Mysterious volcanic ash layer blanketing Mediterranean 29,000 years ago traced to volcano in Naples, Italy

Since the late 1970s scientists have identified the same pre-historic volcanic ash layer in sediment cores extracted from sites ranging across 150,000 square kilometres of the central Mediterranean. This widespread ash layer, dated at 29,000 years ago, blanketed the region and clearly indicated a large volcanic eruption. Whilst the region is well known for its many active volcanoes, such as Mount Vesuvius which famously destroyed Pompeii in 79 AD, scientists had failed to confidently match this older, far-ranging ash deposit to a specific volcano or eruption.

The caldera of the Phlegraean Fields seen from the north with the city of Naples and Vesuvius
 in the background [Credit: Google Earth]

The research, led by Dr Paul Albert, a Research Fellow in the School of Archaeology, has now identified an ash rich-eruption deposit within the city of Naples which was produced by Campi Flegrei volcano and has a chemical composition that matches the prehistoric ash layer traced across the Mediterranean region.

The work was done in partnership with international researchers, including those from the National Institute of Geophysics and Volcanology (INGV), the National Research Council in Italy, the Laboratoire des Sciences du Climat et de l'Environnement in France, and the Berkeley Geochronology Centre in the USA.

The Campi Flegrei caldera cluster [Credit: NASA Earth Observatory image by Jesse Allen
and Robert Simmon, using EO-1 ALI data from the NASA EO-1 team]

“Part of the challenge of reliably attributing this major ash fall event to Campi Flegrei volcano has been that there is limited evidence for a large eruption close to the volcano,” says Albert. “This is in part because a more recent large-scale eruption of the volcano buried the Naples area in a thick ash deposit, largely destroying or concealing the evidence of this older event,” says Albert.

The team used a computer-based ash dispersal model to reconstruct the size of the eruption. “By linking the thickness of the ash deposits found in Naples, to those preserved in cores from across the central Mediterranean, the model was able to demonstrate and provide important constraints on the size of this large magnitude eruption,” says Albert.

View of the eastern part of the populated Campi Flegrei caldera from the Camadoli Hill. The volcano is the source
of the Masseria del Monte Tuff eruption 29,000 years ago. More than 300,000 people live within this active caldera
 that has experienced more than 60 eruptions in the last 15,000 years. One of these recent eruptions formed
the small Nisida cone that is seen lying just off the mainland. The Bay of Naples and Capri
can be seen in the background [Credit: Victoria Smith]

This research positions the timing of this previously un-reported large-scale eruption of Campi Flegrei between two well-known large-scale eruptions of the volcano, at 15,000 and 40,000 years ago, drastically reducing the reoccurrence interval of large magnitude eruptions at the volcano.

The research, published in the journal Geology, also highlights the importance of considering ash fall events preserved well away from the volcano when reconstructing the timing and scale of past explosive eruptions. “Ash fall preserved hundreds of kilometers away from the volcano has been critical here in the identification and reconstruction of this large eruption at Campi Flegrei,” says Albert.

Source: Oxford University [April 25, 2019]