acid rock

Any igneous rock that contains a high proportion of silica (more than 66%). The main silica mineral is quartz, but some feldspars are also quite rich in silica. Acid igneous rocks are usually light in color. The most common is granite. When they are weathered to form soil, they are generally infertile.

Related category

• GEOLOGY AND PLANETARY SCIENCE

Source: U.S. Geological Survey

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