After about 40 minutes of increased seismicity and ground deformation, a fissure eruption within the Reykanes volcanic system began at 2023 on 16 March near the older Sundhnúkagígar crater row on the Reykjanes Peninsula, prompting IMO to raise the Aviation Color Code to Red (the highest level on a four-color scale). According to a news report about 700 people at the Blue Lagoon spa and the few people in Grindavík were evacuated within about a 30-minute period. IMO noted that the fissure quickly lengthened to 2.9 km and that the length and location was similar to the 8 February fissure eruption. The fissure was oriented roughly NE-SW and small fissure segments were aligned in the same orientation but offset at each end. A steam-and-gas-rich plume rose above 3 km; no ash was evident in the plume so IMO lowered the Aviation Color Code to Orange at 2122. Lava fountaining occurred along the length of the fissure and lava flows advanced E, SE, SW, and NW at a rate of about 1 km per hour. By 2210 the S flow was about 200 m from the earthen barriers constructed to protect the E part of Grindavík. Lava advanced NW, curved around the Stóra Skógfell cones, and then flowed SW; by 2220 lava was 700-800 m from Grindavíkurvegur (Road 43) and advancing towards the road at a rate of about 660 m per hour. At around 0030 on 17 March lava flowed W over the road, along the earthen barrier, and towards the water distribution pipe from the Svartsengi power plant; the flow slowed during the morning about 200 m from the pipe, only advancing minimally.

Eruptive activity decreased overnight during 16-17 March. Seismicity significantly decreased with only a few earthquakes recorded after 0300 on 17 March, coinciding with decreased tremor. Lava flows to the S were diverted from Grindavík along the barriers towards the SE. The effusion rate decreased substantially at around 0400, and lava was produced by a segment near the middle of the fissure that was 500 m long. By 1300 lava fountaining was concentrated at three areas along the fissure. The S flow advanced at a rate of about 12 m per hour during 1015-1630 and a few hours later the leading lobe was about 330 m from Suðurstrandarvegur, the main road along the S coast of the Reykjanes Peninsula. During the morning sulfur dioxide emissions peaked at 15,000 micrograms per cubic meter and emissions detected by satellite that day were the highest measured of the recent 2023-2024 eruptions. Sulfur dioxide fluxes were as high as 50 kilograms per second. A news articles noted that some small lava ponds formed near the Grindavík barriers and at the flow near Suðurstrandarvegur. The area of the flow field was an estimated 5.85 square kilometers based on a satellite image acquired at 1456 on 17 March.

The eruption continued at stable levels during 18-19 March. Lava activity was concentrated at a series of vents which had built cones at the S end of the fissure; occasional fountaining was observed. The lava flows that had crossed Grindavíkurvegur and stopped near Suðurstrandarvegur were slow moving. Deformation data suggested that magma continued to flow into the dyke system. According to Almannavarnadeild ríkislögreglustjóra (National Commissioner of the Icelandic Police and Department of Civil Protection and Emergency Management) Grindavík residents were permitted to return to town on 19 March, though it was not recommended that they stay overnight. The Blue Lagoon remained closed.

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), Almannavarnadeild ríkislögreglustjóra (National Commissioner of the Icelandic Police and Department of Civil Protection and Emergency Management), Icelandic National Broadcasting Service (RUV), Simon Carn