The are a mountain range

Mountain range

A mountain range is a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, formation, and age; a component part of a mountain system or of a mountain chain...

 in the Tōhoku region

Tohoku region

The is a geographical area of Japan. The region occupies the northeastern portion of Honshu, the largest island of Japan. The region consists of six prefectures : Akita, Aomori, Fukushima, Iwate, Miyagi and Yamagata....

 of Honshū

Honshu

is the largest island of Japan. The nation's main island, it is south of Hokkaido across the Tsugaru Strait, north of Shikoku across the Inland Sea, and northeast of Kyushu across the Kanmon Strait...

, Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

. The range is the longest range in Japan and stretch 500 km (311 mi) south from the Natsudomari Peninsula of Aomori Prefecture

Aomori Prefecture

is a prefecture of Japan located in the Tōhoku Region. The capital is the city of Aomori.- History :Until the Meiji Restoration, the area of Aomori prefecture was known as Mutsu Province....

 to the Nasu

Nasu

Nasu has multiple meanings.* Nasu people, an ethnic group in China, a traditional name for a region in northern Tochigi Prefecture. It is also in the names of several places in that region:**Nasushiobara, Tochigi, a city**Nasukarasuyama, Tochigi, a city...

 volcanoes at the northern boundary of the Kantō region

Kanto region

The is a geographical area of Honshu, the largest island of Japan. The region includes the Greater Tokyo Area and encompasses seven prefectures: Gunma, Tochigi, Ibaraki, Saitama, Tokyo, Chiba, and Kanagawa. Within its boundaries, slightly more than 40 percent of the land area is the Kantō Plain....

. Though long, the range is only about 35 kilometres (21.7 mi) wide. The highest point in the range is Mount Iwate

Mount Iwate

is a stratovolcano complex located northwest of the city of Morioka in western Iwate Prefecture, Tohoku, Japan. The mountain is listed as one of the 100 Famous Japanese Mountains in a book composed in 1964 by mountaineer/author Kyūya Fukada.-Geology:...

, 2038 metres (6,686.4 ft).

The range includes several widely known mountains: Hakkōda Mountains, Mount Iwate, Mount Zaō

Mount Zao

is a complex volcano on the border between Yamagata Prefecture and Miyagi Prefecture in Japan. It consists of a cluster of stratovolcanoes and is the most active volcano in northern Honshū. The central volcano of the group includes several lava domes and a tuff cone, Goshiki-dake, which contains...

, Mount Azuma

Mount Azuma

is an active stratovolcano in Fukushima prefecture, Japan.It has a conical-shaped crater and as the name "Kofuji" suggests, the shape of Mount Azuma is like that of Mount Fuji...

, and Mount Adatara

Mount Adatara

is a stratovolcano on the island of Honshū in Japan.It is located about 15 kilometres southwest of Fukushima city and east of Mount Bandai. Its last known eruption was in 1996....

.

Naming

These mountains previously formed the boundary between historical provinces of Mutsu

Mutsu Province

was an old province of Japan in the area of Fukushima, Miyagi, Iwate and Aomori prefecture and the municipalities of Kazuno and Kosaka in Akita Prefecture...

 (陸奥国) and Dewa

Dewa Province

is an old province of Japan, comprising modern-day Yamagata Prefecture and Akita Prefecture, except for the city of Kazuno and the town of Kosaka. It was sometimes called .-Historical record:...

 (出羽国). The kanji

Kanji

Kanji are the adopted logographic Chinese characters hanzi that are used in the modern Japanese writing system along with hiragana , katakana , Indo Arabic numerals, and the occasional use of the Latin alphabet...

for the name of the mountain range was created from one kanji

Kanji

Kanji are the adopted logographic Chinese characters hanzi that are used in the modern Japanese writing system along with hiragana , katakana , Indo Arabic numerals, and the occasional use of the Latin alphabet...

of the two provinces, 奥 and 羽, respectively.

Geology

The Ōu Mountains began to form in the Pliocene

Pliocene

The Pliocene Epoch is the period in the geologic timescale that extends from 5.332 million to 2.588 million years before present. It is the second and youngest epoch of the Neogene Period in the Cenozoic Era. The Pliocene follows the Miocene Epoch and is followed by the Pleistocene Epoch...

. They sit over the middle of the inner arc of the Northeastern Japan Arc

Northeastern Japan Arc

Northeastern Japan Arc also Northeastern Honshū Arc, is an island arc on the Pacific Ring of Fire. The arc runs north to south along the Tohoku region of Honshū, Japan. It is the result of the subduction of the Pacific Plate underneath the North American Plate at the Japan Trench...

. This is the result of the Pacific Plate

Pacific Plate

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres, it is the largest tectonic plate....

 subducting under the North American Plate

North American Plate

The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, Bahamas, and parts of Siberia, Japan and Iceland. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust...

. A chain of Quaternary

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

volcanoes along the range forms the volcanic front.