Basic rocks are another large group of igneous rocks. They are usually formed from the intrusion of mantle magma into the Earth’s crust or from the eruption of magma onto the ground surface. The rocks contain 45–53% SiO2, and the main minerals are pyroxene and mafic plagioclase, but the rocks contain little or no quartz. They are dark and have high densities. A common intrusive basic rock is gabbro, and a common extrusive basic rock is basalt. Basalt is an important basic rock that forms landscape features.