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Pyroclastic Flow
Pyroclastic flows are fast-moving currents of hot gas, ash, and rock, which can travel away from the volcano at up to 700 kph. The gas can reach temperatures of up to 1000 degrees Celsius. The flows normally hug the ground and travel downhill, or spread laterally under gravity.
The photograph opposite is of a pyroclastic flow sweeping down the Mayon Volcano in the Philippines in 1984.
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'By this time the courtyard giving access to his room was full of ashes mixed with pumice stones, so that its level had risen, and if he had stayed any longer he would never hadve got out. He was wakened, came out and joined Pomponianus and the rest of the household, who had stayed up all night.'
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At about 2 am a second pyroclastic surge follows the course of the first surge and roars through Herculaneum burying the town and reshaping the shoreline. Pompeii has so far escaped these damaging surges.
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'They debated whether to remain indoors or take their chances in the open. After comparing the risks they chose the latter. As a protection against falling objects they put pillows on their heads tied down with cloths. Elsewhere there was daylight, but they were still in darkness.
My uncle decided to go down to the shore. There, a sheet was spread on the ground for my uncle to lie down, and he repeatedly called for cold water which he drank.'
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At about 6 am a third pyroclastic surge heads this time towards Pompeii. It almost breaches the city walls. In Pompeii volcanic debris has now reached a depth of over two metres.
About an hour later there a fourth surge. Again it is aimed at Pompeii. This time the dust and hot gases envelop the city killing every remaining soul (probably over 1,000).
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'Then the flames and smell of sulphur which heralded the approaching fire drove the others to take flight. Aroused, my uncle struggled to his feet, leaning on two slaves, but he immediately collapsed. I assume that his breathing was impeded by the dense fumes which blocked his windpipe - for it was constitutionally weak and narrow and often inflamed.'
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A fifth surge follows minutes later. Then, at about 8 am, a sixth and final surge, the most powerful of all, sweeps down over Herculaneum and Pompeii and across blackened countryside burying or asphyxiating all it touches. Several thousand people die. The surge almost reaches Misenum across the bay.
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'And now came the ashes, but at first sparsely. Behind us (Pliny the Younger and his mother) an ominous thick smoke, spreading over the earh like aflood, followed us. We had scarcely agreed what to do when we were enveloped in night. To be heard were only the shrill cries of women, the wailing of children, the shouting of men. But the darkness lightened, and then like smoke the cloud dissolved away. Everything appeared changed - covered by a thick layer of ashes like an abundant snow fall.'
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The eruption at last begins to weaken, although ash continues to fall for a day or so. Heavy rains brought on by the effects of the eruption bring mudslides to Herculaneum, completing its burial.
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