Because of their location’s geology, Icelandic volcanoes are known to be directly connected to the Earth’s mantle. New research shows that a volcano in Costa Rica may have a direct connection to the mantle too, despite lying on a much thicker part of Earth’s crust. This discovery suggests that eruptions in a wider range of volcanoes might be predicted by techniques such as earthquake monitoring, something proposed for Icelandic volcanoes. But it also suggests that volcanoes can recharge with magma very quickly, raising the risk of a sudden eruption.
Tectonic plates cover the Earth’s surface. The boundaries between these plates can be dangerous places, with earthquakes, volcanoes, and tsunamis all originating along the places where plates meet. The plates create two types of volcanoes. Those that sit on oceanic crust are directly connected to the mantle (the thick layer between the Earth’s crust and molten outer core); they spew magma directly from it. Others erupt magma that has been heated, mixed and stored in magma chambers over an extended period. These are assumed to lack a direct connection to the mantle.
Icelandic volcanoes are an example of the former, occurring on the mid-ocean ridge of the North Atlantic where crust is constantly being created. In contrast, most volcanoes around the Pacific rim, including the Costa Rican volcano, have been assumed to be examples of the latter.
That, however, turns out not to be the case, as shown by the work of Philipp Ruprecht and Terry Plank of Columbia University, who published their results in Nature. They studied ash ejected during an eruption of the Irazu volcano in Costa Rica, which occurred between 1963 and 1965, to draw this conclusion.
They measured the amount the metal nickel in olivine crystals found in the ash. These crystals preserve variations in nickel content depending on how they are formed within the magma. Thus, studying this variation can reveal where the magma actually came from and how long its journey from the depths took. From their data, Ruprecht and Plank could detect that there was a shallow chamber underneath the volcano that refills with melt from the mantle.
While such chambers are common, what was surprising was the timescale at which the chamber refilled. It took as little as a matter of months, which is rapid for geology. The Irazu volcano is an arc-type volcano, which is formed when oceanic crust dives into the mantle beneath a continental plate, causing melting. In an arc volcano, a quick refill could happen only if the magma chamber is directly connected to the mantle. (These arc-type volcanoes are a subset of “stratovolcanoes” that are capable of huge explosive and devastating eruptions that throw ash high into the stratosphere, more than 10km above the ground.)
According to Sally Gibson of the University of Cambridge, who was not involved in the study, “It has widely been assumed that the so-called ‘arc’ volcanoes magma is the result of stalling and crystallisation of primitive mantle melts in long-lived chambers that exist over a range of depths in the continental crust. Ruprecht and Plank’s findings show that the melts beneath the Irazu volcano must have ascended through the crust very rapidly.”
Gibson suggests that this sort of migration of the magma is similar to that found in Icelandic volcano systems. The Icelandic magmatic movement can be detected by seismic activity in the area, so Ruprecht and Plank’s results suggest that geophysical seismic monitoring of small earthquakes in the deep crust may provide advance warning of eruption risks. Of course, given the speed of refilling seen here, we may have to extend monitoring to volcanoes that were generally thought to be at a relatively low risk of eruption.