The Campania region sits along a tectonic boundary where the African plate is slowly being pushed beneath the Eurasian plate, a process known as subduction. This "Campanian Arc" is home to several volcanoes, but none as active or as famous as Vesuvius (though Mount Etna, in Sicily and not part of the Arc, is Europe's most active volcano) [source: Taylor].
Veusvius is a stratovolcano, a type of volcano known for its steep sides built of alternating layers of hardened lava, ash, pumice and tephra. Stratovolcanoes are also known for their extremely violent eruptions: Mount St. Helens in Washington state and Krakatoa in Indonesia are other famous stratovolcanoes. Pressure within such volcanos builds as heated magma creeping toward the surface diffuses gas into the rocks. When the pressure grows strong enough, it bursts free from the volcano like popping the cork on a shaken champagne bottle. The 79 C.E. eruption of Vesuvius apparently exploded primarily out the top of the cone, but some stratovolcano eruptions explode out the side of the volcano, like Mount St. Helens and possibly some of the earlier Vesuvius eruptions.
The only recorded eyewitness account of the eruption of Vesuvius comes from Pliny the Younger, whose uncle, Pliny the Elder, was in command of a fleet in the Roman navy stationed at Misenum, a town on the northwestern edge of the Bay of Naples. Pliny the Younger was around 17 years old at the time of the eruption — he wrote his letters to the Roman historian Tacitus more than 20 years later [source: Brilliant]. He and his mother were at Misenum when the eruption occurred, while his uncle was killed leading a rescue fleet. He wrote the following in his letters to Tacitus:
"For several days past there had been earth tremors which were not particularly alarming because they are frequent in Campania: but that night the shocks were so violent that everything felt as if it were not only shaken but overturned."
In fact, a severe earthquake 17 years prior severely damaged Pompeii. The ruins of the city show cracked walls and lintels with signs of repair. Pliny the Younger's harrowing account of fleeing Misenum with a crowd of panicked people gives us a peek into what it was like for Pompeiians during the disaster:
"A dense black cloud was coming up behind us, spreading over the earth like a flood ... We had scarcely sat down when darkness fell, not the dark of a moonless or cloudy night, but as if the lamp had been put out in a closed room. You could hear the shrieks of women, the wailing of infants, and the shouting of men ... There were some who prayed for death in their terror of dying. Many besought the aid of the gods, but still more imagined there were no gods left, and that the universe was plunged into eternal darkness forevermore ... Ashes began to fall again, this time in heavy showers. We rose from time to time and shook them off, otherwise we should have been buried and crushed beneath their weight ... At last the darkness thinned and dispersed into smoke or cloud ... We were terrified to see everything changed, buried deep in ashes like snowdrifts."
The citizens of Pompeii and nearby Herculaneum (a smaller town that was also buried by the eruption) evacuated in the hours after the eruption. However, not everyone was able or willing to leave. An estimated 2,000 people died in Pompeii [source: Brilliant]. For decades it was thought that most of them died due to suffocation after being buried in ash — Pompeii was buried 19–23 feet (6–7 meters) deep [source: Kolich]. Modern archaeologists suspect that sudden pyroclastic flows called nuées ardentes, massive surges of heated gases and ash, killed the volcano's victims quickly. The heat was enough to kill them even when they were hiding in buildings, and the surges carried the ash indoors, burying everything. The eruption also destroyed the settlements of Oplontis and Stabiae.