IMO reported that the eruption on the Reykjanes peninsula from fissures that had opened on 18 December near the older Sundhnúkagígar crater row, about 3 km NE of the town of Grindavík, had stopped by the morning of 21 December. There was a significant decrease in seismicity during 19-20 December, and by 1700 on 20 December lava was erupting from only two craters located at the central part of the fissure. Lava mainly flowed E, though to the N of Stóra-Skógfell a flow to the W continued to be fed but was no longer advancing. Eruptive activity declined later that night or early on 21 December and seismicity continued to decline. Scientists on an overflight during the morning of 21 December confirmed that no eruptive activity was visible and that lava effusion at the craters had ceased. Some areas on the lava-flow field were incandescent, and it was possible that lava continued to flow through tubes. At 1459 on 22 December IMO lowered the Aviation Color Code to Yellow (the second level on a four-color scale) noting that there were no active vents. A total of about 12 million cubic meters of lava had erupted, covering an area of about 3.4 square kilometers.

Seismicity persisted in the Grindavík, Sundhnúksgígar, and Svartsengi regions, was variable daily, though low overall. Ground deformation data from GPS and satellite-based measurements indicated that magma accumulation beneath the Svartsengi region continued. A new hazard assessment and updated map went into effect on 22 December; the areas around the fissure were reduced from very high to high, and the assessment for Grindavík was unchanged (remaining “considerable”).

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.

Source: Icelandic Meteorological Office (IMO)