by Sara E. Pratt Tuesday, October 6, 2015
In the 1980s, Tiziana Vanorio was a teenager living in the Italian port city of Pozzuoli west of Naples when the Campi Flegrei volcanic complex that underlies the town and its harbor began to stir. Between 1982 and 1984, the caldera swelled more than 2 meters — the most rapid volcanic uplift ever measured anywhere. The rising seafloor shallowed Pozzuoli’s harbor so that ships could no longer enter. The uplift was followed by a magnitude-4 earthquake and thousands of microquakes that prompted the evacuation of 40,000 residents. Thereafter, the seismicity waned and the residents returned home, but geologists were left with a puzzle: How did the caldera withstand such extreme strain and deformation for so long without rupturing?
Three decades later, Vanorio — now a geophysicist at Stanford University — and co-author Waruntorn Kanitpanyacharoen, a geophysicist at Chulalongkorn University in Thailand who studied the rock at Campi Flegrei while a postdoctoral researcher at Stanford, have discovered the answer: The caldera rock is unusually strong. Its makeup is similar to Roman concrete — renowned for its strength — that was used to construct ports and structures across the Roman Empire, some of which are still partially standing today, including the Pantheon and Coliseum. “This implies the existence of a natural process in the subsurface of Campi Flegrei that is similar to the one that is used to produce concrete,” Vanorio said in a statement.
Vanorio and Kanitpanyacharoen analyzed rock samples cored from the caldera wall that revealed two separate rock horizons above and below the zone of seismicity, the team reported in Science Express. Below the seismic zone lies a basement of calcium-silicate rocks peppered with the mineral actinolite and graphite spherules; above it lies caprock containing pozzolana, the local volcanic ash, which the Roman philosopher Seneca once noted would turn to stone if touched by water.
The researchers propose that both rock types are formed through a series of geochemical reactions driven by fluids heated by the magma lying below the caldera. As hydrothermal fluids percolate up through the basement rocks, actinolite forms and carbon dioxide is released. The carbon dioxide mixes with calcium carbonate and hydrogen within the rock to produce calcium hydroxide, also known as portlandite, a key ingredient in Portland cement. As the lime-rich fluids continue percolating upward, they combine with pozzolana ash to form a concrete-like rock reinforced with fibrous minerals that confer “shear and tensile strength to the caprock, contributing to its ductility and increased resistance to fracture,” the authors wrote.
“This is the same chemical reaction that the ancient Romans unwittingly exploited to create their famous concrete,” Vanorio said, “but in Campi Flegrei it happens naturally.”
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