IMO reported that seismicity increased on 28 May in an area near the Sundhnúkagígar crater row and, along with continuing inflation, possibly indicated rising magma within the Reykanes volcanic system. According to news articles seismicity intensified during the morning of 29 May, prompting the evacuation of workers at the Svartsengi power plant, 35-38 residents of Grindavík (three remained), and visitors and residents at the Blue Lagoon spa area. At 1109 on 29 May IMO reported that the seismic swarm was likely related to a new dike intrusion. The Aviation Color Code was raised to Red (the highest level on a four-color scale) and then decreased back to Orange at 1115 because no additional geophysical data indicated significant changes. An effusive eruption began at 1246 near Sundhnúk, NE of Sýlingarfell, with the propagation of a 1-km-long fissure that produced lava fountains at least 30-50 m high and dense, gas-rich plumes rose along the fissure. Radar data indicated that particulates were present in the plume up to 2 km and gases rose as high as 3 km. By 1415 lava had advanced about 1 km W and to the S, towards Grindavík road. A second fissure opened just W of the main fissure and eruption plumes rose as high as 3.4 km. The extrusion rate was estimated to be 1,500-2,000 cubic meters per second. The fissure continued to propagate and by 1450 it was 3.4 km long. Lava surrounded Hagafell to the E and advanced S towards Melhólsnáma based on an overflight conducted by the Coast Guard. The southernmost part of the fissure was less than 1 km from the lava barriers N of Grindavík, and lava flowed over Grindavík road.

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)