Is volcanology a good subject

They were scenes like from Dante's Inferno: A tremendous clap of thunder shook the cities of Pompeii, Herculaneum and Stabiae on August 24th, 79 AD, followed by a cloud of ash, lava, pumice stones and gases that soon rose to a height of several kilometers, and a short time later it turned like a The shroud began to be laid over the settlements. For hours, slag and lapilli rained down on the houses and caused many roofs to collapse - this is where the first victims came. Shrouded in deep darkness, although it was supposed to be broad daylight, people lit torches and thus started the first fires. In addition, lightning bolts flashed down from the electrostatically charged atmosphere.

The Roman god of fire, Volcanus, seemed to have come to life and knocked with power on the gate to the upper world. Pompeii's residents left their city and for the time being went to safety, but returned to their homeland on the morning of August 25th, as the gods appeared again appeased. But this calm was deceptive, because in reality the mountain of fire only gathered its strength to afflict people a second, now really devastating time: Another explosion caused a hot gas cloud to crash down the flanks of Vesuvius and burned up any life that was in it Put away.

A large part of the probably 12,000 to 15,000 deaths that occurred in Pompeii and the other localities on that day died in this pyroclastic flow, in which solid dust particles are suspended by the hot gases and sucked in air. Temperatures of up to 800 degrees Celsius can prevail inside such a cloud, as was measured in 1992 during a similar eruption of Mont Pelée on the Caribbean island of Martinique. And at a speed of 160 kilometers per hour driven by the low frictional resistance, any attempt to escape seems pointless and pointless.

The subsequent eruptions were ultimately irrelevant, the work of destruction had long been completed. Their ashes and streams of mud only covered the ruins of the cities, but thus preserved them for the next centuries until the scientific rediscovery in the 19th century. Now, after almost 150 years of research, Pompeii is still providing new insights and providing views of future eruptions. Since its last eruption in 1944, Vesuvius has slumbered again, which, like Krakatau, Unzen or Mount Saint Helens, is one of the explosion volcanoes whose chimney is repeatedly closed like a plug by viscous magma until it is forced to free itself from the obstacles .

And that is precisely what makes Vesuvius the most dangerous volcano in Europe - after all, more than a million people in the greater Naples area live at its feet. Knowing how your effusions behave is therefore worthwhile. In order to find out how a pyroclastic cloud makes its way through populated areas, Italian researchers led by Lucia Gurioli from the National Institute for Geophysics and Volcanology in Pisa examined the remains and distribution of igneous rock in the excavated relics of Pompeii [1]. They estimated the possible temperatures of the air currents based on the degree of magnetism that remained in unmelted rocks: the lower it was, the higher the temperature at that time.

On the map and in the computer model that arose from these distribution patterns, it was then shown how the spatial location, such as the shape of the buildings and streets, steered the glow of hell, because the resistance of the houses or the lanes of the alleys, which facilitated the flow, divided those near the ground raging air masses of the cloud in several partial currents and swirled. As a result, in rare cases in the slipstream of walls or houses, air turbulence could have developed, sucking in cooler air or protecting cavities from the heat.

There, in turn, the likelihood of temperatures well below a hundred degrees Celsius, which a person needs to survive, increased. Outside the stone barriers of the city, however, there was no protection from the heat wave, and so the hot mass consumed all refugees there who wanted to get to safety in the direction of the beach or the sea.

A study by Peter Baxter and his colleagues from the University of Cambridge, who examined the eruption of the Soufrière Hills on Montserrat, shows that running away is not always the best alternative for acute volcanic eruptions and pyroclastic currents [2]. This volcano started its activities again in 1995 - after more than 400 years of deceptive calm - but it did not come to a catastrophe until 1997 when several clouds of gas and ash raced over several villages and the capital Plymouth in the south of the island.

According to the findings of the scientists, the heat of the pyroclastics was enough in many places to ignite and destroy wood and other materials, but the gas flow only developed its full destructiveness when objects were thrown up and hit the walls of buildings like projectiles. In large areas, however, the pressure of the cloud - especially at its edges - was comparatively low and, according to Baxter, only caused minor damage to stone houses, provided it could not penetrate through open windows or doors or wooden roofs caught fire : Survival would have been possible there in the relatively cool rooms.

Of course, this did not apply to the core zone of the gas tower, where an estimated pressure of five kilopascals - which corresponds to a weight of around 510 kilograms per square meter - prevailed: Here almost total destruction took place, and buildings and vegetation were down to their foundations and rootstocks shaved off. Apart from material losses, none of the residents was harmed: they were all evacuated after the first signs of the impending disaster. Fortune that in the case of Pompeii was only granted to a few - like Pliny the Younger, who survived the fall of Pompeii at a safe distance in the small town of Misenum.

This article is contained in Spektrum - Die Woche, June 8th, 2005