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.
Pressure-cooked petrology
The rocks of the Hidaka Metamorphic Belt tell a story of extreme heat and pressure. Geologists classify them as high-grade metamorphic rocks, specifically granulites. Granulites form at temperatures exceeding 650°C and at pressures found deep within the continental crust. The minerals within these rocks act as tiny gauges, recording the conditions of their formation.
Important high-pressure indicator minerals found here include pyrope-rich garnet and a sodium-rich clinopyroxene called omphacite. These minerals are anhydrous, meaning they lack water in their crystal structure, a hallmark of high-temperature metamorphism where water has been cooked out. The specific compositions of coexisting minerals allow petrologists to calculate the peak pressure and temperature conditions, which reached 900-1100°C. These rocks, once the deep plumbing system of a volcanic arc, are exposed on a mountainside. The collision effectively created a natural laboratory, presenting a complete section of an island arc's crust for direct study.