The total metallic content of Earth requires distinguishing between pure elemental metal and metal locked within mineral structures. In a geological context, most metals are chemically bound in compounds called minerals that must be processed to yield the pure element. Understanding the total content requires looking far beyond the accessible surface layer to the vast, inaccessible depths. The overwhelming majority of Earth’s metal is concentrated in the planet’s interior, with only a small fraction residing in the crust where human extraction is possible.
Iron and Nickel: The Vast Metallic Core
The total quantity of metal on Earth is dominated by the massive metallic core, which accounts for approximately 32.5% of the planet’s entire mass. This colossal reservoir is almost entirely inaccessible, located more than 2,900 kilometers beneath the surface. The core’s composition is an alloy of iron and nickel, resulting from planetary differentiation where denser materials sank to the center. Iron (Fe) is the primary component, comprising about 85% of the core’s mass, with Nickel (Ni) making up around 5% to 10%. Over 90% of Earth’s total metallic content is locked away here, playing a fundamental role in generating the planet’s magnetic field.
Abundance of Metals in the Earth’s Crust
In stark contrast to the core, the Earth’s crust—the layer accessible to mining—represents only 0.4% to 0.5% of the planet’s total mass. While this layer is relatively metal-poor globally, it contains the metals that form the basis of society. These metals are nearly always found chemically bonded within silicate and oxide minerals. Aluminum (Al) is the most abundant metal in the crust, making up about 8.2% of its weight, typically found in minerals like bauxite. Other abundant metals integrated into the rocky structure include Iron (5.6%), Calcium (Ca) at 4.2%, and Magnesium (Mg) at 2.3%. Liberating these common metals from their stable mineral forms requires energy-intensive processes.
Scarcity and Concentration of Valuable Trace Metals
The metals of greatest economic value, such as gold, copper, and platinum group metals (PGMs), are the rarest, existing only as trace elements in the crust. These valuable metals collectively constitute less than 0.03% of the crust’s mass. For these sparse elements to be profitably mined, they must be concentrated by geological processes into ore deposits. Magmatic processes, such as segregation, concentrate dense metals like PGMs by allowing them to crystallize and settle out of cooling magma. Hydrothermal fluids are another major concentration mechanism, involving hot, metal-rich water migrating through cracks. As these fluids cool, they precipitate metals like gold and copper into veins, creating deposits thousands of times richer than the average crustal rock. Plate tectonics also plays a role by driving fluid circulation that mobilizes and concentrates metals near plate boundaries.
Determining Earth’s Internal Composition
Since the core is physically unreachable, scientists rely on indirect methods to determine its composition and the distribution of metals. The primary technique is seismology, which studies how seismic waves from earthquakes travel through the interior. Pressure waves (P-waves) and shear waves (S-waves) change velocity and direction at material boundaries. The absence of S-waves passing through the outer core confirmed its liquid state, while the faster speed of P-waves through the inner core indicates its high-density, solid nature. The core’s composition is further constrained by analyzing meteorites, specifically chondrites, which represent the bulk, undifferentiated material of the early solar system. High-pressure laboratory experiments also simulate the core’s extreme conditions, testing how iron-nickel alloys behave to validate geophysical models.