水中自破砕溶岩

概要

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Andesitic and dacitic lavas form a certain kind of autobrecciated structure upon entering into the water instead of forming the pillow structure common in basaltic lavas. Such lavas consist of angular, polyhedral blocks ranging in size from a few meters to several centimeters in diameter cemented by comminuted fragments of the same material or by sediments. Matrix of palagonite is sometimes present. Individual blocks are compact and are often traversed by joints which divide them into still smaller polyhedral blocks. In blocks larger than 0.5m., which are usually angular in outline and sometimes spheroidal, closely spaced joints roughly perpendicular to the surface of the blocks are developed, indicating their quenching against the water. In the Pleistocene volcanoes of Izu Peninsula and Hakone, no such subaqueous autobrecciated lavas have been found even along the present sea coast. This fact implies that they have never been uplifted since the beginning of the Pleistocene or have been partly submerged because of the elevation of the sea level since the end of the glacial period.
特定非営利活動法人日本火山学会の論文
1968-12-01