Awe inspiring Mount Vesuvius

It has been a dream since a long time to see a volcano. While I did not exactly see a ‘live’ one, I got a chance to recently see Mt. Vesuvius in Italy. Needless to say, I was super-impressed by the scale of the eruption that had taken place and of what was left behind.

Present day crater

Present day crater

blown off

blown off

Still steaming at places, with life growing in now

Still steaming at places, with life growing in now

and more steam

and more steam

I bought a book about the volcano and its history from one of the shops there. Here, I am typing out some of my favourite and relevant text sections from the book for all to read and to be transported into, however briefly, the world of mountains and imagine Nature’s forces.

Note: It is going to be lengthy!!!

The plain stretches from Mount Massico to the Lattari Mountains (the Sorrento peninsula), from the coast to the Caserta and Piacentini mountains. When the land sank, it became an arm of the sea and then an area of volcanic activity. At first the eruptions took place below sea level, then on land, their debris piling up to form volcanic edifices that can be seen even today. There are areas which were only recently discovered when deep wells were drilled in the search for geothermic energy. The last to to appear, and therefore the youngest of all, was Somma-Vesuvius.

Lava flows were found recently at a depth of about 1,345 m in a well drilled at Trecase, and presumably represent earlier acitivity, some 300,000 years ago.”

“The Somma Vesuvius complex is composed of two concentric volcanic structures, of different size, shape and age, but rising from a common base. The external one, formed in an earlier period was called “Vesuvius” (or Vesvio, Besobio, Besvio, Besuvio) by the Romans. Vesuvius proper has a truncated cone shape with the present day crater at the top.”

“‘The oldest picture of the volcano can be found in some frescoes discovered at Herculaneum and Pompeii; it is shown as a single peak covered with trees and wild vineyards.”

Fresco of Vesuvius

Fresco of Vesuvius

From 79 A.D. on, the Great Cone changed shape and height with each eruption, and its present appearance is the result of its most recent activity, from 1631 to 1944. Vesuvius, which was 1,186 m above sea level before the 1944 eruption, is at 1,276 m today. Once, before the formation of the Great Cone, many scholars believed that Vesuvius (or more precisely Mount Somma) had only one peak, over 3,000 m high, built up slowly by layers of lava and ejecta (from this the name ‘stratovolcano’).

From a stratigraphic study of Somma Vesuvius’ eruptive material, the volcanic activity of the past 17,050 years can be subdivided into 9 great cycles, separated by lengthy periods of quiescence: these are documented by the presence of thick paleosols which separate the piroclastic products of each cycle. Prehistoric artifacts and animal bones found buried under the volcano’s pyroclastic material indicate that this area was populated from early times. The people of Pompeii remember the volcano as quiet, covered with vineyards up to its peak. Its wine was famous and was bottled in terracotta amphoras to be sold in other centres.

The eruption of 79 A.D.

The eruption started on August 24 in the year 79 A.D. As documented by Plinius the Younger, who vividly described the eruption in his letters to Tacitus; here are some excerpts: “…my mother pointed out to me a cloud that had appeared, of an extraordinary size and aspect – a pine tree indicates its form and appearance better than anything else. In fact, rising straight up like a very long trunk, it spread and then branched: I believe pushed first upward by an impetuous puff and then dropped back on itself when this abated, or beaten by its own weight; then it faded away as it spread out: at times white, at times dark and spotted, depending on whether it had blown up earth or ashes.

At the same time ashes began to shower on us, not yet thickly; I turned and saw behind me a thick cloud that pressed upon us like a river, flooding the ground. Let’s go back, I said to my mother, while we can still see, so that we will not be taken unawares along the way, and crushed by the crowds of people that come from behind. As soon as we sat down, night fell; not a cloudy, moonless night, but as if in a closed room when the lights are out. Finally it cleared a bit; nevertheless it did not seem day to us, but rather the foreboding of a nearby fire; only the fire did not come, instead there was new darkness and a new cloud of thick ash. Getting up every once and awhile, we shook the ashes off, otherwise we would have been not merely covered, but buried, by them.

The eruption lasted three days, the sky was clear again on the 26th. Pompeii was covered by 7 metres of ashes and lapilli, Herculaneum by a mud flow (lahar) 15-25 m thick; more than 2,000 lives had been lost.

Models of the eruption:

During the volcano’s long quiescent phase, both the viscosity and the gas content of the magma increased due to the differentiation produced by the slow and continuous cooling. A thick permeable crust of solidified magma prevented contact between the magma (within the magnetic chamber at a depth of 2-5 km in the carbonate formations) and the groundwater, which was of meteoric origin and contained in the limestone. When the pressure of the gases inside the chamber became greater than the load of the rocks above, the eruption began. Meanwhile, with the conduit open, the pressure suddenly lessened, and the resulting expansion of gases in the viscous mass produced violent explosions. Thus the “volcanic pine” (Plinian volcanic column) was formed; the violence of the explosions split and shattered the impermeable crust inside the magnetic chamber, but the groundwater (of meteoric origin) contained and circulating in the limestone could not penetrate it because of the enormous pressure of the gases still present therein.

The first phase of the eruption ended when there was no further pressure from the gases that had accumulated in the magma and that had shot ash, pumice, solid blocks and scoriae mixed with gas more than 17 km into the air. No longer held up by the pressure of the gas, this material began to fall and buried almost  all of Pompeii in just a few hours, while flows of finer ash and water devastated Herculaneum and other towns on the slopes of Vesuvius.

Activity at the mouth of the crater greatly diminished now that the upper part of the magnetic chamber had been emptied.

Present day crater

Present day crater

Once the eruption ceased, many inhabitants of Pompeii, possibly thieves amongst them, returned to the city, which was by now almost totally buried. Meanwhile the grounwater circulating deep within the limestone flowed into the magnetic chamber for about 10 hours; during that time it appeared that the eruption had ceased.

During the final phase the eruption recommenced with violence (at 6.00 a.m., Aug 25), when water which had filtered into the partially empty chamber came into contact with the magma there. This produced an extremely strong increase in pressure within the chamber; the volcano swelled and rose up. The entire gulf shore-line shifted. The volcano spewed a new, violent cloud consisting not only of magma but principally of products of a “freato-magmatic” process.

Catastrophic piroclastic surges, i.e. explosions on the surface of overheated, high energy steam, formed a ring-like cloud of gases and ash around the center of the explosion. The ring spread out horizontally with the destructive speed of a hurricane, descending the slopes of Vesuvius and destroying everything in its path in just a few minutes.

Those people and animals in Pompeii that did not flee died from suffocation, due to the high temperature of the clouds of overheated steam mixed with piroclastic material and other toxic fumes.

The eruption ceased in less than 24 hours, after having completely destroyed the city and ruined the fertile countryside, which was burned by strong acid rains where not covered in a blanket of ash.

It was an unbelievable sight – something so powerful and devastating. Trying to recollect and picture all that I had read and seen then, and imagining peering into the crater as I type this, I feel the same feeling overcome me. One of awe, fear and disbelief at the scale, among others. Deep respect.


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