The ancient Romans had a secret weapon as they established their astonishingly successful empire: access to volcanic rocks whose unusual flexibility inspired enduring building materials. The explanation for these rocks could assist our attempts to replicate Roman concrete, possibly the missing tool in the quest to stop climate change.

The volcanic substance continues to allow unusual geologic behavior in southern Italy. This led Dr. Tiziana Vanorio of Stanford University to explore how the land beneath the Bay of Pozzouli could rise two meters (6.5 feet) in two years from 1982, a rate of uplift never recorded anywhere. The results of the study have been published in the journal Science. 

"The rising sea bottom rendered the Bay of Pozzuoli too shallow for large craft," Vanorio said. On top of that, the city was evacuated after numerous small earthquakes. Vanorio, then a teenager, was one of 40,000 people evacuated.

"Ground swelling occurs at other calderas such as Yellowstone or Long Valley in the United States, but never to this degree, and it usually requires far less uplift to trigger earthquakes at other places," Vanorio said. "At Campi Flegrei, the micro-earthquakes were delayed by months despite really large ground deformations."

Using rocks taken from an early 1980s deep drilling program, Vanorio found that caprock from the Campi Flegrei volcano's caldera contains the fibrous minerals tobermorite and ettringite. These make the rock ductile, or stretchable, when pulled.

^{The fibrous structures of Campi Flegrei​ rocks explain their capacity to lift under pressure. Credit: Tiziana Vanorio}

The fibrous minerals' presence was explained by another mineral, actinolite, which is interspersed with carbonate rocks at the caldera's base. "The actinolite was the key to understanding all of the other chemical reactions that had to take place to form the natural cement at Campi Flegrei," said co-author Waruntorn Kanitpanyacharoen of Chulalongkorn University, Thailand. The interaction of the actinolite, carbonate rocks and pozzolana ash produced an exceptionally resilient rock.

The Romans built their great structures out of a concrete made with very different techniques to those we use today. Its hardiness is demonstrated by the survival of buildings, such as the Coliseum, through many large earthquakes.

Although they would not have seen what was happening deep beneath the Earth, Vanorio thinks the Romans were inspired by seeing volcanic ash mix with seawater, a process Seneca noted produced stone. “Seneca, and before him Vitruvius, understood that there was something special about the ash at Pozzuoli, and the Romans used the pozzolana to create their own concrete,” she said. 

Moreover, Roman mortar was made with crushed volcanic rock, so the local stones represented a resource as well as an inspiration.

We still don't know precisely how the Romans made their concrete. Until recently, the study of their concrete was a curiosity, but it has now taken on life-or-death significance. Currently, 5% of the world's greenhouse gas emissions are associated with the production of Portland cement for concrete, and we lack the obvious replacements that we have for larger sources such as coal.

Roman concrete, however, not only didn't release as much carbon dioxide in the making, it actually absorbed it. Vanorio's work may help us uncover the Roman way, and with it a chance to make our buildings, and our civilization, as enduring as theirs.