Metals are elements defined by shared physical properties, including metallic luster, high electrical and thermal conductivity, and the ability to be permanently deformed without breaking. They are malleable, meaning they can be hammered into thin sheets, and ductile, meaning they can be drawn into thin wires. Although metals are found throughout the planet and beyond, they are concentrated in specific geological environments and chemical forms. Understanding these concentrations requires studying the geological processes that bring these elements together from their dispersed state.
Primary Sources in the Earth’s Crust
The Earth’s crust is the primary and most accessible source of metals for human use. Metals must be concentrated into economically valuable deposits, known as ore bodies, to be mined. These concentrations are formed through geological processes that enrich the metal content far beyond the average crustal abundance.
Magmatic differentiation is one mechanism where metals concentrate during the cooling and crystallization of magma. As molten rock cools, certain minerals crystallize first and sink, creating layers rich in specific metals like chromium or platinum group elements.
Hydrothermal deposition involves hot, mineral-rich fluids circulating through the crust. These fluids dissolve metals from surrounding rocks and then precipitate them into veins or disseminated deposits when they cool or react with other rocks.
Sedimentary processes also form large metal deposits, particularly for iron and manganese. Weathering breaks down rocks, and the resulting metal-bearing solutions are transported and deposited in basins. Banded Iron Formations (BIFs) are ancient, vast deposits created as iron precipitated from the oceans in layers with silica.
Chemical Forms of Occurrence
Metals exist in nature either as pure, unbonded elements or as compounds bonded with other elements. When found in their pure state, they are known as native metals. Only a few metals, such as gold, silver, copper, and the platinum group metals, are commonly found in native form because they resist chemical weathering and oxidation.
The vast majority of metals are chemically bonded with non-metals to form stable mineral compounds, which are the primary source for most industrial metals. Iron is typically found as an oxide mineral like hematite or magnetite. Copper and zinc are frequently mined from sulfide minerals, such as chalcopyrite and sphalerite, where the metal is bonded with sulfur.
Silicates, carbonates, and sulfates are other common mineral classes that host metals. These compounds are often more complex and difficult to process than oxides or sulfides. The chemical form of the metal dictates the complexity and energy required for its extraction.
Metals in Deep Earth and Oceanic Systems
Beyond the accessible crust, two major reservoirs hold enormous quantities of metals: the deep Earth and the hydrosphere. The Earth’s core, composed primarily of iron and nickel, is the single largest metal reservoir on the planet. Early in Earth’s history, the “iron catastrophe” caused heavy, dense siderophile elements to sink toward the center, leaving the crust relatively depleted.
Some heavy metals, including gold and ruthenium, are thought to be slowly leaking from the core and mingling with the mantle material through convection. While this core material is largely inaccessible, its influence on the upper layers is a subject of ongoing research.
The hydrosphere, or Earth’s oceans, also contains metals, mostly in a dissolved state. The most concentrated deposits are found around deep-sea hydrothermal vents, often called “black smokers,” located along mid-ocean ridges.
Superheated water circulates through the oceanic crust, leaching metals like copper, zinc, and iron. When this metal-rich fluid rapidly mixes with cold seawater, the metals precipitate out as sulfide minerals. This forms massive towers and seafloor deposits, representing a significant, though currently mostly unmined, concentration of metallic resources.
Extraterrestrial Sources
Metals have a cosmic origin, delivered to Earth primarily through meteorites, asteroids, and comets. Meteorites are fragments of planetary bodies, often containing high concentrations of iron-nickel alloys. Iron meteorites are essentially pieces of metallic asteroid cores, rich in elements like iron, nickel, cobalt, and platinum group metals (PGMs).
Most of Earth’s original stock of PGMs and other heavy elements sank to the core during early formation. The “late veneer” hypothesis suggests that a significant amount of the precious metals now found in the crust and mantle was delivered by a subsequent bombardment of meteorites and asteroids. This accounts for the unexpectedly high concentrations of elements like platinum and iridium found in the Earth’s crust.