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Hidakasanmyaku Erimo Tokachi NP, Hokkaido
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Along the Samani–Erimo coast, kombu harvesters have dried kelp on angular green gravel from Apoi’s foothills. The peridotite’s rough, open texture helps air circulate; rounded pebbles can make the drying work harder.
The Horoman peridotite complex that includes Apoi contains olivine megacrysts up to 1.6 m long—single crystals nearly as tall as a person, preserved in rocks that originated deep inside Earth.
A Samani Ainu story links Apoi to apeoi, “place where fire is”: villagers lit a summit altar fire and prayed to the kamuy for deer. When deer returned, the mountain’s name recalled the ceremony.
At only 810 m, Apoi supports about 80 alpine plant species, with nearly 20 endemic forms. Cool sea fog and thin peridotite soil help create their unusual habitat; the plant community became a Special Natural Monument in 1952.
Japan’s only known population of the small Hime-chamadara-seseri butterfly lives around Apoi. It was discovered here in 1973 and designated a National Natural Monument in 1975.
About 13 million years ago, continental plate collision helped raise mantle rock as the Hidaka Mountains formed. Apoi is made of the Horoman peridotite complex, bringing a normally hidden part of Earth into view.
Between the 6th and 7th stations, an outcrop beside the trail reveals alternating layers of peridotite and gabbro. The harder gabbro stands out in relief while the brown peridotite recedes.
Apoi-maimai is a tiny endemic land snail shaped by the area’s unusual peridotite habitat. Its adult shell is only about 1 cm across, and it lives around the Apoi massif and Horoman River.
Peridotite often reacts with water and changes into serpentinite as it rises from the mantle. Much of Apoi’s rock escaped that alteration, preserving unusually fresh minerals and textures for scientists to study.
Apoi’s peridotite gets much of its olive-green color from olivine. In gem form, that same mineral is called peridot—the birthstone for August.
Heavy poaching of the endemic Hidaka-sō in the 1990s prompted local residents to form the Apoi-dake Fan Club in 1997. Volunteers now patrol, teach children about conservation, and maintain trails to help protect the plants.
