The most fascinating points of the Mt. Joyama and Katsuragiyama Geosite are the mountains, which are ‘volcanic necks’ or magma supply paths of old submarine volcanoes. Columnar joints are seen on the cliffs as well.
Deep down below the volcano there are paths, through which magma flows. The magma paths that as a result of crustal movement or other factors upheaved and appeared on the surface of the earth are called the "volcanic neck". Joyama, which never fails to attract attention of the visitors to the area with its peculiar craggy slopes, is one of the gigantic "volcanic necks" Izu is so famous for.