PRECURSERS

An earthquake occurred on 63 AD February 5. The earthquake created major damage to Pompeii and Herculaneum, and caused minor damage in Nuceria and Naples, where the emperor Nero was performing in the theatre. According to Seneca, the earthquakes lasted several days until they became milder. This earthquake swarm probably occurred at a shallow depth in the Vesuvian area, given the distribution and extent of the damage.

Seventeen years later, a new wave of heavy earthquakes occurred. Most people ignored them because earthquakes were so common in Campania. But this time, they were the forerunners of the Vesuvius eruption of 79 AD.

THE ERUPTION OF 79 AD

Pliny the Younger's letters to Tacitus and recent volcanic studies provide a background and description of the Vesuvian eruption.

The time the eruption began is uncertain: the two Plinys directly observed the cloud at about 1 PM 24 August 79 AD from a distance of 21 km across the Bay of Naples from Pompeii. The eruption probably began sometime earlier since at about the same hour a messenger sent from Pompeii arrived.

Pliny the Younger said that the volcanic plume resembled a Mediterranean Pine Tree more than anything else. Like a tree, the cloud ascended in a column and at intervals expanded in different branches. It was at one moment white, at another dark and spotted, as if it had carried up earth or cinders.

During the time of the eruption, Pliny the Younger stayed in the proximity of Misenum from where he observed the eruption along with his mother. Her brother, Pliny the Elder, took a boat to go and help save friends across the bay. He died in the attempt.

Between 2-3 PM Pliny the Elder sails to get a better view of the disaster and to rescue a friend whose villa is at the foot of Vesuvius. Unable to land he goes to Stabiae.

Pumice and dust is falling at a rate of 20 centimetres (8 inches) an hour. Most residents survive the downpour because the material is too light to kill them.

6 PM Pliny the Elder arrives at the house of his friend, Pliny the Elder goes to sleep while the downpour of ash and soot continues.

The pumice is now coarser, indicating an increase in the intensity and violence of the eruption. Magma is flowing 10 times more rapidly than before - at 10 million kilograms (22 million lb) a second. This is known as the 'Plinian stage', after the chronicler of the disaster. The cloud column now rises to 32km (20 miles) - six times higher than airliners fly. There are continuous lightning strikes in the cloud

8-9 PM Upon waking after his nap. 'They consulted together as to whether they should hold out in the house, or wander about in the open. For the house now tottered under repeated and violent concussions, and seemed to rock to and fro as if torn from its foundations. In the open air, on the other hand, they dreaded the falling pumice-stones, light and porous though they were; yet this, by comparison, seemed the lesser danger of the two; a conclusion which my uncle arrived at by balancing reasons, and the others by balancing fears. They tied pillows upon their heads with napkins; and this was their whole defence against the showers that fell round them...'

9 PM The rest of the world was still light but at Stabiae there a deeper darkness prevailed than in the most obscure night... They thought it practical to go down to the shore to observe Vesuvius from close at hand if they could possibly put out to sea, but they found the waves still ran extremely high and contrary. There Pliny the Elder, having thrown himself down upon a disused sail, repeatedly called for and drank cold water.

1 AM Twelve hours after the eruption began, the night view of the volcano showed a spectacular display of lightning amid the volcanic cloud spewing from the crater. At times, the dense cloud of ash, smoke, and stone towered 12 miles high. Strong winds aloft blew the material southeast toward Pompeii and nearby Stabiae.

Up until now, the heavy column laden with dust, ash and rock had been supported by the sheer force pushing out of the volcano. But, at about midnight that force momentarily weakened. The superheated cloud collapsed upon itself and started to roll down the side of the mountain.

The leading edge of this pyroclastic flow was a fast-moving stream of hot ash and gases, hurtling downward at terrific speed. The second part was denser, consisting of pumice, rocks and soil, made liquid by temperatures that approached 750 degrees. The glowing cloud failed to reach Pompeii on the first try, but it easily engulfed Herculaneum. Everyone remaining in the town died.

