The Ischia debris avalanche: First clear submarine evidence in the Mediterranean of a volcanic island prehistorical collapse

Sector or flank collapse with related debris avalanches is increasingly recognized as a relatively common volcanic behaviour, in particular, for large, hot-spot related oceanic islands. Here, we report the case of a catastrophic collapse that occurred at Ischia volcanic island in prehistorical times and was driven by the volcano-tectonic uplift of Mt Epomeo, the major relief of the island. The collapse left a subaerial to submarine horseshoe scar on the southern flank of the island and generated a debris avalanche incorporating thousands of giant blocks dispersed as far as 50 km from the island. During the emplacement, part of the debris avalanche evolved into a debris flow covering an area of 250-300 km2. This constitutes the first, clear evidence of a submarine debris avalanche in the Mediterranean Sea. The major collapse was followed, and probably also preceded, by recurrent, less catastrophic terrestrial and underwater failures. Two other undersea hummocky deposits are found north and west of the island and might tentatively be correlated to the major southern collapse. Such volcanic behaviour, previously unknown for Ischia Volcano, has likely triggered tsunami waves over the entire Bay of Naples raising the question of their impact on prehistorical/historical communities.