Iceland straddles a short stretch of the spreading boundary between the North American and Eurasian tectonic plates. New research by Spaans and Hooper explores how mechanical stress caused by the two plates moving apart contributed to magma emplacement during an eruption of Bárðarbunga volcano in late August of 2014.
Researchers have long had a general understanding of the eruption’s mechanics: Two weeks beforehand, magma began traveling underground away and upward from the ice-covered crater of the volcano in a formation known as a dike. Cutting through the existing rock above, the dike traveled roughly northeast for 50 kilometers before stopping beneath the Holuhraun lava field in central Iceland.
Within days, lava began to erupt from a fissure in the lava field. For 6 months, the fissure released record-breaking amounts of sulfur dioxide gas and more lava than had been produced by any other Icelandic eruption in the past 200 years—1.6 cubic kilometers in total.
Although previous research has revealed this detailed timeline of the dike’s path, the mechanisms underlying its formation have been unclear. The researchers investigated the interaction between two factors that helped open the dike and extend it: pressure from the magma flow itself and existing stress from the two tectonic plates pulling away from each other.
To explore this interaction, the researchers constructed a mathematical, mechanical model of dike formation. They based their approach on a previously developed method that keeps the model computationally manageable by considering only relevant boundaries, like dike walls and magma chamber walls, instead of modeling a much larger volume of rock.
The model used measurements of changes to Earth’s surface that occurred during the eruption, which can hint at what happened underground. Some of these changes were detected in radar images of Earth’s surface captured by satellites in a method known as interferometric synthetic aperture radar (InSAR). Other data came from 31 global navigation satellite system (GNSS) stations positioned around the volcano; slight changes in their relative positions indicate surface changes.
The approach revealed that tectonic forces contributed significantly to formation of the dike at the final, northern stretches of its path. However, the magnitude of tectonic stress was much lower along the dike’s initial path away from the volcanic crater, where magma pressure dominated its formation.
These findings suggest that earlier, undetected volcanic activity within the past 50 years released tectonically induced stress near the caldera, whereas stresses farther north were left unrelieved until the 2014 eruption. Past activity could easily have gone unnoticed because it occurred beneath the Vatnajökull ice cap, and sensitive monitoring equipment was only recently introduced to the area.
In addition to shedding light on the past, the ability to model and understand eruptions like this one could aid efforts to predict when and where lava may erupt in the future. For instance, this new research suggests that a new dike arising from the same crater may not necessarily travel in the same direction and would likely erupt closer to the crater itself. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1002/2017JB015274, 2018)
—Sarah Stanley, Freelance Writer