Seasonally Moving Bedrock Block at Metsähovi, in Finland

1. INTRODUCTION According to the monthly levelling observations during four years in Southern Finland at Metsähovi the bedrock block has been found out to move vertically 3.5 mm. The levelling accuracy in such small area is better than 0.1 mm. The vertical displacement has a strong correlation with the temperature of bedrock and the displacement is more than 10 times larger than caused by the thermal expansion alone (Lehmuskoski, 2006). Horizontal movements of this block have not been observed within the measuring accuracy of 0.5 mm. The rock type of moving block is a micmatite granite and its cleavage is typically orthogonal. Mainly the study area is without soil cap, and only a minor part of it is under a thin till layer. We have studied the area in many ways e.g. leveling bolts, GPR, the temperature of bedrock, and a core sample to find out a reason for a movement. So far we have not got an unambiguous answer, but we have eliminated several options. 2. INVESTIGATIONS By measuring the positions of leveling bolts we mapped roughly the dimensions of moving block. Moreover, the study area is measured by GPR with 2 m grid to locate all fractures with low dipping. According to these studies we have located a rock block with approximate dimensions of 30 × 40 × 1 m. Because of strong temperature dependency of the displacement and the fact that the seasonal temperature variation of bedrock is a near surface phenomenon we can conclude that only a shallow part of bedrock is moving. A groundwater table is deeper than the bottom of a moving block and the displacement doesn't have any correlation with a precipitation with time lag or without it. Thus, we can ignore a possibility that hydrostatic pressure could lift a block for instance after rains. As the surface of bedrock is at its lowest position at a winter time neither the frost can be the source of lifting. We drilled five shallow holes to locate exactly the bottom of moving block and to get signs of thermal expanding clays like Vermiculite. From one drill hole we got a core sample and we also image all holes to get better picture of fractures. None of these investigations or samples support that the source of observed displacement could be originated by clays. This is also supported by the fact that those clays do not occur in a granitic rock but prefer in a mafic rock (Deer, 1996). If thermal expanding cannot alone cause the observed movement, could some kind of lever system lift a block along with thermal expanding of rock? We know that vertical movement is the biggest in the center of block. This can be a sign from either the pending of edge fixed block results from the heavy horizontal pressure. We are going to install stretch slips to find out all tensions under which the surface of bedrock is. 3. CONCLUSIONS After intensive studies we have delineated well the shape of moving block. Still further investigations are needed to construct a proper 3D model of moving block. The origin of the displacement has strong temperature dependency and it must locate near to surface, because heat cannot penetrate deeply into rock in the short summer of Finland. As we have no evidence of the presence of any type of clays mechanical lever is the best explanation of movement. After the stretch slip test we know much more about the tensions of rock block. 4. REFERENCES Deer W. A., Howie R. A., and Zussman J., 1996. An Introduction to the Rock-Forming Minerals. Longman, China, pp. 696. Lehmuskoski P., Rouhiainen P., Saaranen V., Takalo M., and H., Virtanen, 2006. Seasonal Change of the Bedrock Elevation at the Metsähovi Levelling Test Field. Nordic Journal of Geodesy. Vol. 3, 1, 58 - 68.