Volcanoes are usually divided into two sorts, those that produce rivers of fire that threaten only the general vicinity and the ones whose explosive eruptions can plunge the world into darkness and unleash enormous tsunamis. However, a new study suggests the truth is more complex, and we shouldn't get too comfortable about the first type.
The manner of a volcano's eruptions is determined by the composition of its magma. Basaltic magmas produce the lava rivers of Hawaii, while more silica-rich lava creates a Vesuvius or Krakatoa. Volcanologists' assumption has been these magmas are fairly constant over time.
However, Dr Michael Stock of Trinity College Dublin led a team to investigate two of the volcanoes that formed two of the Galápagos islands, Wolf and Fernandina, both known for their basaltic lava flows.
Stock and co-authors report in Nature Communications finding a wide variety of chemical compositions. Some lava, tephra, and nodule samples from these volcanoes look like they would be much more at home at Mount St Helens, Washington, site of a famous explosive eruption in 1980, than the shield volcanoes of the islands on which they were found.
As in the more famous Hawaiian example, the Galápagos islands lie over a plume of hot magma that has pierced the oceanic crust, and pushed mountains above the waterline. The plate is moving over the plume, so older islands represent the sites that were once directly above it.
Stock thinks liquids with higher silica contents get trapped in chambers around a kilometer beneath the surface. Near a plume's center basaltic magma rises so fast materials with other compositions are washed out to the point we barely notice them.
However, once they're only at the edge of a hotspot, the rate at which magma rushes up from below declines and may no longer overwhelm the contents of these chambers, making explosive eruptions possible.
“This was really unexpected,” Stock said in a statement. “We started the study wanting to know why these volcanoes were so boring and what process caused the erupted lava compositions to remain constant over long timescales. Instead we found that they aren't boring at all, they just hide these secret magmas under the ground."
Wolf and Fernandina represent no threat, at least on human timescales – they still have ample amounts of basaltic material recharging them. The situation is different further from the plume's center in the Eastern Galápagos, however, where the volcanoes have much lower flux rates. There is a much greater danger that after thousands of years of basaltic lava production, these volcanoes will one day switch to something far more dangerous, and the same could apply to other volcanoes worldwide we currently consider safe.