[1] No volcanic eruption had occurred for 815 years on the Reykjanes Peninsula until 19 March 2021 when a fissure vent appeared in Geldingadalir to the south of Fagradalsfjall mountain.

[12][13] Due to its relative ease of access from Reykjavík, the volcano has become an attraction for local people and foreign tourists.

Plate spreading at the Reykjanes peninsula is highly oblique and is characterized by a superposition of left-lateral shear and extension.

[25] The Krýsuvík volcanic system has been moderately active in the Holocene, with the most recent eruptive episode before the 21st century having occurred in the 12th-century CE.

[45] Reports stated a 600–700-metre-long (2,000–2,300 ft) fissure vent began ejecting lava,[46] which covered an area of less than 1 square kilometre (0.39 sq mi).

[52] Around noon on 5 April 2021, a new fissure, variously estimated to be between about 100 and 500 metres (300 and 2,000 ft) long, opened a distance of about 1 kilometre (0.6 mi) to the north/north-east of the still-active vents at the center of the March eruption.

[56] The lava production of all open eruption fissures in the whole was estimated on 5 April 2021, being around 10 m3/s (350 cu ft/s) [57][58] and is flowing into the Meradalir valleys ([ˈmɛːraˌtaːlɪr̥], "mare dales") via a steep gully.

[68] A number of smaller openings appeared temporarily, one small vent was reported to have erupted near the main crater on 1 July.

[76][77][78] Results from measurements published by University of Iceland on 26 April 2021 showed that the composition of eruption products had changed, to more closely resemble the typical Holocene basalts of Reykjanes peninsula.

[80] While it was known that most mid-ocean ridge (MOR) basalts result from crustal reservoir melts this proved the suspicion that more rarely, mid-ocean ridge eruptions may be supplied from levels deeper than 7 km (4.3 mi), as the near real time lava sampling prevented later lavas confusing the time sequence of the change in composition.

[80] The eruption itself also changed in character at the same time,[81] and was producing lava fountains up to 50 m (160 ft) in height on Sunday, 25 April 2021.

[85] The lava jets have been explained as explosive release of ancient trapped water or magma coming in contact with groundwater.

[92] From the composition of the magma sampled, they also believe that there is a discrete vent feeding the main lava flow from a depth of 17–20 kilometres (11–12 mi) from the Earth's mantle, and may be of a more primitive kind than those previously observed.

[94] The formation of such volcano has not been studied before in real time, and this eruption can offer insights into the working of the magmatic systems.

[13] Two defensive barriers were created starting 14 May as an experiment to stop lava flowing into the Nátthagi valley where telecommunication cables are buried, and further on to the southern coastal road Suðurlandsvegur.

[111] The lava flow can be tracked by helicopter or satellite, for example via radar imaging that can penetrate through the clouds and volcanic smog that had become more frequent in the area by July.

[133] Measurement taken on 27 July indicated that the lava flow had increased again, returned to and possibly exceeding the peak level last seen in June.

[150] Iceland's Department of Civil Protection and Emergency Management stated that no lives or infrastructure were currently at risk from the eruption.

[156] The eruption was not producing large plumes, though it was likely to affect air quality and pollution in immediately surrounding areas.

[184] The latest Icelandic Institute of Earth Sciences statistics revealed on 31 July indicate a notable reduction of the effusive eruption.

[188] Due to the volcanic site's proximity to the town of Grindavík, Vogar and to a lesser extent Keflavík, Keflavík International Airport and the Greater Reykjavík Area, Iceland's Department of Civil Protection and Emergency Management has created protocols for evacuation plans of nearby settlements and in case of gas pollution and/or lava flows.

The ICAO Aviation Colour code has mostly stayed orange (ongoing eruption with low to no ash production).

[200] In addition, if the lava flows travel northwards, an important high-voltage transmission line to Keflavík is in danger of being cut off.

Many parts of the route are extremely steep with uneven rocky ground, as well as being poorly signed due to the recency of the eruption.

Depending on the wind direction, toxic gas pollution can be a risk as well as unpredictable lava flows and new fissures opening up.

Authorities have used Location Based SMS messages to inform and warn tourists travelling to the site to be prepared.

The site is manned during busy periods by the volunteers from the Icelandic Association for Search and Rescue, as well as local police.

The 2023 eruption produced more volcanic gases as well as sparking some of Iceland's largest moss wildfires, creating much more dangerous respiratory risks for hikers.

[211] The area where the volcano first erupted is thought to be the burial site of an early Norse settler Ísólfur frá Ísólfsstöðum [ˈiːsˌoulvʏr frauː ˈiːsˌoul(f)sˌstœːðʏm].

[212] However, a quick archaeological survey of Geldingadalur after the eruption started in 2021 found no evidence of human remains in the area.