The island of St. Kitts is composed almost exclusively of volcanic rocks of andesite or dacite mineralogy. Its geology is similar to that of other volcanic islands in the Lesser Antillean Archipelago. The islands are the summits of a submerged mountain range which forms the eastern boundary of what is known as the Caribbean Tectonic Plate. St. Kitts is oriented northwest southeast, about 80 km long and 16 km wide. The entire island archipelago is geologically young, having begun to form probably less than 50 million years ago, during the Miocene era. Volcanic activity occurred along the ridges of this arc during the Miocene era and has continued since. Nevis is a volcanic island that began its formation in mid-Pliocene times (approximately 3.45 million years ago). Nevis island comprises a number of discrete eruptive centers that range in age from mid-Pliocene to Pleistocene, these prevent any single model of the island’s geological evolution. The geology of Nevis can be subdivided into four informal units: Volcanic of the eruptive centers, volcanigenic rocks, pyroclastics, lahars, fluviatile and lacustrine deposits, and raised beaches.