ロケーション Location

空から見る吉備高原 Kibi Plateau seen from the sky

 吉備高原は,岡山県を中心として兵庫県西部から広島県中部にかけて東西約170km,南北約45kmの領域を占めている(図1).これは東京都の約4倍の広さに相当する.近年,地震波トモグラフィーという地下構造を可視化する探査手法によって,吉備高原は深さ約20kmまで硬い岩盤から構成されることが分かってきた.また,吉備高原の地形と地質の特徴から,少なくとも3,400万年の間,地質学的に安定な陸塊であったことが明らかにされた(Sonehara et al., 2020).

 吉備高原は,その地質も地形も単調だと思われがちであるが,実際には,形成年代も形成環境も異なる多種多様な岩石から成り,いくつもの特徴的な地形を呈する.筆者らは,吉備高原をより良く知るために,ドローンを用いてそこに発達する地質と地形を120〜150m上空から撮影し,記録している.ここでは,その記録の中から,平坦な稜線が連なる高原面,表層に露出する赤色土壌,地質断層が通る直線状の谷,綺麗な放物線を描くU字谷,そしてかつてのカルデラの痕跡を紹介する.

The Kibi Plateau occupies an area of 170 km east-west and 45 km north-south from the western part of Hyogo prefecture to the central part of Hiroshima prefecture centering on Okayama prefecture (Fig. 1). This is about four times the size of Tokyo. In recent years, it has been revealed that the Kibi Plateau is composed of hard bedrock up to a depth of about 20 km by seismic tomography, an exploration method for visualizing underground structures. The topographical and geological features of the Kibi Plateau revealed that it was a geologically stable land mass for at least 34 million years (Sonehara et al., 2020).

Despite the fact that the Kibi Plateau is often thought to be monotonous in terms of geology and topography, in reality it is composed of a wide variety of rocks of different formation ages and formation environments, and exhibits several characteristic topographies. In order to better understand the Kibi Plateau, the authors have taken and recorded the geology and topography that develop there using a drone from above 120 to 150 m. Here, we will introduce a plateau surface with flat ridges, red soil exposed on the surface, a straight valley through which a geological fault passes, a U-shaped valley that draws a beautiful parabola, and the traces of paleo-caldera from the record.