Pago is a

shield volcano A shield volcano is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more viscous lava ...

in the center of

Tutuila Island Tutuila is the largest and most populous island of American Samoa and is part of the archipelago of the Samoan Islands. It is the third largest island in the Samoan Islands chain of the Pacific Ocean, Central Pacific. It is located roughly north ...

, the largest island of

American Samoa American Samoa is an Territories of the United States, unincorporated and unorganized territory of the United States located in the Polynesia region of the Pacific Ocean, South Pacific Ocean. Centered on , it is southeast of the island count ...

in the

South Pacific Ocean South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both west and east. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþa ...

. The volcano was formed by

subaerial eruption In volcanology, a subaerial eruption is any sort of volcanic eruption that occurs on the land surface, or in the open air "under the air". They generally produce pyroclastic flows, lava fountains and lava flows, which are commonly classified in di ...

s between 1.54 and 1.28 million years ago. It has been estimated that the volcano was as high as above sea level. Activity on Pago Volcano ended with emplacement of

trachyte Trachyte () is an extrusive igneous rock composed mostly of alkali feldspar. It is usually light-colored and aphanite, aphanitic (fine-grained), with minor amounts of mafic minerals, and is formed by the rapid cooling of lava (or shallow intrus ...

bodies with ages of 1.03 ± 0.01 Ma. The Pago Volcano

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the str ...

was formed 1.27+-0.02 million years ago. The caldera's dimensions are estimated at in length and in width. The southeast part of the caldera makes up

Pago Pago Harbor Pago Pago Harbor on Tutuila Island in American Samoa is one of the world's largest natural harbors. The capital, Pago Pago, is located on the inner reaches of the harbor, close to its northwesternmost point. It has the highest annual rainfall o ...

, one of the world's largest natural harbors. The northwest rim, Maugaloa Ridge, creates the southern border of the

National Park of American Samoa The National Park of American Samoa is a national park of the United States located in the territory of American Samoa, distributed across three islands: Tutuila, Ofu, and Taʻū. The park preserves and protects coral reefs, tropical rainfores ...

. Mount ʻAlava is part of the

rim of the Pago Volcano.Harris, Ann G. and Esther Tuttle (2004). ''Geology of National Parks''. Volume 1. Hunt Publishing Company. Page 610. ISBN 9780787299705.

References

Volcanoes of American Samoa Tutuila Pago Pago Extinct volcanoes of the United States Polygenetic shield volcanoes {{AmericanSamoa-geo-stub