Outcrop at Onsen Hotel Parking Lot

Here we can observe the strata of the eruption deposits since the formation of the caldera ("Younger Oshima Group"), which fell between 500 and 700 years after the 5th and 6th centuries. From the lower part of each layer, the black pebbles with holes are scoriae, the black sandy grains are volcanic ash, and the brown soil is the ground formed during the period when no eruption occurred, and these three layers are one unit.

Between the black basaltic strata characteristic of Izu Oshima, pure white rhyolitic volcanic ash strata can also be seen. This is the result of volcanic ash from the 838 eruption of Mt. Tenjo on Kozu Island, which fell on the whole island. This white volcanic ash layer is the key to the approximate age of the strata above and below it.

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