Acidic rocks also form a large group of igneous rocks. They mainly form by the intrusion of acidic magma into the country rock or by eruption onto the ground surface. The rocks contain more than 66% SiO2 and higher contents of light-coloured minerals than dark minerals. The light-coloured minerals include quartz, plagioclase, and alkali feldspar, and quartz makes up 1/4 to 1/3 of the minerals. The dark minerals mainly include biotite and hornblende. Granite is the most common intrusive acidic rock, while rhyolite is the most common extrusive acidic rock. They are important rocks that form landscape features.