Abstract
The island-forming Nishinoshima eruptions in the Ogasawara Islands, Japan, provide a rare opportunity to examine how the terrestrial part of Earth’s surface increases via volcanism. Here, the sequence of recent eruptive activity of Nishinoshima is described based on long-term geological and geochemical monitoring of eruptive products. Processes of island growth and temporal changes in the magma chemistry are discussed. The growth of Nishinoshima was sustained by the effusion of low-viscosity andesite lava flows since 2013. The lava flows spread radially with numerous branches, resulting in compound lava flows. Lava flows form the coherent base of the new volcanic edifice; however, pyroclastic eruptions further developed the subaerial volcanic edifice. The duration of three consecutive eruptive episodes decreased from 2 years to a week through the entire eruptive sequence, with a decreasing eruptive volume and discharge rate through time. However, the latest, fourth episode was the most intense and largest, with a magma discharge rate on the order of 106 m3/day. The temporal change in the chemical composition of the magma indicates that more mafic magma was involved in the later episodes. The initial andesite magma with ∼60 wt% SiO2 changed to basaltic andesite magma with ∼55 wt% SiO2, including olivine phenocryst, during the last episode. The eruptive behavior and geochemical characteristics suggest that the 2013–2020 Nishinoshima eruption was fueled by magma resulting from the mixing of silicic and mafic components in a shallow reservoir and by magma episodically supplied from deeper reservoirs. The lava effusion and the occasional explosive eruptions, sustained by the discharge of magma caused by the interactions of these multiple magma reservoirs at different depths, contributed to the formation and growth of the new Nishinoshima volcanic island since 2013. Comparisons with several examples of island-forming eruptions in shallow seas indicate that a long-lasting voluminous lava effusion with a discharge rate on the order of at least 104 m3/day (annual average) to 105 m3/day (monthly average) is required for the formation and growth of a new volcanic island with a diameter on km-scale that can survive sea-wave erosion over the years.
Highlights
IntroductionVolcanic eruptions occur in shallow-water environments in active volcanic regions, such as at convergent plate boundaries, and in hot spots and rift zones in and along oceanic plates
Submarine eruptions in shallow seas have been recently observed in volcanically active regions, e.g., the Zubair archipelago in the southern Red Sea (Xu et al, 2015) and the Tonga archipelago (Cronin et al, 2017), detailed observations are rarely achieved because tiny new islets can be eroded by waves and disappear in a short period without the supply of a large volume of magma to the surface
We study the 2013–2020 Nishinoshima eruption and discuss the formation process for the new volcanic island and the magma supply system beneath the island based on time series of geological and geochemical data obtained via remote observations and land surveys at the volcano
Summary
Volcanic eruptions occur in shallow-water environments in active volcanic regions, such as at convergent plate boundaries, and in hot spots and rift zones in and along oceanic plates. These submarine eruptions often result in a change in the eruptive environment from submarine to subaerial and the production of a new island. The emergence and growth of a new volcanic island provide a rare opportunity to examine how the terrestrial part of the Earth surface increases via volcanism. Such events provide fundamental knowledge concerning the very early stages of growth of a large volcanic island edifice. Even in the Japanese archipelago, only two cases have created new long-lasting (decadal-year-old) islands in historical times before the 2013–2020 Nishinoshima eruption: the eruptions in Nishinoshima, Ogasawara, from 1973 to 1974
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