The average chemical composition of the earth's crust has been determined from tens of thousands of chemical analyses of rocks and minerals taken from the surface or drill holes. The most common elements in the crust by weight are oxygen (46.6%), silicon (27.7%), aluminum (8.1%), iron (5.0%), calcium (3.6%), sodium (2.8%), potassium (2.6%), and magnesium (2.1%). These eight elements account for about 98.5 percent of the weight of the crust. The many other elements from the periodic table make up the remaining 1.5 percent. It may seem surprising that oxygen, which we normally associate with the atmosphere, is the most abundant element in rocks. It is an important part of most common minerals, such as quartz (SiO2) and calcite (CaCO3).