A NE-SW-trending fissure opened at 2314 and propagated NE. Lava fountains rose along the fissure and lava spread laterally from the fissure to the E and W, though most of the advancement was to the W. IMO raised the Aviation Color Code to Red at 2319 and noted that a volcanic plume rose to 2 km (6,600 ft) a.s.l.; the Aviation Color Code was lowered back to Orange at 2354. According to a news article 50-60 people in Grindavík were evacuated along with people at the Blue Lagoon spa and the Svartsengi power station.

IMO confirmed at 0004 on 21 November that no ash was detected in the plume. Gas emissions drifted S towards Grindavík. The Hazard Levels were raised to the highest and second highest levels (on a five-level scale) in five of the six zones, and an updated hazard map was released. Operations at the Keflavík International Airport were not disrupted, though according to a news report at least one flight path to the airport was modified. The eruption had peaked by 0210 on 21 November. The fissure was about 3 km long and no longer propagating; the S end of the fissure was located near Sýlingarfell. The lava effusion rate was an estimated 1,300 cubic meters per second, compared to approximately 2,500 cubic meters per second recorded during the August eruption. The lava front was about 500 m from Grindavíkurvegur. Lava continued to advance W and curved slightly SW around the margins of previous lava flows and earthen barriers constructed to protect the Svartsengi power station and the Blue Lagoon. Workers continued to strengthen the barriers with additional material. Lava flowed over Grindavík road at about 0430 and just before 0800 the lava front reached the Njarðvík Line hot water pipeline; water flow was not disrupted. Lava crossed the Svartsengi power line at around 0900 which caused a power outage in Grindavík and disruptions across the Suðurnes region, based on a news source. IMO noted that by 1155 only the central part of the fissure was active, between Stóri-Skógfell and Sýlingarfell. By midday lava had reached the parking lot of the Blue Lagoon, about 4 km away from the fissure. The lava advanced over the asphalt faster than 100 meters per second during 1209-1335, and engulfed a service building.

Subsidence was detected around Svartsengi. Both seismicity and deformation decreased significantly shortly after the eruption began; only a few minor earthquakes continued to be recorded and deformation was minimal in the vicinity of the fissure. An estimated 10 million cubic meters of lava was effused within the first few hours, about half of what had accumulated in the reservoir since the previous eruption.

Overnight during 21-22 November activity along the fissure had concentrated into three segments, each with a growing cone. Lava from the middle crater flowed W and lava from the end craters flowed mostly E. The effusion rate had decreased significantly and lava advancement on the W part of the lava flow had notably slowed. Lava encountered higher areas of the ground, flowed N, and thickened. One of the hazard zones, Zone 4 which includes Grindavík, was lowered one step to the middle level. On 23 November the eruption continued at a steady rate from three craters and the middle cone remained the most active. No earthquakes were recorded, and subsidence continued to be detected. Lava continued to flow W and inflated along the earthen barriers at Svartsengi and Bláa Lónið. A channel of lava flowed along the S flanks of Stóra-Scógfell.

By 0500 on 24 November activity at the craters subsided, though each continued to produce lava flows. Sulfur dioxide gas was detected in Grindavík during the previous few days. During the morning of 24 November tremor levels sharply declined and continued to slightly decrease during 24-25 November. The eruptive activity remained vigorous and was not decreasing as quickly as recorded during previous eruptions. The northern cone was the most active and lava from the cone flowed E. The southern cone ejected lava above the crater rim through the night but was not active on 25 November. Activity at the middle cone was not seen on 25 November. The surface of the W lava flow near Svartsengi had cooled. The eruption continued during 25-26 November, though both subsidence and eruptive activity decreased. Lava from the northern cone continued to flow E; no movement was detected at the W flow. The total volume of erupted lava was an estimated 43 million cubic meters and covered an area of about 8.5 square kilometers.

Geological Summary. The Reykjanes volcanic system at the SW tip of the Reykjanes Peninsula, where the Mid-Atlantic Ridge rises above sea level, comprises a broad area of postglacial basaltic crater rows and small shield volcanoes. The submarine Reykjaneshryggur volcanic system is contiguous with and is considered part of the Reykjanes volcanic system, which is the westernmost of a series of four closely-spaced en-echelon fissure systems that extend diagonally across the Reykjanes Peninsula. Most of the subaerial part of the system (also known as the Reykjanes/Svartsengi volcanic system) is covered by Holocene lavas. Subaerial eruptions have occurred in historical time during the 13th century at several locations on the NE-SW-trending fissure system, and numerous submarine eruptions dating back to the 12th century have been observed during historical time, some of which have formed ephemeral islands. Basaltic rocks of probable Holocene age have been recovered during dredging operations, and tephra deposits from earlier Holocene eruptions are preserved on the nearby Reykjanes Peninsula.

Sources: Icelandic Meteorological Office (IMO), Icelandic National Broadcasting Service (RUV)