Geography of Guam

Guam is a U.S. territory in the western Pacific Ocean, at the boundary of the Philippine Sea. It is the southernmost and largest member of the Mariana Islands archipelago, which is itself the northernmost group of islands in Micronesia. The closest political entity is the Commonwealth of the Northern Mariana Islands (CNMI), another U.S. territory. Guam shares maritime boundaries with CNMI to the north and the Federated States of Micronesia to the south. It is located approximately one quarter of the way from the Philippines to Hawaii. Its location and size make it strategically important. It is the only island with both a protected harbor and land for multiple airports between Asia and Hawaii, on an east–west axis, and between Papua New Guinea and Japan, on a north–south axis.

The island is a result of the volcanic activity created by subduction of the Pacific Plate under the Philippine Sea Plate at the nearby Mariana Trench, which runs from the east of Guam to the southwest. Volcanic eruptions established the base of the island in the Eocene, roughly 56 to 33.9 million years ago (mya). The north of Guam is a result of this base being covered with layers of coral reef, turning into limestone, and then being thrust 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.

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 capitol 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.

The Mariana Islands lie atop the largely submerged East Mariana Ridge, a part of the Izu–Bonin–Mariana (IBM) Arc. The IBM Arc is a tectonic plate convergent boundary where the west Pacific Plate subducts the Philippine Sea Plate. Guam is actually located on the Mariana Plate, a micro plate between the two. The subduction area is marked by the Mariana Trench, the deepest gash in the earth's surface, which includes three deep spots to the south of Guam. From east to west, these are: Nero Deep, which was the deepest known spot in the ocean from 1899 to 1927 at 9,660 m (31,690 ft); Sirena Deep, the third deepest measured point at 10,714 m (35,151 ft); and Challenger Deep, the deepest point at 10,902 to 10,929 m (35,768 to 35,856 ft).

There have been three major eruptions on Guam. the first, the Facpi formation, in the mid Eocene, laid the base of the island and is still the topmost formation along the southwestern coast. The second eruption created the Alutom formation that is still the topmost strata in the middle of the island. The Mount Alifan-Mount Lamlam ridge is the remnant of the Alutom formation caldera.

The last volcanic layer, called the Umatac formation, was formed by the third and final eruption, which surrounded this ridge in the south of Guam. Meanwhile, volcanic activity alternately submerged areas of the island, which hosted coral reefs, and then lifted those reefs, which became limestone. The island may be divided into four general geophysical regions: the uplifted and relatively flat coralline limestone plateau in the north that provides most of the drinking water; the low-rising hills of the Alutom Formation in the center; the mountains of the Umatac formation in the south; and the coastal lowlands ringing most of the island. Much of the coast is protected by a fringing reef.

Soils are mostly silty clay or clay and may be gray, black, brown or reddish brown; acidity and depth vary. There are four National Natural Landmarks in Guam chosen as exemplars of the island's geology: Facpi Point, Fouha Point, Mount Lamlam, and Two Lovers Point.

Guam occasionally experiences earthquakes; most with epicenters near Guam have had magnitudes ranging from 5.0 to 8.7. Unlike Anatahan in the Northern Mariana Islands, Guam is not volcanically active, though vog (volcanic smog) from Anatahan affects it due to proximity.