 The Republic of Kiribati contains all of the Gilberts, including the island of Tarawa, the site of the country's capital.The Mariana Islands are an arc-shaped archipelago made up by the summits of fifteen volcanic mountains.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.