Mt. Onidake
The bald, conical summit of 315-meter Mt. Onidake (“ogre mountain”) is one of the Goto Islands’ best-known sights. Looming to the south of Fukue Port, it is one of a ring of volcanoes along the eastern coast called the Onidake volcanic group. They are a monogenetic volcano field, the name for a group of comparatively small volcanoes that share the same magma source and each erupt only once. The Goto Islands are one of only three places in Japan where this type of volcano can be found.
The formation of Mt. Onidake began with the eruption of a shield volcano some 5 to 3 million years ago. Shield volcanoes, some of the most massive on earth, are much wider than they are tall, with gently sloping sides. The individual peaks of the Onidake group, including Mt. Onidake, were formed on top of an existing shield volcano when cinder cones began to erupt there around 50,000 years ago. Cinder cones, sometimes called scoria cones, are one of the most common types of volcanoes. They form when violent eruptions expel lava high into the air, where it cools as cinders and chunks of hardened rock. These fall back to earth, building up in a cone shape around the vent. Mt. Onidake is the youngest of these cones, having formed in an eruption about 18,000 years ago.
The mountain is easily recognizable by its grass-covered peak. The pines that once forested its slopes were cut down to build Fukue Castle in the mid-nineteenth century. Pine wood has a high resin content, making it resistant to rot, and such wood was needed to withstand the waterlogged ground of the castle building site. Trees would likely have regrown on the mountain in the subsequent decades, but residents perform controlled burns every three years. This allows mountain grasses and flowers to flourish on the slopes without competing for sunlight. In summer and autumn, Siebold’s lily, Japanese gentian, swallow wort, and other flowers dot the mountainside.
The hike to the top of Mt. Onidake is a relatively easy climb, and hikers can rest at the observation and picnic area about halfway to the summit. From there, the view encompasses the harbor area, the surrounding peaks of the Onidake volcanic group, and a great expanse of sea dotted with small, uninhabited islands off the coast. At night, it becomes a popular site for stargazing, and on fair days the strong sea breeze and absence of trees make the mountain perfect for flying kites. An annual event celebrating traditional baramon kites is held on Mt. Onidake in May.
