The Niidagawa and Mabechigawa rivers flow almost in parallel from northwestern Iwate Prefecture to eastern Aomori Prefecture. Thus, their basins have almost identical distributions of rock, mainly Jurassic sedimentary rocks and greenstones. The main types of rock in the riverbeds are sandstone, chert, mudstone, greenstone, and crystalline limestone, with rare occurrences of granite. Where the Niidagawa and Mabechigawa rivers differ is that more volcanic rock is found in the Mabechigawa river’s bed.

Geologic Map of the Tohoku District (Written in Japanese, PDF)（272KB）

Recent Volcanic Ash of the Tohoku District (Written in Japanese, PDF)（1,000KB）

Mabechigawa River rocks

The sand contains considerable amounts of volcanic glass and volcanic rock fragments. The main Niidagawa River minerals are plagioclase, quartz, albite, and pyroxenes. The same minerals are found in Mabechigawa River sand, but with a far higher proportion of plagioclase than other minerals. That difference results from the distribution of volcanic rocks in the two rivers’ basins. A mineral clearly not of volcanic origin is albite, which was transported from Jurassic rock. Relatively abundant albite are thus observed in the Niidagawa River sands.
>>See the Niidagawa River data


Heavy minerals
Apart from magnetite, the heavy minerals found most often in both rivers’ sands are orthopyroxene and clinopyroxene. Both are from recent volcanic ash. Sandstone, greenstone, granite, and other rocks are found in the river basins, and they contain extremely rare heavy minerals such as zircon or monazite.

There are wide variations in the magnesium content (Mg number) in orthopyroxene from the Mabechigawa River. The Mabechigawa and Niidagawa distributions are, however, almost identical. Both indicate the mixture of a variety of volcanic ashes.
>>See the Niidagawa River data

The Niidagawa and Mabechigawa river sands contain very little monazite; we can, however, make some interpretations from the small sample. There are three peaks in the age of Niidagawa River monazite: 1.8 billion years old, about 200 million years old, and about 100 million years old. The 1.8 billion and 200 million year old monazites are from the Jurassic sandstone. The 100 million year old monazite is derived from Cretaceous granite intruded into the Jurassic complex. The age distribution for the Mabechigawa River is quite similar, except that its sand also includes monazite younger than 50 million years old. It was derived from young granitoid distributed in the Tertiary volcanic belts.
>>See the Niidagawa River data