Mount Vesuvius

• 1. Introduction

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  Mount Vesuvius rises about nine kilometers east of Naples, on Italy's Gulf of Naples in the region of Campania. In AD 79, it sent a column of stone, ash and volcanic gas 33 kilometers into the sky. Within two days, the Roman towns of Pompeii, Herculaneum, Oplontis and Stabiae were gone. More than a thousand people are thought to have died, though nobody knows the true number. The only surviving eyewitness account comes from a young Roman named Pliny, who wrote of his uncle's final hours. Vesuvius never stopped erupting. It remains active today, and it is one of the most closely watched volcanoes on Earth. Roughly three million people now live close enough to be affected if it erupts again. At least 600,000 of them are inside the official danger zone. Why does a single mountain carry that much risk nearly two thousand years later? And what actually happens underground to make it explode the way it does?

• 2. Names And Gods

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  Writers in the late Roman Republic and early Empire called the mountain Vesuvius. Variants such as Vesaevus, Vesevus, Vesbius and Vesvius also appeared, and Greek authors used their own spelling of the name. Naples had been settled by Greeks, who named their own city Nea-polis, or "New City". Oscans farmed the surrounding countryside, Latins competed for the same land, and Etruscan settlements sat nearby. Because so many peoples and languages overlapped in ancient Campania, no single explanation for the name has won out. Scholars have proposed Greek roots meaning "unquenchable" or "hurling violence". Others trace it to an older Indo-European word for "to shine or burn", or to a root meaning "hearth", the same one behind the goddess Vesta's name.

  The historian Diodorus Siculus told of Hercules passing near Cumae on his way to Sicily. There he found the Phlegraean Plain, ground the Greeks called "fiery" because of the mountain nearby. Giant bandits called "the sons of the Earth" lived there, and Hercules, aided by the gods, fought them off. A poet named Martial wrote an epigram in AD 88. In it, he suggested that both Venus, patron of Pompeii, and Hercules were worshipped in the very region the eruption of 79 would devastate. Vesuvius itself became a household god of sorts. It appears as a serpent in the painted shrines, or lararia, found across Pompeii. An inscription from nearby Capua addresses "Iovi Vesuvio", treating the mountain as an aspect of Jupiter.

  None of this godly reputation explains what actually lies beneath the peak: a plumbing system built by the collision of two entire tectonic plates.

• 3. Shape Of A Sleeping Giant

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  Vesuvius formed where the African tectonic plate slides beneath the Eurasian plate, a process called subduction. Water trapped in the African plate's sediment boiled off as it sank deeper, lowering the melting point of the rock above it. The partly melted rock, lighter than the solid stone around it, pushed upward until it broke through a weak point in the crust. Vesuvius is one of several volcanoes in the Campanian volcanic arc. Others include the Campi Flegrei caldera nearby, the island of Ischia 20 kilometers to the west, and undersea volcanoes further south. That arc is the southern tip of a longer volcanic chain running northwest across Italy as far as Monte Amiata in southern Tuscany. Of all of them, Vesuvius is the only one to have erupted in recent history.

  Above ground, Vesuvius looks humpbacked. A large cone called the Gran Cono sits inside the steep rim of an older, collapsed structure known as Monte Somma. The northern ridge of that caldera rim reaches 1,132 meters at a point called Punta Nasone. The main cone's summit stands 1,281 meters above sea level, over 400 meters above the five-kilometer valley of Atrio di Cavallo, the caldera's northern floor. Lava flows have scarred the slopes, but scrub, forest and vineyards cover much of the rest of the mountain.

  That caldera did not appear overnight. It began forming around 17,000 to 18,000 years ago, and Vesuvius had already been building itself for thousands of years before that.

• 4. A Mountain Rehearsing

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  Core samples come from a borehole more than 2,000 meters deep, reaching down into Mesozoic rock. They show the ground beneath Vesuvius has been volcanically active for at least 400,000 years. The oldest eruption material from the Somma caldera sits on top of a layer called Campanian ignimbrite. That rock was blasted out by the nearby Campi Flegrei complex about 40,000 years ago. Vesuvius itself began forming around 25,000 years ago, in what geologists call the Codola eruption. About 19,000 years ago, its eruptions turned into the large, explosive, caldera-forming kind that would eventually produce Pompeii's destroyer.

  Several of those eruptions rank among the biggest the mountain ever produced. The Basal Pumice eruption struck 18,300 years ago, the Green Pumice eruption 16,000 years ago, and the Mercato eruption 8,000 years ago. Around 3,800 years ago, the Avellino eruption reached a similar scale, with a volcanic explosivity index of 6. It buried Bronze Age villages of the Apennine culture, including one near modern Afragola. In May 2001, archaeologists near Nola used plaster casts to recover fence rails, a bucket, huts, pots and goats. They also found thousands of human footprints, all pointing north into the mountains. The villagers had fled in time; their homes did not survive.

