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）

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

Niidagawa River rocks

The Niidagawa sand contains considerable amounts of volcanic glass and volcanic rock fragments. The main minerals are plagioclase, quartz, albite, and pyroxene. 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 is derived from Jurassic sandstone. Relatively abundant albites are thus observed in the Niidagawa River sands.


Heavy minerals
Apart from magnetite, the most frequently found heavy minerals in both rivers’ sands are orthopyroxene and clinopyroxene. Both are from recent volcanic ashes. Sandstone, greenstone, granite, and other rocks are distributed in the river basins, and they contain extremely small quantities of heavy minerals such as zircon or monazite.
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There are wide variations in the magnesium content (Mgnumber) 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.
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The Niidagawa and Mabechigawa river sands contain very little monazite; we can, however, make the 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 Jurassicsandstone. 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 is transported from young granite distributed in the Tertiary volcanic belts.
>>See the Mabechigawa River data