Geohistory - Rocks from Continental to Sea Periods
15-10 mln years ago, after the formation of the Sea of Japan, the land of Oki was part of the seabed. Sediments formed at that time can now be seen on the land. Of special interest is a stratum containing fossils of diatom, a type of plankton covered in a silica shell, seen in almost every water environment on Earth. Once diatoms die, their shells fall to the bottom of the sea and eventually create Diatomaceous earth. Since diatom shells are porous silica, the Diatomaceous earth is light and doesn't conduct heat well. It is thus often used in heat-resistant bricks and grills. In the southern part of Dogo Island, Diatomaceous earth is mined for use in construction.