Introduction to the Landforms and Geology of Japan

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Submarine Topography

Ocean and marginal seas around Japan

The Pacific Ocean and marginal seas surround the Japanese Islands. The Pacific Ocean expands on the east of the islands. The Philippine Sea fringed by the Izu-Bonin, the Southwest Japan, and the Ryukyu Arcs is separated from the Pacific Ocean. The Sea of Okhotsk, the Sea of Japan, and the East China Sea on the west of the Japanese Islands are marginal seas which are situated behind the arcs (between the arcs and the continent). The Philippine Sea is also recognized as a marginal sea.


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Pacific Ocean

The Northwest Pacific Basin expands on the east of the Kuril, the Japan, and the Izu-Bonin Trenches. The basin 5000 to 6000 m deep has a flat floor. The Pacific Plate (oceanic plate) is produced at the mid-ocean ridge (East Pacific Rise) and goes away from there toward trenches. Therefore, the farther the oceanic plate moves from the ridge, the older it become. The seafloor near Japan, which is very far from the mid-ocean ridge, was formed over 100 million ago (in the Jurassic to Cretaceous).

Philippine Sea

The Shikoku Basin is located on the south of the Nankai Trough, the eastern and western borders of which are the Izu-Bonin Arc and the Kyushu-Palau Ridge, respectively. This basin 3000 to 4000 m deep has the rugged surface to the south of the trough, implying seafloor spreading. The basin is shallower than the Pacific Basin because it is much younger with lower density. The Shikoku Basin was formed from 30 to 15 million years ago accompanied by separating the Kyushu-Palau Ridge from the paleo-Izu-Bonin Arc. The northern end of the ridge is situated at the border between the Southwest Japan Arc and the Ryukyu Arc. The Amami Plateau, the Daito Ridge, and the Oki-Daito Ridge are arranged to the west of the Kyushu-Palau Ridge in this order from the north. Basins lie between these plateau and ridges. The Philippine Basin is on the south of the Oki-Daito Ridge.

Sea of Okhotsk

The Sea of Okhotsk has the Chisima Basin 3000 to 3300 m deep in the south part which is extremely flat and covered with thick sediments.

Sea of Japan

The Sea of Japan is separated into the northern part and southern part at the Yamato Rise situated in the middle of the Sea of Japan. In the northern part, the Japan Basin 3000 to 3600 m deep expands with a flat floor, which is covered with sediments 1000 to 1500 m thick. The crust of the basin is the oceanic crust, formed by seafloor spreading 28 to 18 million years ago. The eastern margin of the Sea of Japan is thought of the boundary of plates at which the plate has begun subducting (see 2) marginal sea side).

The southern part has complex topography including the Yamato Basin, the Korean Plateau, and the Tsushima Basin. The crust in the Yamato Basin is thicker than that in the Japan Basin, and the ridges and plateaus consist of blocks of the continental crust. It is thought that this part has spread mainly by stretching and thinning the curst.

East China Sea

In the East China Sea, the Okinawa Trough is along the Ryukyu Arc and the expansive continental shelf shallower than 150 m in depth is situated between the continent and the trough. The Okinawa Trough is a depression about 1200 km long and 150 km wide. Knolls were formed by volcanic activity in the central area of the trough. This marginal sea is unique in Japan because of active crustal movement and volcanism recognized in the Okinawa Trough. 

Topography from shoreline to deep sea floor

profile of seafloor
Fig. Generalized profile of seafloor
in active continental margins

The seafloor around the Japanese Islands geomorphologically divided into continental shelves, continental slopes, trench (forearc side), and basins (backarc side). This submarine configuration is commonly seen in active continental margins where an oceanic plate subducts. On the other hand, in passive continental margins, the seafloor has no trench but may have a continental rise following a continental slope toward an abyssal plain.

Continental shelf

Continental shelves lying between shorelines and continental slopes are gentle slopes, inclining at an angle of much less than 1 degree (on average, about 0.1 degree [2 m/km]). The slopes abruptly become steep at their outer edge (shelf-slope break) at a depth of less than 200 m (average 130 m). The width of continental shelf varies from place to place, the average of which is about 70 km.

Around the Japanese Islands, the continental shelf is 20 to 30 km wide, narrower than its world average, with shelf-slope breaks of the average depth of 140 meters. Continental shelves in the southern most region of the Sea of Japan offshore from the middle part of southwestern Honshu to northern Kyushu, and the northern Sea of Japan and the Sea of Okhotsk around northern Hokkaido are broader than other areas. Narrow shelves are located offshore the western coast of the Oshima Peninsula, around Toyama Bay, and offshore from the Boso Peninsula to the Kii Peninsula. The selves in Toyama Bay, Sagami Bay, and Suruga Bay are extremely narrow, less than 1 km in width in closed-off sections of the bays. Continental shelf slopes are not flat from shorelines through outer edges, but terrace-like topography (submarine terraces) and submarine valleys are found. Submarine valleys are often filled up with sediments derived from land. 

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