Oxygen is the most abundant element in Earth's crust by mass, followed by silicon. Together they account for about three quarters of the crust, mostly within silicate minerals and oxides rather than as pure elements.
The percentages below are average estimates. Continental and oceanic crust differ in composition, and published crust models do not give identical last digits. ChemWhiz uses the crustal-abundance values in the Royal Society of Chemistry periodic table for a consistent comparison.
The Eight Most Abundant Elements by Mass
These eight elements account for about 99% of the crust by mass in this data set. The percentages describe the mass of each element within compounds and minerals, not chunks of the pure substance.
Most crustal oxygen is chemically bonded in silicate minerals, oxides, and carbonates. It is not present mainly as oxygen gas.
Silicon commonly sits at the center of SiO₄ tetrahedra, the structural building blocks of silicate minerals such as feldspars, micas, and quartz.
Aluminum is the most abundant metal in the crust. It occurs in minerals including feldspars, clays, and bauxite rather than as native metal.
Crustal iron occurs in silicates, sulfides, and oxides such as hematite and magnetite. Earth as a whole contains much more iron because the core is iron-rich.
Calcium is common in plagioclase feldspar, calcite, gypsum, and apatite. Limestone is largely calcium carbonate.
Sodium occurs mainly in minerals such as albite and in dissolved salts. Its reactivity prevents native sodium metal from persisting in crustal rocks.
Magnesium is important in minerals including olivine, pyroxene, and dolomite, and is especially prominent in many mantle-derived rocks.
Potassium is concentrated in minerals such as orthoclase feldspar and mica. Potassium-40 also provides one source of natural radioactivity in rocks.
Why Oxygen and Silicon Dominate
Oxygen readily forms strong bonds with silicon and many metals. The resulting silicates organize around SiO₄ tetrahedra that can remain isolated or connect into chains, sheets, rings, and three-dimensional frameworks. Those arrangements produce much of the mineral variety in igneous and metamorphic rocks.
Quartz is silicon dioxide, SiO₂, while feldspars combine silicon and oxygen with aluminum, sodium, potassium, or calcium. A rock can therefore contain several of the top eight elements in just one or two major minerals.
Abundant Does Not Mean Easy to Extract
Crustal abundance counts each element wherever it occurs chemically. Aluminum is plentiful, but producing aluminum metal requires separating aluminum compounds from ore and then using substantial electrical energy. Sodium, potassium, calcium, and magnesium are also too reactive to remain widely as uncombined metals.
Conversely, an element with a low average abundance can form a locally concentrated ore deposit. Whether a resource is practical depends on concentration, mineral form, location, extraction technology, energy, environmental effects, and demand.
The Crust Is Not the Whole Earth
These rankings describe Earth's thin outer rocky shell. They do not describe the atmosphere, oceans, mantle, core, the entire planet, or the universe. Hydrogen is the most abundant element in the universe, while iron becomes much more important when Earth's metal-rich core is included.