The tangled skeletal remains of the people of Herculaneum, reconstructed as found in one of the arches on the former waterfront.
sequences varying from site to site, made possible a comprehensive new understanding of the eruptive process in both time and space. A broad distinction was made between ‘fallout layers’, numbered A-1 to A-9, and ‘pyroclastic surge and flow layers’, numbered S-1 to S-7 and F1 to F-6. This distinction corresponded to a recurring transformation in the nature of the eruption in its later stages. The ‘Plinian phase’ — the umbrella pine, the mushroom cloud, the blanket of darkness that eventually shrouded Pompeii, the ‘fallout’ of ash and pumice — reached its maximum many hours after the start of the eruption. A wide caldera formed at the top of the mountain as ever more rock was blasted away, allowed more material to be ejected, eventually as much as 1.5 million tons a second, the mass of burning rock and gas shooting into the sky at a speed of 900 miles an hour. (One estimate of the total release of energy during the 24 hours or so that the eruption lasted is that it was the equivalent of 100,000 Hiroshima explosions.) As the cloud discharged its burden, it was constantly reloaded with fresh material, the volcanic rain continuing for hours, first white pumice (A-2), then grey pumice (A-3), the pumice mixed with harder, fist-size rock fragments falling with a lethal velocity of 50 metres per second, killing some people in the open streets of Pompeii. By late afternoon, roofs were collapsing under the weight of accumulation, crushing or entombing people sheltering beneath. But the worst was yet to come. Pompeii had been plunged into darkness during the afternoon. Not so Herculaneum, where the effects of the eruption were minimal until the early hours of the following morning. The volcanic cloud had drifted south, not west, and the exposures in the cliffs around the site today reveal no evidence of the fallout deposits that buried Pompeii. Instead, above the beach, there are alternating layers of surge and flow, thin bands of surge, deep bands of flow, six of each.
The Past | April/May 2021