A 13-million-year-old crash
The Hidaka Mountains are the result of a colossal, slow-motion tectonic collision. Around 13 million years ago, two continental plates, the Eurasian Plate and the North American Plate, crashed into each other here. As the plates converged, the eastern plate was forced up and over the western one, a process called obduction. This immense geological event buckled and lifted the land, forming the 150-kilometer-long Hidaka Mountain range.
This mountain range scientifically exceptional is what the collision exposed. The process scraped off and tilted an entire cross-section of the Earth's crust and upper mantle. The rocks exposed in the Hidaka Metamorphic Belt allow geologists to walk across what was once a vertical slice of the Earth, from the shallow crust down to rocks that originated in the upper mantle. Mt. Apoi, a peak within the range, is composed almost entirely of peridotite, a dense, coarse-grained rock that is the primary component of the upper mantle. This allows geologists to study materials and processes that normally occur tens of kilometers beneath our feet. The main fault line from this collision, the Hidaka Main Thrust, is visible today at the surface in places.