Mantle Xenoliths

Mantle xenoliths are pieces of material from the Earth's mantle, which are contained inside other rocks; they were carried together with magma as it rose to the surface. Basaltic rock from 4–2 million years ago on the eastern coast of Dōgo Island features black pyroxene and yellow-green olivine, minerals from the upper part of the Earth's mantle. It is thought that basaltic magma carried them from tens of kilometers beneath the Earth's surface.
Mantle xenoliths can be thought of as gifts from deep inside the Earth. Although they are rarely found along the Sea of Japan coast, they can be found in places such as the Oki Islands, Ichinomegata in Akita Prefecture, and Kurose in Fukuoka Prefecture. These rocks give us important clues about the composition of Earth's mantle beneath the Sea of Japan.