考古学からみた噴火が人類・社会に及ぼす影響

K-AhとATの噴火

抄録

A volcanic eruption can instantly cause total destruction to the surrounding environment and its inhabitants, and may also produce secondary disasters over a much wider area.
The major eruptions in Japanese prehistory were those which produced the Kikai-Akahoya tephra (K-Ah) in 6, 300 BP, and the Aira-Tn tephra (AT) at 21, 000∼22, 000 BP, both of which scattered volcanic ash over an extremely wide area, and influenced climate and vegetation on a global scale. Such immense eruptions devastated their immediate vicinities with ash storms, accompanied by hot gas and pyroclastic flows, possibly killing entire species of animals and plants. In the case of the K-Ah eruption, there is evidence that such destruction extended over a radius of more than 100km, whereas the AT eruption is thought to have been even larger, and affected an even larger area.
The K-Ah eruption occurred at the Kikai caldera in Kyushu, and the resulting thick layer of volcanic ash can be observed within the Jomon Period horizon in southern Kyushu. This deposit is used as a key layer to separate the Earliest and Early Jomon Periods, represented by the Senokan pottery at the bottom of the K-Ah layer and the Todoroki type at the top. The sudden shift in these types of pottery is thought to have derived from the total extinction of the Earliest Jomon population, undoubtedly as a result of the eruption, followed by the arrival of an unrelated group of people after the tragedy. The effects of the eruption reached as far as Shikoku, the Kinki, and the Tokai regions. The earth's surface was probably covered by thick volcanic precipitation, which also deposited on the floors of bays, killing much of the animal and plant food resources. It was during this cataclysmic event that shellmounds containing Shioya type pottery were abandoned, or totally vanished in the Tokai region.
A chronological study of the Paleolithic stone tool assemblages associated with the AT tephra is in progress. It is generally acknowledged that the tool component preceding the AT eruption is homogeneous throughout Japan, but that tool assemblages following the eruption show regional differences, with a greater variety of tool types and an increase in the number of sites. However, in the Musashino region, where one of the first chronologies of Paleolithic culture in Japan was established, this transition in tool assemblage occurs during a much later period (i. e., upper Layer V to lower Layer IV of the Tachikawa Loam sequence), while the AT tephra is observed within Layer VI, associated with knife shaped tools made from obsidian flakes. The gap is several thousand years. To resolve this apparent contradiction, archaeologists must intensively study how the precipitation of large amounts of volcanic ash actually affected the sites and artifacts over a long period of time.