Geographic area overview
Guam (Chamorro: Guåhån) is an island in the Western Pacific Ocean, at the southern end of the Mariana Islands chain. It lies between 13.2°N and 13.7°N of latitude and between 144.6°E and 145.0°E of longitude. The island is 30 miles (48 km) long and 4 mi (6 km) to 12 mi (19 km) wide. With an area of 209 square miles (541 km²), Guam is the largest island in Micronesia.
Guam and the rest of the Mariana Islands arc were created by the collision of Pacific and Philippine tectonic plates. This has also produced the Marianas Trench, a deep subduction zone to the east of the islands, which contains the deepest surveyed point in the world ocean (35,797 feet / 10,911 m deep). The highest point in Guam is Mount Lamlam (1,332 feet / 406 m). Guam is not volcanically active but it does experience occasional earthquakes.
Guam is divided by a prominent geologic fault (Pago-Adelup fault) into two geologically, topographically, hydrologically, and environmentally distinct halves. The northern part of Guam is a raised limestone plateau, while the southern part contains volcanic peaks dissected by river valleys. The northern part of Guam has relatively flat terrain bounded by nearly vertical coastal cliffs, whereas the south is composed primarily of hilly and mountainous terrain with a prominent ridge of high ground running north-south close to the western coast. The slope of terrain is very steep from the ridgeline to the western coast; from the ridgeline toward the eastern coast the slope is more gradual. In terms of hydrology, the limestone terrain of northern Guam has practically no surface drainage. Instead, water is in the subsurface, within a major karst aquifer called the Northern Guam Lens Aquifer (NGLA), which recharges, transfers, and discharges water through underground pathways. In contrast, the south comprises of steep, relatively impermeable, volcanic formations that support many streams and surface water reservoirs. Finally, vegetation differs as well. The north is covered by scrub and remnants of limestone forest in less accessible areas, whereas the south is dominated by savanna vegetation and riverine forests along valleys and ravines.
This on-line atlas is focused on the northern half of Guam.
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