雲仙火山, 眉山の形成過程

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Unzen volcano is a large volcanic complex which started its eruption ca. 0.5Ma at the center of Unzen graben, Shimabara peninsula, northwestern Kyushu, Japan. This volcano consists of many volcanic edifices such as Takadake, Kusenbudake, Fugendake volcanoes etc. These volcanoes are composed mainly of lava domes and thick lava flows of hornblende andesite and dacite. Volcanic history of Unzen volcano is divided into two stages : the older and younger stages. The younger stage is subdivided into three sub-stages which consist of Nodake volcano, Myokendake volcano, and Fugendake and Mayuyama volcanoes, respectively. Mayuyama is an isolated volcano on the eastern foot of Fugendake, and is the youngest among them with an age of ca. 4ka. Mayuyama volcano consists of two adjoining volcanic edifices, Shichimenzan and Tenguyama, which trend north to south. Geological data indicate that there was once a large lava dome of the same size and at the same place of the present Mayuyama. At the beginning of the Mayuyama eruption, uplift occurred around the present site of Mayuyama, and pre-Mayuyama collapsed. The Shimabara debris avalanche resulted from this movement. A small area to the west of Mayuyama tilted and formed the Taruki plateau, a flat uplifted surface. After the growth of Tenguyama lava dome, Shichimenzan, a volcanic spine, was formed at the northern slope of Tenguyama. Due to the growth of Shichimenzan, the northern part of Tenguyama suffered intense shear-stress which resulted to the formation of many faults and lineaments. During the formation of Mayuyama volcano, Mutsugi block and ash flow was generated mainly to the north of Mayuyama. On the bases of two radiocarbon dates, we estimate the eruption age of Mayuyama as ca. 4.6cal kyr BP. Summit lava domes of Fugendake were also generated shortly before the Mayuyama eruption. This means that lava domes at the summit and flank of Unzen volcano were almost simultaneously formed during the 4.6cal kyr BP eruption.
2005-12-30

雲仙火山, 眉山の形成過程

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