The island of Guam is approximately 30 miles in length with a variable width, ranging from 12 miles to 4 miles at its narrowest point. The largest island in Micronesia, Guam has a total land mass of 212 square miles, excluding reef formations. Shaped like a footprint, Guam was formed by the union of two volcanoes. The island has two basic geological compositions. Two-thirds of Guam, the central and northern features, are primarily raised limestones with several volcanic formations at Mount Santa Rosa and Mount Mataguak. The northern clifflines drop precipitately into the sea with an elevation ranging from 300 to 600 feet. The southern features are basically volcanic with an elongated mountain ridge dividing the inland valleys and coastline. The highest point is Mount Lamlam with an elevation of 1,334 feet.

The Peak of a submerged mountain, Guam [satellite photo], rises 37,820 feet above the floor of the Marianas Trench, the greatest ocean depth in the world. When visiting Guam, hiking up one of its mountains is like scaling a peak higher than Everest. A metal object would take 64 minutes to fall through the Marianas Trench, just east of Guam, with a depth of 6.79 miles, where pressure is over 18,000 pounds per square inch. The Marianas Trench was pinpointed in 1951 by the British Survey Ship Challenger, and on January 23, 1960, the manned U.S.N. bathyscaphe Trieste descended to the bottom. On March 24, 1995, the unmanned Japanese probe Kaiko also reached the bottom and recorded a depth of 35,797 feet.

Click volcano map to the left to enlarge (http://oceanexplorer.noaa.gov). Guam straddles the edge of the Philippine Plate with the Pacific Plate thrusting below it -- an area called the subduction zone. The Mariana islands are volcanic products of the magma released at this subduction zone. The melting of rock produces magma containing alot of water at the subduction zone. When this magma reaches the surface, the water expands very rapidly, which is why island-arc volcanoes are so explosive. According to National Geographic, it is the water and sulfur that give these volcanoes their bang.

Among the most volcanically active--and the only submarine volcanic arc in waters under United States jurisdiction--is the Mariana Islands. The Mariana Archipelago region features some 50 submarine volcanic edifices in addition to 11 major subaerial volcanoes dotted along more than 1000 km of arc length. Active hydrothermal sites have been sampled on a few volcanoes in the Mariana Arc: Esmeralda Bank lying west of the island of Tinian and two seamounts in the northern part of the arc called Kasuga 2, Kasuga 3, and one volcano, called East Diamante. The latter volcano, lying 20 nautical miles south of the island volcano Anatahan has arc vent fields which may be the best modern analogues of gold-rich ore deposits currently mined on land. The amazing organisms living around deep geothermal vents are well adapted to extreme pressures.

Guam's Western shoreline faces the Philippine Sea while just a few miles away the Eastern beaches faces the Pacific Ocean. Ancient perpendicular faultlines which collect water, now determine paths of existing tributaries. Guam is the "isle of orthogonal rivers", and Westernmost U.S. territory. It is west of the International Dateline and is 1 day ahead of the U.S.: Hence the slogan "Where America's Day Begins".

Click below topography for detailed image.