[O-05] Poster presentations by senior high school students

[O05-P15] The Investigation to Figure Out How Tsuyama Sea Disappeared

*Yuki Toda¹, *Tetsu Kano¹, *Kota Miyake¹, Hodaka Aoyama¹ (1.Okayama prefectural Tsuyama high school)

Keywords:Paleomagnetism, Astatic magnetometer, Basalt dike, Neogene

The area of Tsuyama was under the sea about 16 million years ago. We investigated this area to figure out the process in which the sea (we named it “Tsuyama Sea”) had disappeared. There are basalt dikes running from north to south (N2°E) around Sara River in Tsuyama. There are mountains running east to west in the north of Tsuyama. We built up a hypothesis that the dikes and the mountains were the results of the movements of the plate in the north being pressed by the plate in the south (Philippine Sea Plate). So we constructed an astatic magnetometer to measure the rocks’ residential magnetization. The results of this measurement show that the magnetic north at the time the dikes were made was similar to the current magnetic north, which means that the dikes were made by the north-south pressure after the Japanese Islands finished moving 15 million years ago. This pressure may also be the factor which the mountains in the north of Tsuyama were raised. Therefore, we thought that the north-south pressure formed basalt dikes and raised mountains in the north of Tsuyama around 15 million years ago. This uplift made Tsuyama Sea separated from Sea of Japan, causing it to disappear.