One of my favourite stops on the first-year disasters field trip is near Whistler, where columnar basalts curve to a central point. Since columnar basalts crack into pentagons perpendicular to the lava’s cooling surface, the standing theory is that this particular chunk of geology cooled under a glacier.
In BC, we’re along a subduction zone. As the oceanic plate subducts (if it’s pulled or pushed is a big debate!) under the continental plate, the oceanic plate melts and the magma fuels volcanism. The basaltic magmas increase in silica content as it passes through (and partly melts and mixes with) the continental plate (andesitic magmas) before feeding volcanism along the coast. Mount Saint Helens and Mount Baker are part of our volcanic chain, with characteristically large, explosive eruptions capable of sending ash into the upper atmosphere (and thus impacting global climate).
Iceland is both above a hotspot and straddling the mid-Atlantic rift zone, a double-source for basaltic low-silica magmas, but in some locations as the melt passes through the tiny bit of continental plate, the silica content rises, and with it, the explosivity of the lava. Iceland was on watch this weekend, fearing a sub-glacial eruption and the resulting explosion of steam, melt, flooding, and mudslides. Instead, a small eruption took place… but in a volcano that has been a precursor to eruptions in a larger nearby volcano each of its past three eruptions (spanning 1,000 years).