The Sanbagawa Metamorphic Belt

The Ara River flows down from the Chichibu Mountains in Saitama, Japan and works its way across the Kanto Plains to Tokyo Bay where it empties into the Pacific Ocean. During its journey through the mountains, Arakawa (its proper name in Japanese) incises its way through various ancient layers of rock, and at Nagatoro Town, the river has worked itself through a fault and carved out an impressive gorge in some very beautiful crystalline schist. The site is a popular scenic attraction, and the so-called Iwadatami (rock tatami) is a natural treasure.

The schist present at Nagatoro is part of Japan’s largest metamorphic belt. Known as the Sanbagawa Metamorphic Belt, this continuous strip of rock continues from the western shores of Kyushu Island, across the Island of Shikoku, past the Nanki Peninsula and along Honshu until it bends deep inland, arcs around and approaches the coast again. The metamorphic rocks here were once sediments on the ocean floor and located near the subduction zone of the oceanic plate. These sediments accreted over the period of about 130 to 120 Ma and the time of subduction is estimated at about 116 Ma. The sediments underwent metamorphism under high pressure and temperatures but were subsequently thrust toward the surface again at a relatively high speed at first, then gradually their ascent slowed down until they reached the surface about 50 Ma.

In more recent history, the Izu Peninsula, growing from the Higashi Izu monogenetic volcano field and colliding with the Island of Honshu, pushed its way into the island and bent the various belts of metamorphic rock northward. Thus the waters of Arakawa flowing north and far from the ocean cut through the metamorphosed sediments of Jurassic times. Depending on the type of protolith, the schist may appear a metallic blue/grey, smoky blue, light grey, greenish grey, or brown. Many of the rocks have crystalline bands running through them, giving them beautiful patterns.

Because of the abundance of exposed rocks here, Nagatoro is known as the birthplace of Japanese geological study.