Mantle Xenolith is not the name of a type of rock. In the case of mantle xenoliths found in Oki along the East Coast of Dōgo Island, the name of the rocks themselves would be peridotite and pyroxenite.
‘Xenolith’ comes from Ancient Greek, and means ‘foreign rock’. Mantle xenoliths are rocks from the Earth’s upper mantle that became enveloped in magma when it erupted from deep underground.
In the Oki Islands, mantle xenoliths are found within basalt lava, which erupted 4 million to 400,000 years ago.
The Earth’s interior can be roughly divided into three parts. The crust is from the surface to 6 – 30km deep. Next is the mantle which is up to 2900 km deep. The core is a further 6400 km deep. The upper part of the mantle is up to 660 km deep and is composed mainly of peridotite. In Oki, basalt magma erupted from a depth of tens of kilometers to several hundred kilometers into the upper mantle. This magma erupted at an incredible speed (estimated 36km/hr) carrying with it peridotite from the upper mantle.
This rock contains information about the nature of the mantle underneath the Oki Islands. This same type of rock is rarely seen on the coast of the Sea of Japan (it is also found in Ichinomegata in Akita Prefecture and Kurose in Fukuoka Prefecture). This is thought to be a clue to understanding how the Sea of Japan was formed.