2. The Hidaka Mountains and Mt. Apoi were formed by plate tectonics

Theme A:
Learning about and enjoying the earth's transformation from peridotites

The appearance of part of the earth's deep mantle on the surface is attributable to dynamic movement inside the planet. The earth's surface is covered with a dozen or so huge slabs of rock called tectonic plates, which are around 100 kilometers thick and composed of the crust and upper mantle. As they move at a rate of several centimeters a year, they collide or move apart at their boundaries. The earth's continents are believed to have formed as a result of such plate movement (a concept known as plate tectonics theory).

The Japanese archipelago is in one of the world's most tectonically active spots because four plates meet there. This makes the region prone to earthquakes and the effects of volcanic activity. Among these four plates, the boundary between the Eurasian Plate and the North American Plate, which is today located along the archipelago in the eastern part of the Sea of Japan, is considered to have previously run across the island of Hokkaido. The collision of these plates formed the island's foundations, and another collision some 13 million years ago (during the Neogene Period in the Cenozoic Era) caused the edge of the North American Plate to the east to be thrust up onto the Eurasian Plate to the west, resulting in the formation of the Hidaka Mountains. The plate collision pushed part of the mantle at the bottom of the plate up onto the earth's surface, creating the peridotite-rich Mt. Apoi.

Due to the plate's upward thrust, the Hidaka Mountains, which include Mt. Apoi, have a sequence of geological strata with rocks from the shallower part of the plate to its deeper part (i.e., the upper mantle) lying sideways from east to west. The global rarity of this configuration gives the area scientific value for the insight it provides into the earth's interior.