Off all the planets in the solar system, it is only earth that has continents. How this continental crust was formed, we don’t really know. Clues for solving this mystery are gradually being revealed through the continuous surveying in the ocean stretching from the Izu Peninsula to the Mariana Archipelago. This year, the Ocean Drilling project is also scheduled to start in earnest. We talked to team leader Yoshihiko Tamura of the Institute for Research on Earth Evolution, who is on the forefront of research into the mystery of continental crust formation.
| Interviewee： Yoshihiko Tamura Project IBM & Crust Evolution Research Teams, Institute for Research on Earth Evolution, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) |
World’s first discovery of primary magma
The oceanic crust on the seafloor is mainly composed of basalt, and the continental crust which forms the continents is mainly made up of andesite. Only how andesite is produced from magma and how continental crust is formed has long been a big mystery.
“To solve this mystery, we first need to understand what kind of magma is initially formed when the mantle melts,” says Yoshihiko Tamura. However, the composition of regular magma undergoes changes while it makes its way through magma chambers until it erupts at the surface. Getting hold of freshly formed (primary) magma is essential to its research.
In 2013, this primary magma was discovered by Tamura and his research team. In a submarine survey near the island of Pagan in the Mariana Archipelago pillow lava was retrieved from 2,000 m under water, which on analysis turned out to consist of undifferentiated magma, closely resembling primary magma. It appeared to have directly erupted onto the seafloor without passing through magma chambers.
The island of Pagan is one of the volcanoes in the oceanic arc called the Izu-Bonin-Mariana arc (IBM arc). This area, which stretches over 2,800 km from the Izu Peninsula to the Bonin Islands and the Mariana Islands, has a great number of submarine volcanoes and island volcanoes placed in an arc that continue to be active, in the subduction zone where the Pacific plate subducts under the Philippine plate. Nishinoshima, which was in the news last year when a new island appeared, is one of these. With the plate subduction large volumes of water and sediment are brought deep into the earth. This makes the mantle more likely to melt, and magma is formed. The rising of this magma creates the many submarine volcanoes.
Analysis of the primary magma found that two types of primary magma had erupted without having blended. From this we can conclude that there are two types of substances that are supplied to the mantle from the subduction plate, that these melt the mantle independently from each other, and that they produce two different primary magmas. By getting hold of primary magma we have been able to gain a detailed understanding of the origin of magma, something which we have only been able to infer up till now.
Planned drill sites for Project IBM
Structure of the subseafloor crust at the drill sites
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