Samoa hotspot

The Samoa hotspot is marked 35 on map.

Diagram showing how islands are formed by hotspots

The Samoa hotspot is a volcanic hotspot located in the south Pacific Ocean. The hotspot model describes a hot upwelling plume of magma through the Earth's crust as an explanation of how volcanic islands are formed. The hotspot idea came from J. Tuzo Wilson in 1963 based on the Hawai'i volcanic island chain.

In theory, the Samoa hotspot is based on the Pacific Tectonic Plate traveling over a fixed hotspot located deep underneath the Samoan Islands.^[1] The Samoa hotspot includes the Samoan Islands (American Samoa and Samoa), and extends to the islands of Uvea or Wallis Island (Wallis and Futuna) and Niulakita (Tuvalu), as well as the submerged Pasco banks.^[2]

As the Pacific Plate moves slowly over the hotspot, thermal activity builds up and is released in magma plume spewing through the Earth's crust, forming each island in a chain. The Samoa islands generally lie in a straight line, east to west, in the same direction of the tectonic plate 'drifting' over the hotspot.^[3]

Mt Matavanu eruption on Savai'i island, 1905.

A characteristic of a “classic” hotspot, like the Hawaii hotspot, results in islands located further from the hotspot being progressively older with newer and younger islands closest to the fixed hotspot, like the Loihi Seamount, the only submarine volcano which has been studied in detail by scientists. The scientific research from Loihi has resulted in a 'Hawai'i' model for hotspots primarily limited to the information gathered from the Hawai'i islands.^[4]

However, the Samoa hotspot is currently an enigma for scientists.^[5] In the Samoa Islands, the eastern most island of Ta'u and the western most island of Savai'i have both erupted in the past 150 years. The most recent eruption on Sava'i occurred with Mount Matavanu (1905–1911) and on Ta'u in 1866.^[6]

Vailulu'u

In 1975, geophysicist Rockne Johnson discovered the Vailulu'u Seamount, 45 km east of Ta'u island in American Samoa which has since been studied by an international team of scientists. Within the summit crater of Vailulu'u is an active underwater volcanic cone called Nafanua,^[5] named after a war goddess in Samoan mythology. The study of Vailulu'u provides scientists with another possible model for hotspots^[4] as an alternative to the Hawai'i hotspot model.

An important difference between Vailulu'u and Loihi in Hawai'i, is a total lack of tholeiitic basalt compositions at Vailulu'u^[4] although both are located at the easternmost point of their respective island chains.

The northern Tonga Islands (Vava'u and Niuatoputapu) are moving away from Fiji on the Australian plate at rates of about 130 mm/yr and 160 mm/yr, respectively, while Niue and Rarotonga on the Pacific plate are approaching the Australian plate at about 80 mm/yr. This implies that Pacific plate is tearing at the corner of the trench-transform boundary at a rate that is the sum of these two (160 + 80) 240 mm/yr.^[7]

References