International research team dates most recent eruptions – these occurred in short pulses

The West Eifel Volcanic Field comprises more than one hundred maars – circular lakes formed by explosive eruptions. Until now, little was known about when these volcanic eruptions occurred. A research team from Heidelberg University, Curtin University in Perth (Australia), and the University of Göttingen has now reliably dated the most recent eruptions of the maar volcanoes. Their studies show that many of them were active in short pulses across large parts of the West Eifel at nearly the same time. The strongest and most recent of these eruption episodes occurred about 25,000 years ago. The new findings on the dynamics of the West Eifel maar volcanoes are significant for research into other volcanic fields of this type as well as for hazard assessment.

Maar volcanoes are among the most common volcano types worldwide. They were first scientifically described in the West Eifel, from whose maars this type of volcano takes its name. Until now, it has been virtually impossible to reliably determine their age using conventional geochronological methods. The scientists from Heidelberg, Perth, and Göttingen have now applied the (uranium-thorium)/helium dating method to zircon crystals from crustal xenoliths – rock fragments from the Earth’s crust that are carried along by rising magma and thus reach the surface during a volcanic eruption. “This workaround was necessary because the Eifel magmas themselves do not contain zircon, whereas crustal xenoliths typically contain a great deal,” explains Anne Sturm, a doctoral candidate conducting research at the Institute of Earth Sciences at Heidelberg University. The dating method relies on the radioactive elements uranium and thorium contained in the zircon crystals. These elements decay at a known rate to form helium, which escapes at high temperatures but becomes trapped within the crystal as it cools. “By measuring how much helium is stored in the zircon, we can determine the time when the crystal and the surrounding rock must have cooled – that is, the time of the eruption,” says the junior researcher.

Using zircon (uranium-thorium)/helium dating, the research team has now been able to establish, for the first time, a consistent age model for several of the best-known maars in the West Eifel. The analyses show that the maar volcanoes of the West Eifel were last active in several short pulses. During these, numerous volcanoes erupted almost simultaneously across large parts of the West Eifel. Three of these eruptive episodes occurred approximately 75,000, 48,000, and 25,000 years ago. During the most recent and most violent phase of eruptions – which occurred during the coldest period of the last Ice Age – the volcanic lakes known as the Dauner Maare were formed, as well as the Pulvermaar, a site of significant tourist interest. According to the scientists, the onset and peak of these eruption phases coincide with a significant drop in sea level in the North Sea basin. Their chronology of the eruptions of the West Eifel maars thus contradicts previous models, which link volcanic activity to climate warming during interglacial periods related to sea level rise.

“Our research also shows that, contrary to previous assumptions, continental volcanic fields are not subject to steady volcanic activity but can become active in short and sometimes very intense bursts,” emphasizes Prof. Dr. Axel Schmitt, a scientist at Curtin University, who conducts research on isotope geology and petrology as an affiliate of the Institute of Earth Sciences at Heidelberg University, his former home institution. According to the geoscientist, these findings can also serve as a basis for improved volcanic hazard assessments, since many continental volcanic fields are densely populated or developed for tourism.

The German Research Foundation (DFG) and the Australian government, as part of the National Collaborative Research Infrastructure Strategy (NCRIS), funded the research. The results have been published in the journal “Contributions to Mineralogy and Petrology”.