 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.The Marianas were politically divided in 1898, when the United States acquired title to Guam under the Treaty of Paris, 1898, which ended the Spanish–American War.Spain then sold the remaining northerly islands to Germany in 1899.