北海道北部, 利尻火山の形成史
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概要
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Rishiri volcano is situated off the northern coast of Hokkaido, Japan. Based on mode of eruption, migration of main vents, petrological features of the eruptive products and new K-Ar ages, the activity of the volcano can be divided into three main stages: Early, Middle, and Late. The Early and Late stages can each be further subdivided into two stages. The Early stage (<0.2-0.04 Ma) was characterized by the formation of a stratovolcano and flank lava domes which were aligned along a 17km-long NW-SE trend. The stratovolcano consisted of thin lava flows and pyroclastics, mainly of andesite with a small amount of basalt, whereas lava domes were dacite. During the Middle stage (ca. 0.04 Ma), thick lava flows and pyroclastics erupted from central vents and formed a main stratovolcano. The products were mainly andesite, with lesser dacite. After formation of the stratovolcano (<0.04 Ma), many flank vents (15 km-long NW-SE trend) were active in different eruption types, such as plinian, strombolian, and phreatomagmatic. The rocks were mainly basalt, with subordinate andesite and rhyolite. The most recent eruptions occurred several thousands years ago, but no volcanic activity including fumaroles can be observed at present. Eruption rate increased from >0.1 to >0.4 km^3 (DRE)/ky during the Early stage, and also >0.4 km^3 (DRE)/ky in the Middle stage. The rate then decreased from >0・35 to >0・09 km^3 (DRE)/ky in the Late stage. Estimation of magmatic temperature was made from the presence or absence of hornblende in dacites. This suggests that magmatic temperature increased from the Early to Middle stages, and then decreased in the Late stage. Temporal variations in both eruption rate and magmatic temperature suggest that the volcanic activity of Rishiri volcano can be explained by ascent and cooling of a single heat source (diapir). Based on the diapir model and dormancy of activity during recent several thousands years, it seems that the life of Rishiri volcano as an active volcano is virtually over.
- 特定非営利活動法人日本火山学会の論文
- 1999-03-05
著者
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