The island is volcanic in origin, lying along the subduction fault where the South American Plate slides beneath the Caribbean Plate. Martinique has eight different centres of volcanic activity. The oldest rocks are andesitic lavas dated to about 24 million years ago, mixed with tholeiitic magma containing iron and magnesium. Mount Pelée, the island's most dramatic feature, formed about 400,000 years ago. Pelée erupted in 1792, 1851, and twice in 1902. The eruption of 8 May 1902, destroyed Saint-Pierre and killed 28,000 people in 2 minutes; that of 30 August 1902 caused nearly 1,100 deaths, mostly in Morne-Red and Ajoupa-Bouillon.