Studies on the Neogene subaqueous lavas and hyaloclastites in southwest Hokkaido 西南北海道における新第三紀の水中溶岩およびハイアロクラスタイトの研究
Studies on the Neogene subaqueous lavas and hyaloclastites in southwest Hokkaido
Southwest Hokkaido is situated in the extension of the inner belt of the North Honshu Arc and is dominated by the Neogene to Quaternary volcanic rocks. The Early Miocene volcanic rocks are mostly subaerial products, whereas the subsequent Neogene volcanic rocks are mainly submarine andesitic products diagnostic of island arcs. They are divided into lavas and volcaniclastic rocks. The lavas are subdivided into pillow lobes, lava lobes, sheet flows and pseudo-pillow lavas. The volcaniclastic rocks are classified into hyaloclastites, pyroclastic rocks and epiclastic volcanic rocks. Particularly, the hyaloclastites occupy the large proportion of the Neogene volcanic rocks in Southwest Hokkaido. The pillow lobes are cylinder or tubular and mostly andesitic. Their surface structures are recognized as ropy wrinkles, spreading cracks, tensional cracks and contraction cracks. The former three are formed during the growth of the pillow lobes, whereas the latter two are made after the emplacement. The ropy wrinkles are analogous to those on a subaerial pahoehoe lava. The corrugations are formed by internal convection or scratching. The spreading cracks are formed across or along the elongation of the pillow lobes, and play the most important role in their growth. The tensional cracks are produced by further supply of the interior magma even after stopping of the growth of the pillow lobes. The contraction cracks are formed by gradual cooling of the pillow lobes. The internal structure of the pillow lobe is characterized by joints, crusts and vesicles. The joints correspond to the contraction cracks and are radial, irregular and tortoise-shell in shape. The crusts are produced by shear jointing during the growth of the pillow lobes. The vesicles are ellipsoidal to spherical. The inner vesicles are larger than the outer ones. Pipe vesicles develop in places along the boder zone of the pillow lobes. Microscopic features of the andesitic pillow lobes are intersertal or intergranular texture in the inner part, and quench crystal overgrowth on the phenocrysts or microphenocryst in the outer part. The lava lobes are chracteristic of andesitic or rhyolitic subaqueous lavas. They are ellipsoidal a few meters across and are composed of crystalline cores and glassy margins grading outward into hyaloclastites. The cores are characterized by flow layers conformable to the outlines, and radial rude joints. Sheet flows and small lava lobes develop from the glassy margins. The lave lobes are formed by expansion of the sheet flows due to further supply of the internal magma.The pseudo-pillow lavas are one of andesitic subaqueous forms. They consist of polyhedral blocks of tens centimeters to a few meters across. They are formed by development of quench joints. The hyaloclastites are divided into (A) and (B) types. The (A) type is basaltic to andesitic monolithologic breccia and usually includes pillows (ellipsoids with quench glass). These pillows are formed by disintegration of pillow lobes or apophysis-like feeder dyke. Particularly, those concentrated at the margins of the apophysis-like feeder dykes are called concentric pillows. The (B) type is andesitic to rhyolitic monolithologic breccia. It is a product formed by brittle fracturing of the pseudo-pillow lava, lava lobe, massive feeder dyke, caused by quenching with water. Massive hyaloclastites are accumulated around the lavas and feeder dykes, and grade laterally into stratified hyaloclastites which are foreset-beds showing a primary dipping of 20°-30°, in places overlain by topset-beds. Each foreset-bed displays reverse or reverse-to-normal grading, suggesting that it was formed by a debris flow. The Neogene subaqueous lavas and hyaloclastites in Southwest Hokkaido are highly vesicular and in places associated with subaqueous pyroclastic rocks, such as scoriaceous agglomerates and pyroclastic pillow breccias, suggesting that they are the products of shallow submarine volcanism.
Hokkaido University（北海道大学）. 博士(理学)