American Samoa
American Samoa ( sm, Amerika Sāmoa, ; also ' or ') is an unincorporated territory of the United States located in the South Pacific Ocean, southeast of the island country of Samoa. Its location is centered on . It is east of the Internationa ...
is part of the broader geology of the Samoan island chain.
Geologic History & Origins
Except for Rose Atoll, the islands in the chain are geologically young, having formed within the last few million years, likely in the
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Quaternary. Preliminary mapping took place in the 1940s and 1950s, with a hiatus until the 1980s. Situated near the southwest margin of the Pacific Plate, American Samoa exhibits high volcanic mountains and submerged reef banks. Throughout the island chain, islands get larger to the west, with the eroded Rose Atoll in the east.
The island chain is inferred to be related to hotspot volcanism, with alkali basalt rather than a
tholeiitic magma series
The tholeiitic magma series is one of two main magma series in subalkaline igneous rocks, the other being the Calc-alkaline magma series, calc-alkaline series. A magma series is a chemically distinct range of magma compositions that describes the ...
.
On Tutuila,
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...