AVALANCHE-ASSOCIATED MASS FLOWS DAMAGED POMPEII SEVERAL TIMES BEFORE THE VESUVIUS CATASTROPHIC ERUPTION IN THE 79 C.E

The ancient town of Pompeii, in southern Italy, is situated southeast of Naples at the base of Somma-Vesuvius volcano. Historical data show that the town had a discontinuous development until the I century B.C.E. and then flourished until it was seriously damaged by powerful earthquake tremors in 62 C.E. While still under repair seventeen years later, Pompeii was destroyed and completely buried by pyroclastic deposits from the catastrophic Vesuvius eruption of 79 C.E.

Mass-wasted deposits triggered by flood flows were discovered in and around Pompeii under the 79 C.E. pyroclastic deposits. We associate the discontinuous development of Pompeii with these sediment gravity flows that damaged the city long before its final demise by the Vesuvius eruption. Evidence for this, in new site excavations and drill cores, includes at least three downslope-directed depositional episodes channeled via a now-buried river valley that formerly extended to Pompeii. One such mass flow may have been partially responsible for urban decline between the 5th and 3rd centuries B.C.E. when climatic conditions in southern Italy were characterized by cool-humid phases involving increased rain storms.

Ancient gravity flows discovered here are similar to those recently triggered by intense rains and that destroyed towns near Pompeii. On 5 May 1998 avalanche flows of water-saturated tephra-rich colluvium, including boulders were released after about 30 hours of strong continuous rainfall (100-180 mm) from the surrounding calcareous highlands, merged and moved downslope at a rate of ~50 km/hour. The landslides evolved to hyperconcentrated flows incorporating remobilized volcaniclastic sediment with carbonate debris. These recent high-density mass flows killed more than 150 persons, destroyed numerous dwellings, and thickly blanketed the streets of the town of Sarno, 14 km E of ancient Pompeii.

Geoarchaeological study shows that the Gulf of Naples region, in the past as at present, has been more susceptible to episodic damage from hydrological hazards of avalanches and mass flows than to earthquakes and volcanism.