National Park of American Samoa - Geologic History

Geologic History

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. The eastern islands of Ofu and Ta'u, which are consequently youngest, are still in the process of being built. 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 sine broken into angular fragements 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. T'au 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 hot spot 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.