pumice ranging between high, which rained on Pompeii to the southeast but not on Herculaneum upwind. The chief energy supporting the column came from the escape of steam superheated by the magma, created from seawater seeping over time into the deep faults of the region, which interacted with magma. Subsequently, the cloud collapsed as the gases expanded and lost their capability to support their solid contents, releasing it as a pyroclastic surge, which first reached Herculaneum but not Pompeii. Additional blasts reinstituted the column. The eruption alternated between Plinian and Peléan six times. Surges 3 and 4 are believed by the authors to have buried Pompeii. Surges are identified in the deposits by dune and cross-bedding formations, which are not produced by fallout. Another study used the magnetic characteristics of over 200 samples of roof-tile and plaster fragments collected around Pompeii to estimate the equilibrium temperature of the pyroclastic flow. The magnetic study revealed that on the first day of the eruption a fall of white pumice containing clastic fragments of up to fell for several hours. It heated the roof tiles up to . This period would have been the last opportunity to escape. The collapse of the Plinian columns on the second day caused pyroclastic density currents (PDCs) that devastated Herculaneum and Pompeii. The depositional temperature of these pyroclastic surges ranged up to . Any population remaining in structural refuges could not have escaped, as gases of incinerating temperatures surrounded the city. The lowest temperatures were in rooms under collapsed roofs. These were as low as . The two Plinys The only surviving eyewitness account of the event consists of two letters by Pliny the Younger to the historian Tacitus. Pliny the Younger describes, amongst other things, the last days in the life of his uncle, Pliny the Elder. Observing the first volcanic activity from Misenum across the Bay of Naples from the volcano, approximately , the elder Pliny launched a rescue fleet and went himself to the rescue of a personal friend. His nephew declined to join the party. One of the nephew's letters relates what he could discover from witnesses of his uncle's experiences. In a second letter, the younger Pliny details his own observations after the departure of his uncle. The two men saw an extraordinarily dense cloud rising rapidly above the peak. This cloud and a request by a messenger for an evacuation by