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Towada-Hachimantai NP, Aomori
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Idodake’s summit holds a 250 m-wide, 40–50 m-deep explosion crater. Its eastern wall exposes a lava dome shattered by steam-driven blasts, with debris layers containing blocks up to 4 m across.
Snow can outline Idodake’s summit crater as a heart. Near the peak, ice gathers on trail posts and ropes in clusters nicknamed ebi no shippo—“shrimp tails”—like tiny winter sculptures.
Idodake may have erupted most recently from its south flank, not its summit: geologists link scoria near the saddle toward Ōdake to a separate, roughly 150 m crater.
Heavier hiking in the 1960s–70s trampled alpine plants from Idodake’s south slope. Restoration began around 1985; volunteers now track the slow return, tending mats and barriers that can be damaged each winter.
Idodake belongs to North Hakkoda, a younger group of volcanoes that grew within the older, roughly 9 km-wide Hakkoda Caldera. Huge eruptions formed the caldera about 1 million–400,000 years ago.
Along the Ōdake–Idodake–Akakuradake ridge, a park ranger recorded the Yamase—a cold wind from the Pacific—colliding with the crest and dividing the landscape into two visibly different-looking sides.
