The rocks at the surface of the modern Earth are broadly divided into two types: felsic and mafic. Felsic rocks are generally relatively low density—for a rock—and light in color because they are made from whitish minerals rich in silicon and aluminum. Half Dome in California is made of granite that is a felsic rock. Mafic rocks, in contrast, are relatively high in density and dark in color because they contain minerals rich in iron and magnesium; Giants Causeway in Northern Ireland is made of basalt, which is a mafic rock.
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Source: Phys.org