Intermediate rock

In terms of chemical composition, this is an igneous rock with a silica (SiO2 ₎ content of about 55-65%. It has an intermediate composition between basic and acidic rocks. Volcanic rocks include dacite, andesite, and trachyte, while plutonic rocks include quartz diorite and diorite. Looking at igneous rocks as a whole statistically in terms of silica content, there are relatively more acidic and basic rocks, while there are slightly fewer intermediate rocks. This is because granite and basalt are produced in greater quantities than andesite and diorite. However, in tectonic zones such as Japan and the western United States, intermediate rocks such as andesite and diorite are produced in greater quantities than in other stable regions.

[Mitsuo Hashimoto]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

化学組成のうえで、シリカSiO₂の量が約55～65％の火成岩。塩基性岩と酸性岩の中間的組成をもつ。火山岩では石英安山岩、安山岩、粗面岩など、深成岩では石英閃緑(せんりょく)岩や閃緑岩がこれに含まれる。火成岩全体をシリカの量で統計的にみると、酸性岩と塩基性岩が比較的多いのに対し、中性岩はやや少ない。これは安山岩や閃緑岩に比べて、花崗(かこう)岩と玄武岩が大量に産するためである。しかし、日本やアメリカ西部のような変動帯では、安山岩や閃緑岩のような中性岩が、他の安定した地域に比べて多量に産出する。

［橋本光男］

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