火山の年代

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The significance to study the age of volcanism is stressed and discussed with some examples published. With the knowledge of age of volcanic activity, we can infer the duration of volcanism, eruption rate and so on. These factors give important constraints on the mechanism of volcanism. Ages of Hawaiian Islands have been much studied and the systematic increasing of ages from Kilauea to the northwestern islands has been explained by a moving plate over a "hot spot". Rather short volcanic activity (mostly less than 1 m.y.) and larger eruption rate (more than 10^4 km^3/m.y.) for each island seem to be compatible with this model. Some exceptions like recent volcanic activity in the Oahu Island, however, may need further explanations. Canary Islands are characterized with longer volcanic activity, including many intermittent volcanic dormancies for several islands. Although hot spot has also been suggested under Canary Islands, it does not seem to explain the patttern of volcanic activity sufficiently for these islands. Oceanic islands have often maintained the volcanic activities for more than 10 m.y. (e.g. Mauritius Island). In such cases, plate model may request the existence of magma source in a moving plate or magma spots in the asthenosphere along the locus of the island. Seamounts in the Northwestrn Pacific seem to be rather younger than the surrounding ocean floor, which may also suggest the existence of some mechanism to form magma in a moving plate. According to the examples of the Deccan Traps and the Columbia River Plateau, the plateau formation seems to have occurred in rather short time (say, less than 5 m.y.) with great euruption rate (10^5 km^3/m.y.). Although some regions with assumed hot spots (e.g. Iceland, Hawaiian Islands) have a little less eruption rate (10^4-10^5 km^3/m.y.) than that of a continental plateau, their volcanic activities continue for longer time, resulting often in producing greater volume of lavas in total. Isolated islands have an eruption rate of the order of 10^2 km^3/m.y., which exceeds that of seamounts. Ridge system and/or "hot spot" may be among the rest most extensive volcanism on the earth and maintain the volcanic activity for rather long time. However the character of a hot spot is still not clear and it is imperative to study the mechamism to keep these systems which is also responsible for causing many volcanisms on the earth.
特定非営利活動法人日本火山学会の論文
1975-12-25

著者

兼岡一郎
東京大学理学部地球物理学教室

火山の年代

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