Granite is an intrusive rock with few contaminants, while basalt is an extrusive rock with many contaminants. Granite is the principal component of the continental crust, while basalt is the principal component of the oceanic crust.
Granite is an intrusive rock that is formed when magma cools inside the crust. This slow cooling gives time for crystals to grow, making it more coarsely grained than an extrusive rock. This coarse grain makes granite a favorite of rock climbers. Basalt, on the other hand, has a smooth texture from rapid cooling, usually by water.
Granite is a felsic rock, meaning that it has a high silicon content; it is mostly made out of quartz, mica and feldspar. Basalt, as a mafic rock, contains more calcium oxide, manganese oxide and iron compounds than granite. Its high iron content gives it magnetic properties, and it may show signs of rust when exposed to air. These impurities make it roughly 10 percent heavier than granite.
Because it cools quickly, basalt is often found in formations where granite is not. While granite often takes the form of monoliths or boulders, basalt is often found in roughly hexagonal columns. These columns are created from the shock of cooling, as cracks propagate through the material.