Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Shaul Hurwitz, research hydrologist with the U.S. Geological Survey.

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The city of Naples (Napoli) in southern Italy is surrounded by three active volcanoes: Vesuvius, Ischia, and Campi Flegrei (Phlegrean (or burning) Fields). More than 3.5 million people in the metropolitan area live within about 30 km (19 mi) of one of these volcanoes. The volcanoes are monitored by the National Institute of Geophysics and Volcanology (INGV) Vesuvius Observatory (Osservatorio Vesuviano in Italian)—the first volcano observatory in the world, established in 1841. The observatory uses many of the same monitoring methods as the Yellowstone Volcano Observatory, including measurements of seismicity, deformation, gas emissions, and thermal activity. The Italian Civil Protection Department is the agency responsible for the communication and mitigation of volcanic hazards and risk.

Vesuvius (Vesuvio) is a stratovolcano south of Naples with many towns on its flanks. The volcano was constructed within a large caldera that formed incrementally beginning about 17,000 years ago. Eight major explosive eruptions have taken place since, often accompanied by large pyroclastic flows and surges, such as during the 79 CE eruption that buried the Roman towns of Herculaneum and Pompeii and that were described in letters written by Pliny the Younger. Intermittent eruptions since 79 CE were followed by a period of frequent long-term explosive and effusive eruptions between 1631 and 1944.

Volcanism at the island of Ischia began more than 150,000 years ago, and the most recent eruption occurred in 1302 CE. The largest eruptions took place between 74,000 and 55,000 years ago, culminating with the formation of a small (compared to Yellowstone!) caldera. In the following millennia, volcanism alternated between periods of reduced and periods of sustained activity. In the last 2,900 years, 34 effusive (lava) eruptions accompanied by some explosive activity have had variable impacts on the island’s environment and human settlements. The magmatic system of Ischia is still active, as manifested by historic volcanism, active fumaroles, and thermal springs.

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Campi Flegrei, to the west of Naples, generated the largest volcanic eruption to occur in Europe in at least the last 200,000 years. The upper half of the collapse caldera, with a diameter of about 13 kilometers (8 miles), is exposed on land, with the town of Pozzuoli within the center, while the lower half is submerged beneath the Bay of Naples and the Tyrrhenian Sea. The caldera formed during the enormous Campanian Ignimbrite eruption 40,000 years ago with a volume of 181–265 km³ (43–64 mi³) classified 7 on the Volcanic Explosivity Index. That eruption covered much of southern Italy and the eastern Mediterranean region with thick layer of volcanic ash. A major eruption 15,000 years ago produced the Neapolitan Yellow Tuff, covering an area larger than 1,000 km² (386 mi²) with an estimated volume of about 40 km³ (about 10 mi³) of erupted magma. While impressive, the volumes of magma are significantly less than the volume of magma erupted during the formation of Yellowstone caldera, which was about 1000 km³ (240 mi³).

Records of deformation at Campi Flegrei go back to Roman times. Marble columns of a 2,000-year-old at the Pozzuoli marketplace provide a remarkable record of long-term uplift and subsidence patterns. The lower thirds of the columns are pocked with holes from burrowing sea mollusks, which means the marketplace was built when the land was above sea level, subsided by about 7 meters (23 feet) and was inundated by seawater, and then rose out of the water again—all within about the last 2,000 years!

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Since the Neapolitan Yellow Tuff eruption there have been dozens of small eruptions, leaving small craters scattered around the caldera. The most recent eruption, which formed the Monte Nuovo (new mountain) cone, occurred in September 1538 and lasted for six days. The eruption followed a century of uplift, several years of frequent earthquakes, and increased gas emissions. Prior to the Monte Nuovo eruption, there was an eruption hiatus of 3,000 years, but more than 70 eruptions occurred between around 14,000 and 3,500 years ago.

More recently, several major unrest episodes at Campi Flegrei were recorded by modern instruments. Uplift episodes accompanied by increased earthquake activity occurred between 1969 and 1972 and then between 1982 and 1984, when uplift of the caldera totaled 1.85 meters (6 feet). By comparison, the largest uplift episode measured at Yellowstone during the past few decades between 2004 and 2009 was 0.25 meters (10 inches). The 1982–1984 unrest led to the evacuation of over 40,000 residents from the town of Pozzuoli due to earthquake activity that destabilized the landscape and damaged buildings. Since 2005, Campi Flegrei has uplifted by 1.1 meters (3.6 feet), with more than 18 centimeters (7 inches) since July 2022.

The volcanologists of INGV use many of the same tools to monitor Vesuvius, Ischia, and Campi Flegrei as YVO volcanologists use in Yellowstone. And INGV and USGS volcanologists frequently collaborate, learning from one another so that we can better understand how the volcanoes in our respective countries work!  For more information on the volcanoes of the Naples region and their activity, check out the INGV Osservatorio Vesuviano website at https://www.ov.ingv.it/.