Dennis Geist

During the voyage of the H.M.S. Beagle (1831–1836), Charles Darwin had the opportunity to visit and observe volcanoes and volcanic deposits at several archipelagos in the Atlantic, Pacific, and Indian Oceans and also in the Andes. In the Galápagos Archipelago, he described hydrovolcanism and what we now know as pyroclastic surge deposits. Darwin also noted the relation between scoria and lava in Strombolian deposits and the role of effective viscosity on lava surface morphology. He observed volcanic eruptions in the Andes from a distance and speculated on the role of earthquakes in triggering eruptions. Darwin's field work on Ascension, the Cape Verde Islands, Mauritius, and the Azores included detailed work on volcanic bombs and the spatial relationship between basalt and evolved lavas. Much of his focus was on making detailed observations at the outcrop scale to deduce large-scale geologic processes, such as the relation between magmatism and mountain building and the distribution of magma in the deep Earth. While many of Darwin's theories involving volcanism did not stand the test of time, in contrast to his theories on coral reefs and the origin of species, they exemplify his genius at developing novel hypotheses and making observations to test those hypotheses.

The lavas associated with mantle plumes may sample domains throughout Earth’s mantle and probe its dynamics. However, plume studies are often only able to take snapshots in time, usually of the most recent plume activity, leaving the chemical and geodynamic evolution of major convective upwellings in Earth’s mantle poorly constrained. Here, we report the geodynamically key information of how the lithology and density of a plume change from plume head phase to tail. We use iron stable isotopes and thermodynamic modeling to show that the Galápagos plume has contained small, nearly constant, amounts of dense recycled crust over its 90-million-year history. Despite a temporal evolution in the amount of recycled crust-derived melt in Galápagos-related lavas, we show that this can be explained by plume cooling alone, without associated changes in the plume’s mantle source; results are also consistent with a plume rooted in a lower mantle low-velocity zone also sampling primordial components.

Using recent advancements in high-performance computing data assimilation to combine satellite InSAR data with numerical models, the prolonged unrest of the Sierra Negra volcano in the Galápagos was tracked to provide a fortuitous, but successful, forecast 5 months in advance of the 26 June 2018 eruption. Subsequent numerical simulations reveal that the evolution of the stress state in the host rock surrounding the Sierra Negra magma system likely controlled eruption timing. While changes in magma reservoir pressure remained modest (<15 MPa), modeled widespread Mohr-Coulomb failure is coincident with the timing of the 26 June 2018 moment magnitude 5.4 earthquake and subsequent eruption. Coulomb stress transfer models suggest that the faulting event triggered the 2018 eruption by encouraging tensile failure along the northern portion of the caldera. These findings provide a critical framework for understanding Sierra Negra’s eruption cycles and evaluating the potential and timing of future eruptions.