This study uses on-land and offshore geological and structural data to demonstrate that a huge lateral collapse involved the SE flank of Nisyros volcano. The collapse beheaded the summit part of the volcano and also involved the submarine portion of the slope, producing a large debris avalanche deposit with a volume of about 1 km³ which has been recognized on the sea floor. On-land, stratigraphic and structural data indicate that a thick succession of lava flows (Nikia lavas) was emplaced in a huge horseshoe-shaped depression open seaward and extending below the sea. The magma-feeding system in the volcano, pre-dating and following the collapse, was structurally influenced by a dominant NE-SW direction, which is perpendicular to the newly-recognised sector collapse. The NE-SW structural trend is consistent with the regional tectonic structures found offshore around Nisyros and with the related NW-SE extension direction. We suggest that the lateral magma pressure produced by repeated magma injections along tectonic discontinuities contributed to destabilise the volcano flank. The occurrence of a pyroclastic deposit that mantled the scar left by the collapse suggests that a magma batch might have been injected inside the volcano and triggered the collapse. The lavas of the pre-collapse edifice have been deposited in alternating submarine and subaerial environments, suggesting that vertical movements might also be a major triggering mechanism for large lateral collapses. Recognition of this phenomenon is particularly important in recent/active island or coastal volcanoes, as it can trigger tsunamis. © 2008 Elsevier B.V. All rights reserved.

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