page_title: National Park of American Samoa
section_title: Geology
hierarchical_section_title: National Park of American Samoa / Geology
caption_attribution_description: English: Samoan Tectonic setting
context_page_description: The National Park of American Samoa is a national park in the United States territory of American Samoa, distributed across three islands: Tutuila, Ofu, and Ta‘ū. The park preserves and protects coral reefs, tropical rainforests, fruit bats, and the Samoan culture. Popular activities include hiking and snorkeling. Of the park's 13,500 acres, 9,000 acres is land and 4,500 acres is coral reefs and ocean. The park is the only American National Park Service system unit south of the equator.
context_section_description: The volcanic islands of Samoa that dominate the acreage of the national park are composed of shield volcanoes which developed from a hot spot on the Pacific Plate, emerging sequentially from west to east. Tutulia, the largest and oldest island, probably dates from the Pliocene Epoch, approximately 1.24 to 1.4 million years ago, while the smaller islands are most likely Holocene in age.
  The islands are not made up of individual volcanoes, but are rather composed of overlapping and superimposed shield volcanoes built by basalt lava flows. Much of the lava that erupted has since broken into angular fragments known as breccia. The volcanoes emerged from the intrusion of basaltic dikes from a rift zone on the ocean floor during the Pliocene Epoch, and were heavily eroded during the Pliocene and early Pleistocene Epochs, leaving behind trachyte plugs and exposed outcrops of volcanic tuff throughout the park. Ta'u island, the youngest of the islands included within the national park, is all that remains from the collapse of a shield volcano during Holocene time. This collapse produced sea cliffs over 3,000 feet high on the north side of the island, some of the highest such escarpments in the world.
  While the Samoan islands have not shown evidence of volcanism for many years, the Samoa hotspot beneath the islands continues to give indications of activity, with a submarine eruption detected just east of American Samoa in 1973. The Vailulu'u Seamount, located east of Ta'u, is a future Samoan island developing from submarine lava flows, continuing the eastward progress of volcanic development from the hotspot below the islands. The lava flows forming the seamount have been dated by radiometric methods to between 5 and 50 years, during which time the seamount has risen 14,764 feet from the ocean floor.
  Evidence exists of past submarine and surface landslides as a result of weathering and other forms of erosion of the rocks and soil making up the islands. On Ta'u island, an inland escarpment known as Liu Bench (a feature of mass wasting) threatens to slump into the nearby ocean, an event which could produce a tsunami strong enough to devastate the island of Fiji to the southeast.
  Olivine basalts were extruded from a N. 70° E. trending rift zone, oriented along the current Afono and Masefay bays of Tutuila, in the Pliocene or earliest Pleistocene.  The Masefau dike complex and talus breccias are remnants of this rifting.  Development of the Taputapu, Pago, Alofau, and Olomoana shield domes followed long parallel fissures followed, but when the Pago and Alofau summits collapsed, calderas were formed.  Thick tuffs were deposited in the Pago caldera, and the southern rim was buried by lavas composed of picritic basalts, andesites, and trachytes.  Subsequent erosion in Early to Middle Pleistocene enlarged the calderas, the Pago River in particular carved a deep canyon, the forerunner of today's Pago Pago Bay.  A submarine shelf formed from the erosional runoff, allowing for the development of coral reefs before the island was submerged 600 to 2000 feet. Sea level fluctuations continued in the Middle to Late Pleistocene.  A barrier reef formed, was submerged 200 feet, before emerging 50 feet, leaving sea caves above sea level.  Leone volcanics erupted in recent time generating tuff cones undersea, such as Aunuu Island, and cinder cones on land.  The pahoehoe flows buried the submerged barrier reef, enlarging the island by 8 square miles.  The island has since emerged another 5 feet.
  Ofu and Olosega are the remains of a single basaltic volcano, 4 miles north to south and 6 miles east to west, which formed in the Pliocene to Early Pleistocene.  Remnants of one half of the caldera, ponded flows, form the north center portion of Ofu.  The steepest cliffs, 600 feet high, are found on this north coast.  The Ofu-Olosega island group formed along the same N. 70° W. trending rift which formed Tau, another single basaltic dome.  The remnant of Tau's caldera is found on the south coast.  A 2000-foot cliff marks the north coast of this island.
  Upolu f
caption: Samoan tectonic setting
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