Recent volcanism on the Reykjanes Peninsula, Iceland

The Reykjanes Peninsula (RP), or rather its volcanism, could be seen as a transition from the ocean ridge volcanism of the Mid-Atlantic Ridge to the hot spot volcanism of the Iceland Plume. Historic volcanic activity in the RP suggest a roughly 1200 year volcanic cycle during which all main volcanic systems there are active periodically during a longer time span of approximately 400 years. These volcanic periods are followed by volcanic quiescence lasting about 800 years. A prospect for the RP is thus intermittent volcanism there for the coming decades, or even centuries. Key to understanding the ongoing eruptions in the RP is to understand where the magma comes from and how it is transported through the crust. This is also important for predicting which systems are likely to erupt in the near future. We show by analyzing the seismicity and with seismic tomography that the magma first erupted on the 19 March 2021 came from a reservoir below 9 km depth in the Fagradalsfjall Volcanic Lineament (FVL). Two eruptions in the same region during 2022 and 2023 followed. In late 2023 volcanism shifted about 4 km to the west to the Sundhnúkur Volcanic Lineament (SVL). Geodetic data has shown that magma accumulated in a shallow reservoir (at about 4-5 km depth) below the Svartsengi geothermal power plant prior to the eruption. Continuous geodetic monitoring shows the inflation of this reservoir between the nine eruptions that has occurred in the SVL since then. An outstanding question is if there is a common source for this magma, and where it is located. Again, with studying the seismicity and refining the tomographic model we show that magma feeding the reservoir beneath Svartsengi is coming from the same source located beneath the FVL where the first three eruptions occurred. This suggest that the two volcanic lineaments (FVL and SVL) are connected, and the system is in fact a two-chamber system. For furthering our understanding of magma transport through the crust to eruption it is important to have good knowledge of geometry of the magma plumbing system, level of major storage zones and the recurrence history of magma injection pulses.