 The Centre Hills in the central part of the island and the Silver Hills in the north are older volcanic massifs related to the subduction zone.Seismic activity had occurred in 1897–1898, 1933–1937, and again in 1966–1967, but the eruption that began on 18 July 1995 was the first since the turn of the 20th century in Montserrat.When pyroclastic flows and mudflows began occurring regularly, the capital, Plymouth, was evacuated, and a few weeks later a pyroclastic flow covered the city in several metres of debris.