  The volcano's last eruption before AD 79 may have come in 217 BC. Roman writers later described earthquakes that year and a sun dimmed by gray haze. Plutarch wrote of the sky catching fire near Naples, and the poet Silius Italicus described Vesuvius thundering with flames worthy of Mount Etna. Both men, though, were writing about 250 years after the fact. After that, the mountain fell quiet for nearly three centuries. Romans described its slopes as covered in gardens and vineyards, craggy only at the very top. The historians Strabo, Vitruvius and Diodorus Siculus all wrote of its volcanic past. Pliny the Elder, though, never mentioned it in his own Natural History.

  Nothing in those old accounts prepared anyone for what happened next. The latest evidence suggests the eruption came sometime after the 17th of October, in AD 79.

• 5. Two Days In AD 79

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  On its first morning, nothing seemed unusual to the era's only eyewitness, watching from across the bay. By midday, an explosion threw a towering column of ash and pumice into the sky, and the ground below began to vanish under falling debris. Rescue attempts were still possible then. Sometime that night or early the next day, pyroclastic surges began racing down the volcano's slopes. They were fast, dense and searingly hot, flattening buildings and altering the coastline itself. Tremors shook the region and stirred up a mild tsunami in the Bay of Naples. By late afternoon on the second day, it was over, leaving only a weak, hazy sun.

  Scientists later found the eruption unfolded in more than one phase. The first column of ash and pumice rose between 15 and 30 kilometers high. It rained down on Pompeii but not on Herculaneum, thanks to the wind. Its power came from seawater that had seeped into deep faults, meeting magma and flashing into superheated steam. When that column collapsed, it unleashed a pyroclastic surge that struck Herculaneum first, sparing Pompeii for a while longer. The eruption swung between two violent styles six times in total, and the third and fourth surges are thought to have finally buried Pompeii.

  A study of more than 200 roof-tile and plaster fragments from around Pompeii measured just how hot things got. On the first day, falling white pumice, some chunks up to 3 centimeters wide, heated roof tiles to 140 degrees Celsius. That was the last real chance to escape. The pyroclastic surges of the second day reached depositional temperatures up to 300 degrees Celsius, hot enough that shelter indoors offered little protection. Even the coolest recorded spot, a room under a collapsed roof, still measured around 100 degrees Celsius.

  By 2003, researchers had recovered around 1,044 body-shaped casts from ash deposits in and around Pompeii, plus the scattered bones of about 100 more victims. At Herculaneum, the remains of roughly 332 people have been found, including 300 discovered together in arched vaults in 1980. Thirty-eight percent of Pompeii's dead lay in ash-fall deposits, mostly inside collapsed buildings. The rest were found in surge deposits, likely killed by suffocation and flying debris. Herculaneum was closer to the crater but spared the ash fall. It was buried instead under 23 meters of surge material, and people caught on its old shoreline died instantly of thermal shock. Pliny the Elder was one of the few victims known by name; a Herodian prince named Agrippa and his wife were the only other named casualties.

  Only one voice from that day survives to tell it firsthand: a Roman named Pliny, in letters to the historian Tacitus.

• 6. The Two Plinys

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  From Misenum, about 35 kilometers across the bay, Pliny the Elder spotted an extraordinarily dense cloud rising above the peak. He commanded the Roman fleet stationed there. A messenger soon arrived from Rectina, wife of Tascius, who lived near the volcano's foot and could only escape by sea. Pliny launched the fleet's galleys to evacuate the coast. Then he set out himself in a light ship toward Stabiae, about 4.5 kilometers from Pompeii, to rescue her.

  Crossing the bay, Pliny's ship ran into thick showers of hot cinders, pumice and rock. When his helmsman urged turning back, Pliny answered, "Fortune favors the brave," and pressed on. At Stabiae, he and his companions watched what looked like flames on the mountain. They stayed the night until falling debris threatened to trap them inside. They fled to open fields, tying pillows to their heads for protection. The wind kept the ships from launching. Pliny sat down on a spread sail and, when the others rose to flee, could not get up even with help. He died there; his body was found the next day, without any visible injury.

  Back in Misenum, his nephew was jolted awake that night by a tremor and fled with his mother to the courtyard. More tremors near dawn sent the wider population running. A black, lightning-streaked cloud rose up, which the younger Pliny compared to sheet lightning. It blotted out the morning and swallowed the view of Point Misenum and the island of Capri. Ash rained down as he tried to escape along the road, and he had to keep shaking it off his clothes. By evening the fall had stopped and the sun showed weakly, so he and his mother went home to wait for word of his uncle.

