Abstract
Extension joints with a peculiar shape of hemispherical lens were observed in the Narukawa Andesite, Nanae, Hokkaido. The size of the joints ranges from 5 to 46cm in diameter and from 0.7 to 2.4cm in depth. The occurrence of the hemispherical joints is restricted on the surface of platy and/or columnarjoints of the andesite lavas. The joint surface morphology is characterized by a plumose pattern, which is distinctive at the joint fringes. The hackle marks are also common around plagioclase megacrysts, which can be observed on the bottom of the hemispherical lenses. In some cases the jointing stays on the half ways, where the lenses remain on the platy surface of lavas. Taking all the modes of occurrence into considerations, the hemispherical lens-shaped joints are explained as extensional cracks initiated by a residual strain within individually jointed blocks of plates and/or columns in the andesite lavas.
