The Mariana Islands (also the Marianas) are an arc-shaped archipelago made up by the summits of fifteen volcanic mountains in the north-western Pacific Ocean between the 12th and 21st parallels north and along the 145th meridian east. They are south of Japan and north of New Guinea, and form the eastern limit of the Philippine Sea. The islands were named after Spanish Queen Mariana of Austria in the 17th century, when Spain started the colonization of the archipelago. They form the northern part of the western Pacific subregion of Micronesia, and are composed of two U.S. jurisdictions: the Commonwealth of the Northern Mariana Islands and, at the southern end of the chain of islands, the territory of Guam.
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 island chain arises as a result of the western edge of the Pacific Plate moving westward and plunging downward below the Mariana plate, a region which is the most volcanically active convergent plate boundary on Earth. This subduction region, just east of the island chain, forms the noted Mariana Trench, the deepest part of the Earth's oceans and lowest part of the surface of the Earth's crust. In this region, according to geologic theory, water trapped in the extensive faulting of the Pacific Plate as serpentinite, is heated by the higher temperatures of depth during its subduction, and the pressure from the expanding steam results in the hydrothermal activity in the area, and the volcanic activity which formed the Mariana Islands.
The Mariana Islands are the southern part of a submerged mountain range that extends 1,565 miles (2,519 km) from Guam to near Japan. Geographically, the Marianas are the northernmost islands of a larger island group called Micronesia, situated between 13° and 21°N latitude and 144° and 146°E longitude.