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Geography and environment

Guam is 30.17 miles (48.55 kilometers) long and 4 to 12 miles (6 to 19 kilometers) wide. It has an area of 212 square miles (549 square kilometers). It is the 32nd largest island of the United States. It is the southernmost and largest island in the Mariana Islands, as well as the largest in Micronesia. Guam's Point Udall is the westernmost point of the U.S., as measured from the geographic center of the United States.

The Mariana chain of which Guam is a part, was created by collision of the Pacific and Philippine Sea tectonic plates. Guam is located on the micro Mariana Plate between the two. Guam is the closest land mass to the Mariana Trench, the deep subduction zone that runs east of the Marianas. Volcanic eruptions established the base of the island in the Eocene, roughly 56 to 33.9 million years ago. The north of Guam is a result of this base being covered with layers of coral reef, turning into limestone, and then being thrust upward by tectonic activity to create a plateau.

The rugged south of the island is a result of more recent volcanic activity. Cocos Island off the southern tip of Guam is the largest of the many small islets along the coastline. Guam's highest point is Mount Lamlam at 1,334 feet (407 meters) above sea level. If its base is considered to be the nearby Challenger Deep, the deepest surveyed point in the Oceans, Mount Lamlam is the world's tallest mountain at 37,820 feet (11,530 m).

Politically, Guam is divided into 19 villages. The majority of the population lives on the coralline limestone plateaus of the north, with political and economic activity centered in the central and northern regions. The rugged geography of the south largely limits settlement to rural coastal areas. The western coast is leeward of the trade winds and is the location of Apra Harbor, the capital Hagåtña, and the tourist center of Tumon. The U.S. Defense Department owns about 29% of the island, under the management of Joint Region Marianas.