The Goto Islands are home to many volcanoes, and the largest island, Fukue, has a number of volcanic fields. One of the most prominent of these is the Mt. Onidake volcano group, comprised of eleven monogenetic basalt volcanoes (Daienji, Koba, Nagate, Masuda, Shimosakiyama, Hinodake, Kamiozu, Shirodake, Sakiyamabana, Midake-Usudake, and Onidake). The oldest of these was active roughly half a million years ago, while the youngest, Onidake, was active as recently as 18,000 years ago.
In its early stages, Onidake produced lava flows and scoria falls. Scoria is a type of volcanic rock with many cavities caused by gas bubbles. Later, the vent generated wide-area scoria falls, and in the final phase it released several separate lava flows. During this period of activity, multiple scoria cones were created.
A 315-meter-high cinder cone covered with grass is all that remains. From the summit you can see the flattest stretch of land in the Fukue district. This plain, which includes much of the business center of Goto City, Fukue Port, and Fukue Airport, is a product of the considerable lava flow from the Onidake volcanic group.
Onidake has a gentle slope, and it is a relatively short, easy hike to the top. That vantage point offers a pleasant view not only of Goto City, but also of the surrounding area and many of the Goto Islands. Lower down, there is a picnic area and a lookout spot. Few places in these islands can rival Onidake for both geologic history and scenic beauty.