A fossilized magma chamber exposed at the surface when a volcanic island slammed into Honshu 5 million years ago. Walk through what was once a subterranean magma reservoir, frozen solid and tilted on its side.
Collision of worlds
The Tanzawa Mountains are the result of a slow-motion tectonic collision. This area was once the middle crust of a volcanic island, part of the Izu-Bonin-Mariana (IBM) arc, which is a 2,800-kilometer-long chain of volcanoes. For millions of years, the Philippine Sea Plate carried this island arc northward, until it began to collide with Honshu, the main island of Japan, around 15 million years ago. The specific block of crust that now forms the Tanzawa mountains slammed into Honshu between 5 and 6 million years ago.
This immense geological impact built mountains. It crumpled and uplifted the crust, pushing rocks that had formed deep underground to the surface. Over millions of years, the upper layers of the volcano eroded away, exposing the solidified magma chamber that once fed it. This exposed body of intrusive igneous rock is known as the Tanzawa plutonic complex. It shows the geologic processes that happen miles beneath an active volcanic arc.
A cross-section of a magma chamber
The rocks of the Tanzawa plutonic complex are primarily tonalite and quartz diorite. Tonalite is a coarse-grained intrusive rock high in plagioclase feldspar and quartz, with amphibole and biotite as common dark minerals. These rocks formed when magma cooled slowly, between 4 and 5 million years ago, deep within the crust of the Izu-Bonin-Mariana arc. The slow cooling allowed large, visible crystals to grow, a characteristic feature of plutonic rocks.
The tectonic forces of the collision tilted the entire block of crust, laying the ancient magma chamber on its side. This tilting provides geologists with a unique cross-sectional view. By walking through river valleys like the Kurokura-gawa or the Shirasawa-gawa, one can essentially traverse from the roof to the floor of a fossilized magma reservoir. The best exposures are found in these riverbeds, where water has polished the tonalite, revealing its mineral composition and structure. The complex stretches approximately 25 kilometers from east to west and 10 kilometers from north to south. Analysis of the rocks reveals they cooled at an astonishingly fast rate for a pluton, up to 658°C per million years, suggesting the entire complex was rapidly uplifted soon after its formation during the collision.
💡Fun Facts
The Izu-Bonin-Mariana arc is colliding with Honshu at a rate of about 2 to 6 centimeters per year.
The rocks of the magma chamber solidified from magma with temperatures around 1050°C at pressures below 1.2 GPa.
The name "tonalite" comes from its type locality near the Tonale Pass in the Italian Alps, first named in 1864.
The Shirasawa River, one of the best places to see the pluton, may get its name, which means "white river," from the light-colored, granite-like sand eroded from the exposed tonalite.
The area is a natural park, accessible at any time, but best and most safely explored during daylight hours.
Admission
Free
Accessibility
Accessing the exposed rock requires hiking along riverbeds, which involves walking on uneven surfaces and boulders. Sturdy, waterproof footwear is essential. The terrain is not suitable for wheelchairs or visitors with mobility issues.