Press Releases

November 1, 2007
The Japan Agency for Marine-Earth Science and Technology

Discovery of New Continental Crust Formation in Izu-Bonin Intra-oceanic Arc Scientific grounds for extending Japanese Continental Shelf


The Institute for Research on Earth Evolution (IFREE: Mr. Yoshio Fukao, Director-General), the Japan Agency for Marine-Science and Technology (JAMSTEC: Mr. Yasuhiro Kato, President) had conducted the crustal structure survey (*1) in Izu-Ogasawara region in 2004 and 2005 and clarified for the first time that volcanoes (including marine volcanoes) there forming continental curst although the most part of Izu-Ogasawara Arc (*2) is under the seafloor. This achievement is expected to be an important scientific reason for extending continental shelf.
This result will be published on the American science journal "Geology" on November 1 (Japan standard time).


Geological findings of continental crust existence and its continuity from Japanese territory is significant scientific grounds for extending continental shelf. As for existence and distribution range of continental crust (*3), information could not be obtained since ample crustal structure survey had not been conducted at sea area around Japan, especially in southern area.


Survey had been done from July 8 to July 28 in 2004 and from July 8 to August 1 in 2005 at 1,000km traverse line (Fig.1) from Sagami bay to north off of Kita Iwo-jima, by deploying ocean bottom seismometers (OBSs)(Pic.2) at 213 points and generating sound wave on ocean surface with a large-capacity air-gun (Pic.3) utilized Research Vessel KAIYO (Pic.1). Data of sound wave through crust and mantle were recorded on OBSs and analyzed. And inner structure of crust and mantle were studied through sound wave velocity distribution under the seafloor.


Clarified subsurface structure under the tandemly-arrayed volcanoes located from Sagami bay to northern Kita Iwo-jima down to approximately 35-kilometer depth.
This survey provided new findings as described below.

Continental crust composed mainly from granitoid or andesitic crust along with volcanoes array which is continuing more than 1,000 kilo meters from Japan archipelagoes.
Its thickness varies along with volcanoes array and the max is right under the volcanoes. In other words, continental crust is formed centering on volcanoes. (Fig.2)
Crust between Sagami Bay and Tori Shima is thick but crust under marine volcanoes between Tori Shima and Kita Iwo-jima is thin. However, the crust components are similar to typical continental crust regardless of volcanoes size. (Fig.3)
There is mixed layer of crust and mantle between crust and mantle, which is considered to be formed in time of continental crust formation. Hence, the mixed layer of crust and mantle need to be back in mantle, if the structure is to be the typical continental crust in a strict sense. (Fig.2, 3)

From these reasons, it is concluded that volcanoes array at Izu-Ogasawara Arc, large or small, are existing as production plants of continental crust.

Future prospects and Significance
To know particulars about continental crust extensity toward transverse direction of Izu-Ogasawara Arc, combine results from several traverse lines and clarify three-dimensional distribution of continental crust. It is possible to verify continental crust structure of Izu-Ogasawara Arc if we could actually obtain rocks that forming the crust and analyze them by drilling many locations including areas surrounding volcanoes utilizing Deep-sea Drilling Vessel "Chikyu".
Achievements from this survey brought geological grounds which is very important to make a point of extending continental shelf starting from volcanoes array on Izu-Ogasawara Arc. It suggests that continental crust also exist in Kyushu-Palau Ridge, since that had been formed by being divided from Izu-Ogasawara Arc. So that it can be geological ground for extending continental shelf starting from Kyushu-Palau Ridge. With data of bathymetric feature, these geological findings are considered to be potent reasons on extending continental shelf.

(*1) Conducted as "the Crustal Structure Exploration for Continental Shelf Territorial Delimitation" from 2004, based on "the Future Fundamental Policy for Continental Shelf Delimitation" designed in August 2003 ("the Basic Policy for Continental Shelf Delimitation" from August 2004) at "the Commission in the Concerned Government Ministries on the Continental Shelf Exploration" (now succeeded to "the Executive Meeting of General Maritime Policy Headquarter").

(*2) Series of volcanic arc of 1,200 kilo meters from Izu-island to southward. There are Oh-shima, To-shima, Nii-jima, Kouzu-shima, Miyake-jima, Mikura-jima, Hachijo-jima, Aoga-shima, Tori-shima, Soufugan in Izu Island chain, Bonin Islands, Nishinoshima, Kita Iwo-jima, Iwo-jima, Minami Iwo-jima. It is considered to be formed by subduction of Pacific plate under the Philippine sea plate.

(*3) Continental crust is composed of relatively light rocks such as granite or andesite. Normally it is thicker than 35 kilo meters. On the other hand, Oceanic crust is composed of heavier rocks such as basaltic rocks and normally it is thin crust around 6 kilometer-thick.

Fig.1 Survey sea area. Survey of subsurface structure had been conducted along the black and blue line area.

Cross section of subsurface structure from Sagami Bay to Kita Iwo-jima
A. Seismic velocity structure obtained by subsurface structure survey
B. Geological translation
C. Thickness of continental crust has been calculated based on data from subsurface structure survey. It is clear that the thickness is max right under the volcano.

Fig.3 Structural comparison of typical continental crust and crust under volcanoes on Izu-Ogasawara Arc. It shows that crustal structure components of Aoga-shima (Izu Arc) and Suiyo seamount (Ogasawara Arc) are similar to typical continental crust. Crust of Aoga-shima would likely to be typical continental crust when it grows 150%. However, the mixed layer of crust and mantle need to be back in mantle, if the structure is said to be the typical continental crust in a strict sense.

Pic.1: Research Vessel KAIYO. Equipped four air guns on each side.

Pic.2: Ocean-bottom seimographs (OBSs). Deployed on the seafloor.

Pic.3: Air gun apparatus.
Left: Shooting acoustic wave in the water.
Right: Putting air guns into the water. Long black object is the float and two silver cylinders are air guns.


(For the study)
Dr. Shuichi Kodaira,
Group Leader, Research Program for Plate Dynamics
Mr. Katsura Shibata
Manager, Research Promotion Office
(For Publication)
Mr. Shinji Oshima, e-mail:
Manager, Planning Department Press Office