  In one of his letters, the younger Pliny guessed that poisonous gas had killed his uncle, reacting badly with weak lungs. But the companions who traveled with Pliny the Elder breathed the same air and survived. Modern historians think a stroke or heart attack, in a heavyset man already struggling to keep up, is a more likely explanation.

  Down in Pompeii and Herculaneum, countless others had no fleet, no light ship and no letter to record what happened to them.

• 7. Centuries Of Fire

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  Vesuvius erupted again in 203, during the lifetime of the historian Cassius Dio. In 472, it threw enough ash into the sky that some fell as far away as Constantinople, 1,220 kilometers off. The eruptions of 512 were severe enough that the Gothic king Theodoric the Great exempted the mountain's slope-dwellers from taxes. More eruptions followed in 787, 968, 991, 999, 1007 and 1036, the last of these producing the first recorded lava flows. By the end of the 13th century the volcano had gone quiet again, and its slopes filled back up with gardens and vineyards.

  That calm ended in December 1631, when a major eruption buried villages under lava and killed around 3,000 people. Mudslides called lahars added to the damage. Frequent eruptions followed for centuries, six times in the 18th century, including 1779 and 1794, and eight times in the 19th, notably in 1872. Since 1750, seven of Vesuvius's eruptions have lasted more than five years each, a record matched only by Mount Etna over the same 270 years.

  On the 5th of April 1906, an eruption killed more than 100 people and released more lava than any other Vesuvian eruption on record. It struck just as Italy prepared to host the 1908 Summer Olympics; the funds went to rebuilding Naples instead, and the Games moved to London. Vesuvius then stayed active from 1913 to 1944, its crater slowly filling with lava.

  That long spell ended in the eruption of March 1944. It destroyed the villages of San Sebastiano al Vesuvio, Massa di Somma and Ottaviano, and part of San Giorgio a Cremano. Lava stayed inside the rim until the 18th of March, then overflowed and destroyed nearby villages through the 22nd. A final explosive burst on the 24th sent up an ash plume and a small pyroclastic flow. A few kilometers away, the United States Army Air Forces' 340th Bombardment Group was stationed at Pompeii Airfield near Terzigno. Falling ash damaged the fabric control surfaces, engines, windscreens and gun turrets of its B-25 Mitchell bombers, destroying an estimated 78 to 88 aircraft.

  Vesuvius has not erupted since. But nothing about its geology suggests that silence is permanent.

• 8. Living With Vesuvius

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  Vesuvius's largest eruptions release around 1 cubic kilometer of material; one of them buried Pompeii and Herculaneum. Eruptions on that scale tend to follow gaps of a few thousand years. Smaller sub-Plinian eruptions, releasing about 0.1 cubic kilometers each, recur every few hundred years. Between 1631 and 1944, smaller eruptions came every few years, each releasing far less material. Roughly, the volcano's potential eruption size grows by about 0.001 cubic kilometers for every year it stays quiet. Eighty years of rest, for instance, could set up an eruption of about 0.08 cubic kilometers.

  Officials base their emergency planning on a worst case similar to the 1631 eruption, which reached a volcanic explosivity index of 4. In that scenario, pyroclastic surges could sweep across roughly 7 kilometers of the volcano's slopes. Prevailing winds would carry ash mainly toward towns to the south and east, as far as Avellino or Salerno. The government's plan assumes two to twenty days of warning. It calls for evacuating everyone in the so-called red zone within about seven days. Most evacuees would then be sent elsewhere in the country, possibly for months.

  In 1984, officials evacuated 40,000 people from the nearby Campi Flegrei volcanic complex as a precaution, though no eruption followed. Since then, authorities have worked to shrink the population living in Vesuvius's red zone. They have demolished illegal buildings, created a national park to block new construction, and paid residents to relocate. The goal is to cut evacuation time to two or three days by the 2020s or early 2030s.

  The Osservatorio Vesuviano in Ercolano now tracks the mountain constantly, using seismic and gravity sensors, GPS, satellite radar and gas readings from its fumaroles. As of July 2024, the observatory classified Vesuvius at a Green Alert Level. Its low-energy tremors are blamed on rocks settling inside the crater, not rising magma.

  The land around Vesuvius became a national park on the 5th of June 1995. Visitors can still walk to the summit along a network of paths, though vehicles must stop 200 meters short. A funicular railway first carried passengers up the mountain in 1880, until the eruption of March 1944 destroyed it. The Neapolitan song "Funiculì, Funiculà" was written to celebrate its opening, and people still sing it long after the tracks themselves disappeared.

  The song "Funiculì, Funiculà" is still sung today, long after the railway it celebrates and the volcano that destroyed it both fell quiet, for now.