The Fires

by Sigríður Hagalín Björnsdóttir

The magma will acidify and become more viscous as it accumulates, such that if it’s been biding its time under the crust, it has the potential to erupt explosively, even though magma on the Reykjanes Peninsula is generally mafic and tends to have a low viscosity. Mafic and low viscosity: that’s by far the likeliest outcome. But then we also need to allow for the possibility that the lava will come into contact with a considerable quantity of water, particularly in the hot spring area at Krýsuvík. We must always be prepared for some explosive activity, although I think it’s unlikely that that would characterize this eruption.

As magma cools down slowly, underground at depth, crystals of minerals grow. That changes the viscosity of the magma and its chemical composition. The longer it sits underground, the more ends up like lumpy sticky rock porridge, which can trap its gases. When that magma reaches the surface, the eruption is more explosive and destructive because of those trapped gases, then if it had been simple mafic, runny magma. I was also taught that any eruption that interacted with water or wet rock was 4 times as explosive than if no water was involved, because of the expansion of water into steam.