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American Samoa - Geography

The volcanic Samoan Islands chain, which includes both American Samoa and the independent nation of Samoa, lies at the northern end of the Tonga Trench, a highly active seismic zone. American Samoa is located in an active seismic zone and is prone to earthquakes. Tsunamis may occur after a strong earthquake and can travel long distances across the Pacific. American Samoa is a group of five volcanic islands located in the South Pacific Ocean (roughly 14° S and 170° W) approximately halfway between Hawaii and New Zealand. American Samoa is an unincorporated territory of the United States. American Samoa is the southernmost U.S. territory, centered at about 14°S latitude. It is located 3,700 kilometers southwest of Hawai’i and 2,574 kilometers northeast of New Zealand.

Tutuila Island possesses an excellent natural deepwater harbor at Pago Pago, which rests at the base of Mt. Matafao, the second highest peak in the territory at 653 m. The highest peak is Lata Mountain at 966 m on Ta’u Island. Ofu, Olosega, and Ta’u Islands form the Manu’a Islands group. Swains Island is an atoll without a channel connecting the central lagoon and the open ocean, whereas Rose Island is a typical atoll with free exchange of water between the lagoon and open ocean.

The total land area of American Samoa is 76 square miles. There are two coral reefs that are part of the territory: Rose Island and Swains Island, which are low, tropical atolls. Tutuila is the major island of American Samoa, accounting for 69% (55 square miles) of the territory's land area and 97% of the total population. The topography of Tutuila is rugged, volcanic terrain. Aunu’u Island is located one mile off the southeastern shore of Tutuila and has a land area of 375 acres. It is a small volcanic island with a very small population (under 500).

The Manu’a group of islands, Ofu, Olosega, and Ta’u, are approximately 70 miles east of Tutuila. Similar to Tutuila and Aunu'u, the Manu'a Islands have a rugged, volcanic terrain. Ofu and Olosega are connected by a bridge. The Island of Ta’u has a slightly larger population.

The Exclusive Economic Zone (EEZ) of American Samoa encompasses a variety of geologic, morphologic, and tectonic environments. The sediment-covered, deep-water Samoa Basin occupies most of the eastern half of the EEZ. Seamounts of unknown age penetrate through the sediment in the Samoa Basin, mostly near the southern and northern margins of the EEZ. A small piece of the Manihiki Plateau, a feature about 110 million years old, occupies the northeast corner of the EEZ and includes several seamounts.

The eastern end of Samoa Ridge occupies the west-central part of the EEZ. This ridge is composed of the geologically young (1.5 million years old to recent) volcanoes that form all the islands within the American Samoa EEZ, except Swains Island. Tutuila Island has a maximum age of about 1.5 million years, and Ta’u Island has had recent volcanic activity. The hot spot (stationary magma chamber deep in the Earth) that created the entire Samoa island chain and ridge (in both Samoa and American Samoa) lies about 45 kilometers east of Ta’u Island (part of the Manu’a Islands) and its active sea-floor manifestation is called Vailulu’u seamount.

This seamount is both volcanically and hydrothermally active. Part of Robbie Ridge occupies the northwest corner of the EEZ and includes Swains Island and several seamounts. The age of Robbie Ridge may be similar to that of Manihiki Plateau. The southwest margin of the EEZ is sediment covered and abuts the outer (eastward) margin of the Tonga Trench.

The American Samoa EEZ is essentially unexplored for cobalt-rich crusts, nodules, phosphorites, hydrothermal deposits, and precious coral. Although resource evaluations are not possible, environments are present that offer targets for likely mineral occurrences. Cobalt-rich iron-manganese crusts likely occur on the several large and many small seamounts located in the Samoa Basin, Manihiki Plateau, and Robbie Ridge. However, the distribution and morphology of the seamounts in the EEZ are poorly known, and the grade and tonnage of the metal-rich crusts that may occur on those seamounts are unknown, because no research cruises have been dedicated to their

study. Analyses of only two samples of cobalt-rich crusts collected within the EEZ are available. Neither of those samples is from a location likely to contain the highest potential cobalt-crust resource. The two analyses indicate low contents of total copper plus nickel plus cobalt (1.0 and 0.6 percent). However, the most economically important of those three metals is cobalt, and it is quite high in one of the samples (0.7 percent).

Iron-manganese nodules likely occur on the sediment surface of the Samoa Basin, where sediment accumulation rates are less than 5 millimeters per 1000 years. However, the distribution, grade, and tonnage of the nodules are not known. Of the 12 sediment samples collected in the American Samoa part of the Samoa Basin, 6 contained nodules along with the sediment. The abundances of nodules in those samples were estimated as low to medium.