This study records that Pompeii, long before its final devastation by the 79 A.D. Vesuvius eruption in southern Italy, was damaged by several mass gravity flows. Composition of the deposits indicates that they were derived from volcaniclastic cover of carbonate highlands positioned 14 km NE of the city. Stratigraphic and petrologic analyses of sections in excavations and drill cores in and near Pompeii record the presence of three downslope‐directed debris flows dated between 8th and 2nd century B.C. Some of these deposits were channelized via a stream bed that once extended from high reliefs to Pompeii. It is proposed that one of these events may have been partially responsible for urban decline during the 4th century B.C. These mass flows are interpreted as having been triggered primarily by intense rains in a manner similar to those that have occurred and destroyed towns in this region during the past 50 years. Our investigation shows that Pompeii and adjacent areas in the past, much as in recent time, have likely been most frequently susceptible to episodic damage by avalanches and mass flows of volcaniclastic material resulting from hydrological effects rather than from volcanic events, earthquake tremors, and societal disruptions such as wars.

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Studies of some 70 bore holes around ancient Pompeii, on the southwestern slope of the Somma-Vesuvius volcano, allow the reconstruction of Holocene environments earlier than the A.D. 79 eruption. This eruption produced about 10 km3 of pyroclastic material that buried the Roman cities of Pompeii, Herculaneum, and Stabiae and promoted a shoreline progradation of 1 km. The Sarno coastal plain, in a post-Miocene sedimentary basin, has been affected by Somma-Vesuvius volcanic activity since the late Pleistocene. At the Holocene transgressive maximum, the sea reached an area east of ancient Pompeii and formed a beach ridge (Messigno, 5600 and 4500 14C yr B.P.) more than 2 km inland from the present shore. Progradation of the plain due to high volcanic supply during the following highstand resulted in a new beach ridge (Bottaro-Pioppaino, 3600 14C yr B.P.) 0.5 km seaward of the Messigno ridge. Ancient Pompeii was built as the shoreline continued to prograde toward its present position. Deposits of the A.D. 79 eruption blanketed the natural levees of the Sarno River, marshes near the city and on the Sarno's floodplain, the morphological highs of Messigno and Bottaro-Pioppaino beach ridges, and the seashore. That shore was probably 1 km landward of the present one.

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