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Grabens, or valleys formed during extensional tectonic events, are common but rarely observed during formation. In November 2023, inelastic surface deformation formed abruptly along Iceland's plate boundary in Grindavík. We documented graben formation in real‐time through satellite mapping (InSAR), seismicity, GNSS data, repeated lidar surveys, and field mapping. Five normal faults and ∼12 fissures ruptured the surface delineating two grabens separated by a horst, a context not present in other contemporary case studies. The graben normal faults slipped rapidly (over hours) and maximum surface motions coincided with the occurrence of turbulent seismic swarms in both space and time. Although 3 eruptions took place ∼15 km northeast of Grindavík from 2021 to 2023, attributed to magma intrusions (i.e., dikes), none of these also formed grabens. Thus, the Grindavík grabens shows evidence for tectonic origins. Real‐time monitoring of these phenomena provide insight into graben formation on Earth and potentially on other planets. Plain Language Summary: Valleys known as grabens, typically caused by extensional tectonic events, are a common geological feature, yet their formation is rarely observed. In November 2023, such two grabens suddenly emerged along the boundary between tectonic plates in the town of Grindavík, Iceland. The grabens were shaped by tectonic activity expressed by a swarm of earthquakes and associated ground deformation and resulted in normal faults rupturing the surface and cutting through houses, linear infrastructure including water pipelines and roads within Grindavík. We document the grabens and horst in real time using satellite mapping (InSAR), geodetic GPS data, repeated drone surveys, field mapping, and seismic recordings from a local seismic network. The combination of these data provide unprecedented level of precision in characterizing surface and sub‐surface deformations, especially when maximum surface motions correlate with a turbulent seismic activity in both space and time. Critically, following the formation of the grabens, a series of intensive through short‐lived eruptions happened in December 2023, January 2024, February 2024, March, and May 2024 within the northernmost portion of the grabens. Our observations provide valuable insights into the natural laboratory of Iceland and contribute to the understanding of plate tectonics both on Earth and other planets. Key Points: The 2023 seismic swarm and extension lead to a graben forming that coincides in time and space along Iceland's plate boundaryDuring 2021 to December 2023 only one tectonic graben formed, although 4 eruptions have taken placeThe graben in Grindavík, Iceland is wider at the surface than most recently documented grabens (∼4,500 m vs. <1,000 m) [ABSTRACT FROM AUTHOR]

The Reykjanes Peninsula (RP) hosts several volcanic lineaments that have been periodically active over the last 4000 years. Since 2021, following a ca. 800‐year quiescence, eight eruptions have occurred on the RP, with more expected in the future. To better understand the origins of this renewed volcanism and help forecast future eruptions, we examine (i) if the ongoing volcanism is fed from a single or multiple magma storage zone(s) or from several smaller reservoirs and; (ii) where the zone(s) are located (i.e. mantle or lower or upper crustal depths). Using major and trace element geochemistry, oxygen isotopes, and seismic tomography we rule out a single, RP‐scale, deep‐seated magma storage zone. Instead we propose the presence of a ca. 10‐km‐wide region of crustal‐level (9–12 km) magma accumulation beneath the Fagradalsfjall volcanic lineament that fed both the 2021–23 eruptions of the Fagradalsfjall Fires and the 2023–24 eruptions of the Sundhnúkur Fires. [ABSTRACT FROM AUTHOR]