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Location and Physical Geography

A satellite photograph of Guam
A satellite photograph of Guam

The Pacific island of Guam, the southernmost island in the Mariana Archipelago, is a U.S. unincorporated territory located at 13o 28΄N, 144o 45΄E.  It is 1,500 miles (2,414 km) south of Japan, 3,800 miles (6,116 km) west of the main Hawaiian Islands, and 900 miles (1,448 km) north of the Equator. Guam is the largest and most populated island in Micronesia with an estimated population of 173,500 (2007 census). It has a land mass of 216 square miles (560 km2), excluding coral reef formations, and a maximum elevation of approximately 1,329 feet (405 meters). The island is approximately 30 miles (51 km) long and 9 miles (15 km) wide, narrowing to 4 miles (7 km) at the center. Guam is the westernmost U.S. territory, west of the International Dateline, and is one day ahead of the U.S.

Guam is a volcanic island formed by the union of two volcanoes. It is the peak of a submerged mountain rising 37,820 feet (11,528 meters) above the floor of the Marianas Trench, which has the greatest ocean depth in the world at 36,201 feet (11,033 meters) or 6.79 miles in the Challenger Deep. The majority of the island is surrounded by a coral table reef with deep water channels. The coral reefs are well developed with reef flats as wide as 1,969 feet (600 meters). Coastline areas are characterized by sandy beaches, rocky cliffs, and mangrove forests. 

The northern half of Guam is a relatively flat, uplifted coralline limestone plateau, sloping gently westward, with steep coastal cliffs and narrow coastal plains. It is overgrown with tangan-tangan (Leucaena leucocephala), a scrawny tree planted on Guam after World War II to cover the denuded land and prevent erosion of the exposed soil. Northern Guam is the site of the island’s principal aquifer. There are low-rising hills in the center and mountains in the south. This topography has numerous watersheds drained by 96 streams.

Southern Guam is made up of rolling volcanic hills and mountains, where high waterfalls cut through the landscape. Mount Lam Lam in the south rises to a maximum elevation of 1,334 feet (407 meters) above sea level. Measured from its base at the bottom of the Marianas Trench to its top, it may be the highest mountain on Earth at approximately 37,200 feet (11,339 meters).

Major Features Map of Guam

Map of Guam
Map of Guam (Image: NOAA/Ken Buja)