There are regular volcanic eruptions on the Reykjanes peninsula. This is because it sits on the Mid-Atlantic Ridge, which makes the region one of the most volcanically active areas in the world.
The Mid-Atlantic Ridge is a tectonic boundary where two continents are moving apart. Imagine that the whole of Europe is moving away from North America, and in the gap between these two tectonic plates lots of volcanoes are to be found.
This is exactly what’s happening at Reykjanes.