When earthquakes reveal the internal plumbing of volcanoes

An international research team has deciphered the seismic signals from a volcano like no other: Tanzania’s Oldoinyo Lengai, the only volcano in the world to produce "cold" lava. Thanks to previously unpublished seismic data, the study - in which Corentin Caudron (Faculty of Science, WEL Research Institute investigator) took part - reveals the hidden mechanisms of this unique volcanic system.

Located in Tanzania’s Rift Valley, the Oldoinyo Lengai volcano has fascinated volcanologists for decades. Its special feature is that it emits "cold" lava - at around 500°C - composed of natrocarbonatites, a material that is virtually unique on Earth. By comparison, the usual temperature of lava coming out of the crater is between 700°C and 1,200°C. While the chemistry of these unique lavas has been extensively studied, the underground structure of the volcano has remained largely unknown until now.

Now, thanks to a temporary deployment of seismic sensors by the University of Mainz (Germany), this has been partly achieved. By analyzing the signals recorded, the team of German, American and Belgian researchers were able to identify very long and complex vibrations, known as tremors. They can last several minutes and are complex to decipher, not least because it’s hard to pinpoint their origin.

Scientists have, however, succeeded in locating them at different depths in the Earth’s crust: some at around 10 km, linked to magmatic outgassing and the upwelling of molten carbonatite through the Earth’s crust, others deeper, up to 20 km, probably reflecting deep magmatic injections into a more superficial reservoir causing resonances, comparable to those of a musical instrument.

These signals, which last for minutes, have the potential to shed light on the structure of volcanoes and magma upwelling. Comparisons between different volcanic systems, with varying chemical compositions, could provide crucial information on pre-eruptive and co-eruptive commonalities and differences, or predict when an eruption will end. In the future, this study could lead to a better understanding of this mysterious seismic signal and, ultimately, help volcanic monitoring agencies.