IMO reported that a new eruption began in a location between Stóra-Skógfell and Sundhnúkur, within the Reykjanes volcanic system, after about a month of increasing seismicity and inflation and about a week of more intense seismicity and larger earthquakes. A 20 August report noted that seismicity had increased during the previous few days and that the estimated volume of magma that had accumulated beneath the surface was greater than it was prior to any previous eruption in the region. Two earthquakes with magnitudes greater than two were recorded during 19-20 August, making a total of six earthquakes with similar magnitudes recorded during the previous week. An earthquake swarm began at 2048 on 22 August and increased pressure was identified in borehole data; these data indicated that a dike had begun to propagate. At 2111 IMO raised the Aviation Color Code to Orange (the second highest level on a four-color scale).

A fissure eruption began at 2126 on 22 August and within four minutes a gas plume had risen 1 km high and began drifting S. The Aviation Color Code was raised to Red at 2130, and a news source reported that possibly more than 1,000 people were evacuated from the Blue Lagoon Spa area. The fissure produced lava fountaining along its roughly SW-NE axis and propagated mostly NE, and by 2230 was 3.9 km in length. According to a news source observers noted that the lava fountains were likely taller than those from the previous eruptions in the area. Seismicity was concentrated near the N end of the fissure system. Lava flows spread E and W, towards Grindavíkurvegur; flows did not head towards Grindavík. At 2145 the Aviation Color Code was lowered back to Orange because ash was not detected in the gas-rich plumes rising from the fissure. A M 4 earthquake recorded at 2237 was located about 3 km NE of Stóra-Skógsfell. As magma propagated from the reservoir to the surface a maximum of around 40 cm of land subsidence was recorded.

As the fissure opened to the NE, activity along the S end of the fissure declined. The fissure consisted of at least two segments; the main segment that had opened first and a second shorter segment that was offset to the NW from the end of the original segment; the total length of the fissure was about 7 km long according to a news source. Seismicity rapidly decreased at around 0400 on 23 August. By 1000 the vigor of the eruption had decreased and was confined to the northernmost fissure segment. Explosive activity at around 1030 produced dark ash plumes that rose 1 km and local tephra fall based on webcam views. Lava was mostly moving to the NW, N of Stóra-Skógfell. By 1600 the rate of both the seismicity and ground deformation had decreased, and activity along the main fissure was low. Lava flows had caused extended wildfires in the area. Sulfur dioxide plumes from the eruption drifted more than 1,000 km SE towards Scotland. The lava extrusion rate on 24 August was estimated to be around 100 cubic meters per second, lower than the estimated rate of 1,500-2,000 cubic meters per second during the beginning phase of the eruption.

During 25-26 August the activity was centered in one area at the N part of the fissure. Lava advanced N in two main channels though the advancement had slowed considerably. The lava extrusion rate had decreased to an estimated tens of cubic meters per second. Modeling suggested that 17-27 million cubic meters of magma had erupted, covering an area totaling 15.1 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)