The islands are part of a geologic structure known as the Izu-Bonin-Mariana Arc system and range in age from 5 million years old in the north to 30 million years old in the south (Guam). The islands are formed as the highly dense and very old western edge of the Pacific plate plunges downward to form the floor of the Mariana Trench and carries trapped water under the Mariana plate as it does so. This water is super-heated as the plate is carried farther downward and results in the volcanic activity which has formed the arc of Mariana Islands above this subduction region.