is a rocky island off the northern coast of Natsudomari Peninsula in Mutsu Bay. The island is a part of Hiranai in

Aomori Prefecture is a prefecture of Japan in the Tōhoku region. The prefecture's capital, largest city, and namesake is the city of Aomori. Aomori is the northernmost prefecture on Japan's main island, Honshu, and is bordered by the Pacific Ocean to the east, ...

. The island has of coastline and an area of . It is a part of the prefecture-maintained,

Asamushi-Natsudomari Prefectural Natural Park is a Prefectural Natural Park on the north coast of Aomori Prefecture, Japan, overlooking Mutsu Bay. Established in 1953, the park spans the borders of the municipalities of Aomori and Hiranai. It encompasses Asamushi Onsen and the coastline of ...

Geography

The island lies only 200 meters north of Natsudomari Peninsula in Mutsu Bay. It was once possible to walk from the island to the peninsula at low-tide, but erosion made this impossible over time. The island is forested on its southern half, while its northern half is covered in grassland. The grassland was used for pastoral farming before the island was designated a natural park.

Geology

Ōshima is a

sill Sill may refer to: * Sill (dock), a weir at the low water mark retaining water within a dock * Sill (geology), a subhorizontal sheet intrusion of molten or solidified magma * Sill (geostatistics) * Sill (river), a river in Austria * Sill plate, a ...

that was formed underground when a volcano dominated the landscape of Natsudomari Peninsula during the

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

. The rock of the Ōshima sill is made up of

dacite Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...

and

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...

History

During the time the Tsugaru clan ruled the area, the island was known as , reflecting that it was viewed as a cape historically rather than an island due to being able to walk to the island during low-tide. In May 1949, a lighthouse was built on the northern side of the island. The island was designated as a part of Asamushi-Natsudomari Prefectural Natural Park in June 1953.

Access

No automobile traffic is allowed on Ōshima. There is a parking lot along Aomori Prefecture Route 9 and bus stop for the Hiranai Town Bus on the northern tip of Natsudomari Peninsula that serves the vicinity of the island. The island is accessible from the peninsula by a concrete

footbridge A footbridge (also a pedestrian bridge, pedestrian overpass, or pedestrian overcrossing) is a bridge designed solely for pedestrians.''Oxford English Dictionary'' While the primary meaning for a bridge is a structure which links "two points at a ...

that connects to a trail that spans the length of the island.

References

External links

*{{in lang, ja}
夏泊半島（大島・椿山）Natsudomari Peninsula, Ōshima- Hiranai Town Website
Islands of Aomori Prefecture Hiranai, Aomori Sills (geology)