Akan-Mashu Three-Caldera Landscape was added to Your Favorites!
Akan-Mashū NP, Hokkaido
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Akan-Mashū brings 3 major volcanic collapse basins together: Akan, Kussharo and Mashū, each with a lake bearing its name. Japan’s Ministry of the Environment notes that this close grouping of volcano-and-lake pairs is nationally rare.
The calderas formed on very different timelines: Akan began forming hundreds of thousands of years ago, Kussharo took shape about 130,000–100,000 years ago, and the youngest, Mashū, formed only about 7,000 years ago.
Kussharo Lake covers about 80 km², making it Japan’s largest caldera lake. It fills a vast volcanic basin inside the Kussharo Caldera.
Akan Lake was once part of a larger body called Ancient Lake Akan. About 10,000 years ago, eruptions of Mount Oakan divided it, helping create today’s Akan Lake, Panke-to and Penke-to.
Akan’s himemasu, or kokanee, are landlocked relatives of sockeye salmon. The park’s story explains that volcanic activity trapped their ancestors in the lake; nutrient-rich hot springs help feed the plankton that sustains its fish.
When much of Kussharo Lake freezes, hot springs bubbling up from the lakebed keep some patches of water open. Whooper swans migrating from Siberia use these unfrozen areas to rest and feed through winter.
In 1931, a transparency measurement at Lake Mashū reached 41.6 m—a historic record that remains unbeaten. The lake has no inflowing rivers, and its steep caldera walls help keep its shores largely untouched by development.
Some summer cloud seas over Mashū and Kussharo begin as fog near the Pacific coast. Warm air carries it inland, where the caldera basins help hold the cloud layer in place.
At Atosanupuri, an old railway rail from the sulfur-mining era has been engulfed by a growing tree and lifted along with its trunk. The nearby active volcano’s Ainu name means “Bare Mountain,” a fitting description of its gas-scorched slopes.
The tiny Kamui-shu Island in Lake Mashū is just the visible top of a lava dome formed by later volcanic activity. Local Ainu folklore tells a different story: the island is a grandmother who became stranded there while searching for her lost grandchild.
In deep winter, Kussharo’s frozen surface can buckle into long ridges called omiwatari. Researchers observing the lake found that cooling opens cracks, new ice forms in them, and later warming pushes the ice sheets upward.
Geothermal warmth gives Wakoto Peninsula, on Kussharo Lake, a curious distinction: it is home to the northernmost population of min-min cicadas. Nearby ground warmed by volcanic heat helps create a habitat unlike the surrounding cold landscape.
