Felsic and mafic rocks
Also called: Acid and basic igneous rocks · Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT
Meaning
Igneous rocks (rocks formed when molten rock cools) are grouped by how much silica they contain. Felsic (acid) rocks have more than about 65% silica. They are rich in feldspar and quartz, so they are light-coloured and less dense. Mafic (basic) rocks have less silica and more iron and magnesium, so they are dark and heavy. This matters for volcanoes, because more silica makes lava thicker, which makes eruptions more explosive. Low-silica mafic lava is runny and spreads out in wide sheets.
Example
The pale granite of the Peninsular Gneiss area of Bengaluru, Karnataka, is felsic. The dark basalt of the Deccan Traps on the Maharashtra plateau is mafic. It came from runny lava that spread in flat sheets.
Don't confuse with
- Intrusive vs extrusive: felsic or mafic describes what a rock is made of. Intrusive or extrusive describes where it cooled: inside the crust or at the surface. Granite is felsic and intrusive. Basalt is mafic and extrusive.