Their model sheds light on how Galápagos volcanoes grow, which has been a subject of debate since Darwin’s time, and may also help in forecasting the locations of future eruptions, adding to the vast scientific knowledge acquired by study of this iconic island chain.
In the study, Marco Bagnardi, a doctoral candidate at the UM Rosenstiel School of Marine & Atmospheric Science (RSMAS) and visiting scientist at USGS' Hawaiian Volcano Observatory, analyzed surface deformations on Fernandina from European Space Agency (ESA) ENVISAT satellite images acquired just two hours before the 2009 eruption. He sought to explain why Hawaiian and Galápagos volcanoes, while similar in some respects, show different eruptive patterns. Eruptions from Hawai‘i's volcanoes tend to occur along narrow rift zones that extend radially from the summit, while the shields of western Galápagos volcanoes have eruptive fissures that circle the summit near the calderas but are oriented radially on the flanks below.
Bagnardi found that magma began moving away from Fernandina's reservoir as a sill, or horizontal feature, before rotating vertically and erupting in a fissure perpendicular to the summit on the volcano's southwest flank. A sill also fed the 2005 eruption at Fernandina, but that magma moved upward to form a fissure circling the volcano's summit closer to the top. These data suggest that sills feed all eruptive activity in the Galápagos but that they rotate in different ways as they propagate toward the host volcano's surface.
"Our findings have literally turned our perspective by 90 degrees," Bagnardi said. "For decades, we thought that eruptions in the Galápagos were starting with the intrusion of vertical blade-like bodies. We now know this is not the case."
"Without knowing why the fissures are arranged the way they are, we had no way to infer the geologic history of the volcanoes and how they evolved over time," said USGS geophysicist Mike Poland, a co-author of the study. "This research allows scientists to better model how the volcanoes grow and behave, especially with respect to past and future activity," Poland said. "You can't move forward with solving the puzzle because you are missing a key part of the story. This work fills in a major missing piece and will allow better interpretations of a multitude of parameters about Galapagos volcanoes."
UM RSMAS is part of a long-term satellite-based monitoring program of the Galápagos volcanoes using Interferometric Synthetic Aperture Radar (InSAR), a technique that uses repeat satellite observations to measure millimeter-scale ground displacements. Bagnardi credited the "extremely fortunate" timing of data collected by the ENVISAT satellite, which passed over the island just when magma was already on its way toward the surface. Fernandina erupts every four to five years, on average, and the satellite passed over the archipelago only once every several days, he said.
The team theorizes that Hawaiian and Galápagos volcanoes behave differently because neighboring volcanoes of the Galápagos grew concurrently and did not affect one another as they formed. Hawaiian volcanoes, however, grow sequentially, meaning that older volcanoes control the structural development, including eruptive patterns, of younger edifices.
Based on the relations between deformation and eruptions at Fernandina in 1995 and 2005, the authors argue that the next eruption of Fernandina will be from a fissure circling the volcano's summit on the southwest side of the caldera in the area uplifted by the 2009 sill intrusion.
"Unfortunately, we are still not able to predict the timing of future eruptions in the Galápagos," Bagnardi said. "However, providing a forecast for the location and type of eruption is a step in the right direction."
Satellite imagery used in the study was also provided by the Japanese Space Exploration Agency (JAXA). This research was supported by the National Aeronautics and Space Administration (NASA) and the National Science Foundation (NSF).
The paper in Earth and Planetary Science Letters, "A New Model for the Growth of Basaltic Shields Based on Deformation of Fernandina Volcano, Galápagos Islands," by Marco Bagnardi, Falk Amelung and Michael P. Poland, is available online.