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Fuji-Hakone-Izu NP, Tokyo
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In 2000, Oyama’s summit collapsed into a caldera about 1.6 km wide and roughly 500 m deep. The mountain’s height fell from 814 m to 775 m—a 39 m loss.
Before 2000, Hachodaira on Oyama’s summit was a favorite hiking escape, with a naturally steaming spot called Oyama Sauna. The collapse swallowed that fumarole and the familiar highland landscape.
Every other year, Toga Shrine’s portable shrine spends 6 days circling Miyakejima, staying overnight at shrines in all 5 districts before returning home. The island-wide festival links communities across the island.
About 2,500 years ago, the huge Hachodaira eruption opened a summit caldera. Later lava and scoria filled it, building today’s Oyama; in 2000, the summit collapsed into a new caldera in almost the same place.
Earthquakes migrated west to an offshore eruption, then returned beneath Oyama. As the summit sank, Japan’s Geospatial Information Authority measured the collapse volume growing by about 15 million m³ a day through early August.
After the 2000 eruptions, sulfur dioxide from Oyama kept about 4,000 islanders away. Evacuation orders were lifted only on Feb 1, 2005, after 4 years and 5 months.
A 1940 fissure on Oyama’s northeast flank built Hyōtan-yama in about 22 hours. It began as 2 cinder cones; waves later wore away the seaward cone, while a red cliff still marks the old shoreline.
Toga Shrine once stood near Oyama’s eighth station. Local accounts say an eruption burned its main sanctuary, after which the shrine moved twice before reaching its present home much lower on the island.
Not every Oyama eruption starts at the summit: in 1983, a fissure opened on the southwest flank. Lava reached Ako within about 2 hours, burying or burning hundreds of homes.
On Oyama’s eruption-scarred slopes, fallen leaves and twigs helped new plants return. Researchers found covered ash stayed softer and moister, while bare ash hardened into a crust that made germination harder.
After Oyama’s eruption stripped the upper slopes, foresters considered Ohbayashabushi, a tree whose root bacteria fix nitrogen. Islanders had long planted it on volcanic wasteland and used its wood for fuel.
