What overcame Pompeii and Herculaneum was a pyroclastic flow--hot gases, dust and near molten rock moving at high speed. Wikipedia defines pyroclastic flow thus: A pyroclastic flow (also known scientifically as a pyroclastic density current) is a fast-moving current of hot gas and rock (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gas can reach temperatures of about 1,000 °C (1,830 °F). That also did for almost 30,000 people on the island of Martinique in 1902, when the volcano Pelée erputed.
The photograph was taken after the initial, explosive eruptions. After 1945, the devastation from Pelée was compared to the atomic bomb attacks on Hiroshima and Nagasaki. Photographs of the effects of Pelée were taken within minutes of the eruption, and constitute the earliest records of the effects of a pyroclastic flow event. Roman soldiers on guard and people on the street in Pompeii and Herculaneum were trapped (and preserved) by an event which struck them before they even knew there had been an eruption.