Reinterpretation of the volcanological and historical evidence shows that the eruption of Vesuvius in A.D. 79 consisted of two main phases. The initial 18 to 20-hour Plinian phase caused extensive pumice-fall south of the volcano, resulting in the slow accumulation of a pumice layer up to 2.8 m. thick over Pompeii and other regions to the south. Much of the population fled the area during this non-lethal phase. On the second day of activity the Peléan phase occurred, when a series of nuées ardentes or hot ash-avalanches swept down the south and west flanks of the volcano, affecting the region as far as Misenum, 30 km. to the west. The first of two nuées ardentes which inundated Pompeii over-whelmed and asphyxiated those who remained above ground in the city and their bodies were immediately interred in the fine-grained deposit. The effects of the Peléan activity were even more severe west and north-west of Pompeii, resulting in burial of the cities of Oplontis and Herculaneum by a series of nuée ardente deposits.