Its most famous eruption occurred on August 24 in AD 79. At that time, there were a number of Roman cities spread around the vicinity of the volcano - most notably Herculaneum and Pompeii. The citizens of these cities had grown used to the frequent earthquakes in the area, some of which were quite large. On February 5 in AD 62, for example, a large quake caused extensive damage to Pompeii. Interestingly, Seneca the Younger wrote in his work Naturales quaestiones (Natural Questions) that a flock of 600 sheep died as a result of this earthquake from pestilent air (see section xxvii in the above link) clearly linking the event to the release of volcanic gases from Vesuvius.
Starting on August 20 in AD 79, a swarm of small earthquakes started occurring in the region. Today, seismologists would recognize this as being due to the movement of magma in the subsurface of Vesuvius. To the Romans, they were simply an annoyance that one learned to live with in the Campania region.
Morning of August 24 was the same as any other but around mid-day, the mountain blew its top and the eruption lasted into the second day. At first ash began to fall but this was soon followed by pyroclastic clouds of glowing hot ash and searing gases rushing down the mountain to blanket the plains below. Nothing in their path survived.
The eruption of Vesuvius was phreatic. This type of eruption occurs when magma, molten rock, rises up to meet subsurface groundwater aquifers. The water immediate flashes to steam and violently explodes. This energy forces the ash (fragmented rock) and hot gases (water vapor and exsolved carbon dioxide from the magma) high into the atmosphere as shown above. This type of eruption, with the material shooting straight up into the sky, is called a Plinian eruption (we'll see why in a minute).
When the energy from the gases weakens, the column collapses and the ash and superheated gases fall downward and follow the slopes of the volcanic cone to rush outward (sometimes well over 100 mph) as glowing clouds (nuee ardentes) or pyroclastic flows. At 1,000° C, they kill everything in their path. Based on volcanic sediment studies, Vesuvius apparently cycled at least six times between these Plinian eruptions and pyroclastic surges.
While Pompeii, downwind of Vesuvius at the time, was affected most by the fallout of 4 to 6 meters (13 to 20 feet) of ash and pumice, Herculaneum was hit hard by the pyroclastic flows which went to the northwest. It was buried under 20 meters (50 to 60 feet) of ash. Anyone who didn't flee these cities immediately after the eruption was entombed forever. Other Roman towns, less well known today, were also affected (e.g. Oplontis in the map above).
It's hard to estimate casualties, but some claim as many at 16,000 Roman citizens died over those two days of the eruption. Today, millions of people live in that area around Vesuvius (see my post from yesterday).
The great thing about the eruption of Vesuvius is that we have an eyewitness account by the Roman Pliny the Younger who was 17 at the time. He wrote two letters to the historian Tacitus describing what he saw. His uncle, Pliny the Elder, was in charge of the Roman fleet at Misenum (see map above) across the Bay of Naples. He launched ships to rescue survivors and himself landed at Stabiae to rescue friends. Strong onshore winds prevented him from leaving and they attempted to flee by land where Pliny (middle-aged, reportedly "corpulent", and suffering from asthma) died of unknown causes (overcome by noxious fumes or possibly a heart attack).
Pliny's description of the eruption still make fascinating reading.
On the 24th of August, about one in the afternoon, my mother desired him to observe a cloud which appeared of a very unusual size and shape. He had just taken a turn in the sun, and, after bathing himself in cold water, and making a light luncheon, gone back to his books: he immediately arose and went out upon a rising ground from whence he might get a better sight of this very uncommon appearance. A cloud, from which mountain was uncertain, at this distance (but it was found afterwards to come from Mount Vesuvius), was ascending, the appearance of which I cannot give you a more exact description of than by likening it to that of a pine-tree, for it shot up to a great height in the form of a very tall trunk, which spread itself out at the top into a sort of branches; occasioned, I imagine, either by a sudden gust of air that impelled it, the force of which decreased as it advanced upwards, or the cloud itself, being pressed back again by its own weight, expanded in the manner I have mentioned; it appeared sometimes bright and sometimes dark and spotted, according as it was either more or less impregnated with earth and cinders. [Read this letter - Read the next letter]
Now you see why these eruptions are called "Plinian."
Tomorrow I'll talk more about the geology of this area of Italy.