Drobne struktury spękaniowe w porfirze z Miękini

Wojciech Jaroszewski




So far, minor structures on joint surfaces (Fig. 1) have been reported to occur in sedimentary rocks mainly. Such structures have, however, been found by the present author also on joints by cooling, an porphyry at Miękina in the vicinity of Cracow. These are represented by both plumose structure and fringe joints (Fig. 13). The structures are related to vertical joints (Fig. 2), but their orientation frequently points to a development in horizontal direction. In connection with this, the present author theoretically analyses the directions of the development of joints by cooling in volcanic massifs. According to the results obtained by the author, the horizontal components of the directions of development of these joints, and, thereby, the same components of the directions indicated by the minor structures on joint surfaces, should point to the position of the centre of effusive mass (situation of volcanic neck). The previous observations made by the author are in accordance with this conclusion. The author is of the opinion that such a situation can be of practical importance, particular as concerns determination of directions of exploitation in volcanic rock quarries. Minor structures in magmatic rocks can also yield data on the form of a magmatic body from before its partial destruction by erosional processes, on its thermal character, on the relative age of the systems of joints by cooling, and on other problems.


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