Sequential phreatomagmatic eruptions during the lateral propagation of giant dyke swarms

Giant dyke swarms are widely developed on Earth, Venus, and Mars and form by lateral propagation of constituent dykes over hundreds to thousands of kilometers. The failure of these dykes to erupt magma at surface partly accounts for their long-range propagation. However, the lack of preservation of the upper regions of most terrestrial dykes is a barrier to fully explaining the reasons for this upward arrest below ground level. Here we use 3-D seismic and well data to document the near surface expression of dykes comprising the Mull Dyke Swarm from the southern North Sea (UK). The upper tips of these dykes are overlain by linear arrays of craters with deeply eroded bases. Craters are flanked by a tephra apron comprising reworked chalky sediments. The well data allow us to exclude crater forming mechanisms based on collapse into a void but are consistent with those linked to phreatomagmatic eruption. We propose that the rapid cooling of dyke magma during the phreatomagmatic eruptive process was an important contributory factor in the upward arrest of dyke propagation, preventing surface extrusion and hence promoting lateral propagation.