December 26, 1997 Tsunami: White River Valley, Montserrat

A volcanic debris slide of 60 million cubic meters occurred in the White River Valley, Montserrat, on December 26th (named the Boxing Day Collapse). On the night of the eruption there were reports of a wave inundating the Old Road Bay area, 10 km from the landslide site. A small tsunami was generated, probably by the steep flow front of the debris avalanche, possibly assisted by the pyroclastic flows as they entered the sea at the mouth of the White River Valley. The tsunami wave was refracted around the coastline of Montserrat,and achieved considerable run-up in Old Road Bay. The wave was estimated to have been about 1 m higher than the road which lies 2-m above water level, and to have moved inland a maximum distance of 80 m. A variety of objects, including a small wooden boat, a roof to a shelter, and a stone table were displaced several meters inland and a large log was carried even further by the wave. Impact marks up to 1 m were also on the side of palm trees facing the sea. The grass was oriented in such a way as to indicate the retreat of the wave. An observer reported seeing the sea move out and then back in, which is typical of a landslide-generated tsunami.(reference #1315)

The largest single event of the first phase of the eruption occurred on 26 December 1997, when the southern wall of the old summit crater failed due to loading by the lava dome. This generated a debris avalanche and resulted in the rapid collapse of the active lava dome, unleashing a powerful pyroclastic density current and a volcanic blast, as well as generating a small tsunami. (reference #1705)