Various types of rock, metamorphic, sedimentary, and volcanic, are distributed in the Mukawa river basin. Dominant cobbles in the riverbed are sandstone, chert, mudstone, serpentinite, and greenstone; granite and gabbro are seen only rarely

Geology of Southern Hokkaido (PDF)
Geological Map of Hokkaido (Written in Japanese)（548KB）

Recent Major Volcanic Eruptions in Western Hokkaido (PDF)
Volcanic Ash of Hokkaido (Written in Japanese)（244KB）

Mukawa River rocks

The sand includs many volcanic lithic fragments, including volcanic glass. The dominant minerals are, however, serpentine, quartz, and plagioclase. The serpentine is derived from the serpentinite bodies in the river basin; the quartz is originated from sandstone and granite. The plagioclase and orthopyroxene are of volcanic ash origin.


Heavy minerals
Apart from magnetite, almost all the heavy minerals are orthopyroxene and Ilmenite. They are of volcanic ash origin, probably originating in a 17th century ashfall. Spinel, which is rich in chromium, was also found in quantity; it is derived from the serpentinite bodies. For other minerals, extremely small amounts of garnet and zircon are included.


Rare Minerals
As nonmagnetic heavy minerals, zircon and pyrite are found. An attempt at dating for monazite was made, but only extremely small amounts of monazite could be collected.

This diagram shows the chemical composition of Mukawa River orthopyroxene in terms of its magnesium content (Mg number). Almost all the grains analyzed had a Mg number of 60, a result indicating a single or similar volcanic eruptions for their source. In other regions, the Mg numbers are dispersed; that indicates a mixture of various kinds of volcanic ash. Orthopyroxene with a Mg number in the area of 90 is found in the river basin serpentinite in and may be derived from there; we concluded that it is so minor that being obscured by overwhelming majority of recent volcanic ash

Age distribution of monazite
Monazite, a mineral originally crystallized in granite and gneiss, indicates their age of formation. Most of the monazite found in the river sand derived, however, from old sandstones.
There are two peaks in the age of Mukawa River monazite: 1.8 billion years old and under 300 million years old. The 1.8 billion year old and about 200 million year old monazites were from the Cretaceous and Paleogene periods, respectively. When those old sandstones formed, the Sea of Japan did not exist; ancient sand particles are provided from the Asian continent.
Since age determination of younger monazite is technically difficult, more detailed classification is not possible. It is thought, however, that the monazite younger than 50 million years old originated in Hidaka metamorphic belt granite and gneiss in the river basins.