BLUE BACKS Shima wa dousite dekirunoka (How do islands form? - Volcanic eruption, and from birth to extinction of islands)

In recent years, several volcanic eruptions have occurred in the seas around Japan, including those at Nishinoshima, Fukutoku-Oka-no-Ba, and Io-To in the Ogasawara Islands. A particularly large-scale submarine eruption occurred at the Hunga-Tonga=Hunga-Ha'apai volcano (Hunga volcano) in the Kingdom of Tonga in the South Pacific in January 2022, causing widespread global effects. This eruption not only produced a massive volcanic plume and tsunamis but also led to the disappearance of portions of the existing islands and the formation of a caldera. As a result, the shape of the volcano and the surrounding environment underwent significant changes.
 
Volcanic eruptions in oceans are common in our planet, which contains abundant water. However, because oceanic volcanoes are often located in places that are difficult to access, witnessing the entire process from eruption to the formation of land is rare. As seen in the eruption at Nishinoshima, where magma and seawater interacted, and the eruption at the Hunga Volcano, which caused a huge tsunami, the effects of seawater on submarine eruptions can cause phenomena that are different from those of volcanic eruptions on land, and the nature of the disasters can also be different. The eruptions observed on islands and submarine volcanoes in recent years may provide important clues to the clarification of the origin of volcanic activity that repeatedly occurs in oceans worldwide and be an aid in planning the mitigation of disasters caused by such eruptions.

In this book, the author shares the latest research results on the birth and disappearance of volcanic islands, the characteristics and mechanisms behind volcanic eruptions that have occurred there, and the impact of eruptions on human society based on his fieldwork at the volcanoes he visited, including the recent cases of Nishinoshima and Fukutoku-Oka-no-Ba. The first half of the book focuses on the phenomena that occur from the birth to the growth of volcanic islands, while the second half focuses on the processes that occur from the maturation to the collapse of volcanic islands.
 
Large-scale eruptions in the ocean have the potential to have a greater influence on the Earth's surface environment than previously thought, including the atmosphere and oceans. However, phenomena involving the creation and destruction of land are rare. It is no exaggeration to say that we were fortunate enough to observe recent large-scale eruptions in the oceans, and we should make the most of this opportunity to improve our understanding of volcanic phenomena. The study of volcanoes, known as volcanology, has made great progress in the last half-century and has contributed to the elucidation of eruption mechanisms and mitigation of volcanic disasters. Although research on submarine volcanoes is ongoing, the causes of the diverse eruption phenomena that occur in various marine environments have not yet been fully elucidated. In this context, we are gaining new insights into not only the characteristics of the eruptions that occur and the nature of the magma that drives the eruptions, but also the individual phenomena that can cause disasters, such as volcanic plumes, pyroclastic flows, pumice drifts, and tsunamis. The research on these phenomena covered in this book will not only help us understand the origins of the volcanic islands that comprise the Japanese archipelago but also provide an approach to the question of how human society, which has been built on these islands, can coexist with volcanoes.
 

(Written by MAENO Fukashi, Associate Professor, Earthquake Research Institute / 2025)