Note: A pyroclastic flow is defined as: A ground-hugging avalanche of hot ash, pumice, rock fragments, and volcanic gas that rushes down the side of a volcano as fast as 100 km/hour (60 miles/hr) or more.

Pliny the Younger and his mother have stayed in Misenum, across the Bay of Naples. 'That night the earthquakes became so violent that one might think that the world was not being merely shaken but turned topsy-turvy. My mother flew to my chamber and they moved outside and sat in the forecourt of the house.

Another town at the foot of the volcano and even closer to the crater, Herculaneum, had been spared a heavy fall of ash because it was upwind. The residents thought they were safe. They weren’t.

2 AM A second pyroclastic flow surged down the mountain. Again, it failed to reach Pompeii. But the steady rain of rocks and ash were getting to be more of a hazard than those at ground level were prepared to bear. Remaining residents, who were still huddled in their houses, gathered what belongings they could collect and started out of the city. Yet, some remained.

In the proximity of the volcano these phenomena caused severe damage and deaths. However, most people living in the area had escaped during the day before, in the first phase of the eruption, before the high eruption column had deposited about 4 meters of pumice lapilli around and in Pompeii during the early morning hours of the 25th. Pompeii was mostly inhabited by animals like horses, which were unable to walk in the thick cover of lapilli, or watch dogs, which were left to guard houses. So only a few people died because of the ash fall.

6 AM. During the night of the first day of the eruption, and for most of the morning of the next day, the houses of Misenum where shaken by earthquakes that caused much panic. Pliny the Younger and his mother left town and climbed the "Monte di Procida" hill. On the top of the hill, wheeled-charts on flat land were shaken back and forth even if chocks were placed against the wheels. Seismic activity even that far away was magnitude 5.

At dawn on the 25th from their vantage point they saw the sea retreating. This retreat of the sea is probably related to a tsunami which came with the climax of the eruption.

A cloud reached the "Monte di Procida" hill, across the bay, where Pliny the Younger and his mother were. A dense haze followed us like a stream flowing on land and night fell on us, not like a night with clouds or without stars, but like the night in a closed place without a lamp.

Third surge cloud. This is much larger than the last. It runs out of energy just as it reaches the northern walls of Pompeii. The air in the city becomes increasingly hard to breathe, and the ground shakes with increasingly fierce earthquakes. Some try to flee but are faced with 2.4m (8ft) of hot ash and pumice.

7:30 AM Fourth surge cloud. The remaining inhabitants of Pompeii - 1,044 bodies have been found within the city walls - are killed within two minutes by the ash-and-dust-filled wind that sweeps over them at over 300km (185 miles) an hour with a temperature of at least 100°C (212° F).

8:30 AM Pliny the Elder remains on the shore with his companions. '· Flames, and a strong smell of sulphur, which was the forerunner of them, dispersed the rest of the company to flight; Pliny the elder they aroused. He raised himself up with the assistance of two of his slaves, but instantly fell; some unusually gross vapour, as I conjecture, having obstructed his breathing and blocked his windpipe.

Fifth surge cloud. This sweeps down through Herculaneum and Pompeii and into the countryside, cutting down many who had thought they had escaped. Possibly as many as 10,000 Pompeii residents - half the city's population - die. Then the cloud - looking like a black tidal wave - hit the headland of Misenum, just as it ran out of energy.

Again, Pliny the Younger and his mother were subjected to another cloud. Again they experienced the obscurity, and again the ash was dense and heavy. It took them some time to shake away the ash as we could have been covered and choked by its weight.

Pompeii had joined Herculaneum, 12 miles away, in death. Two further surges that evening sealed the cities in an earthen tomb.

Pompeii was buried under nine feet of ash, and a deep, hard layer left by the surges. Herculaneum, much closer to the mouth of the volcano, was buried to a depth of 65 feet or more, in a deposit as tough as concrete.