Metals and minerals are most often found in Earth’s crust, concentrated in specific geological settings: along tectonic plate boundaries, within igneous rock formations, in sedimentary basins, and on the deep ocean floor. Where they end up depends on how they formed, what rocks surround them, and what geological forces pushed them into concentrated deposits over millions of years.
What Earth’s Crust Is Made Of
Just ten elements make up over 99% of Earth’s crust by weight. Oxygen alone accounts for 46.6%, followed by silicon at 27.7%. After that come aluminum (8.1%), iron (5%), calcium (3.6%), sodium (2.8%), potassium (2.6%), and magnesium (2.1%). Titanium and hydrogen round out the top ten at less than 1% combined. These elements don’t sit around in pure form. They bond together into minerals, and those minerals make up rocks. The specific combination determines what you can extract from any given location.
Almost all common rocks are silicates, meaning silicon and oxygen form their backbone. The other elements show up in varying proportions depending on the rock type. Rocks low in silica tend to be richer in iron and magnesium, while rocks high in silica contain more sodium and potassium. This basic chemistry drives where economically valuable metals concentrate.
Tectonic Plate Boundaries
The most important metal deposits on Earth cluster along the edges of tectonic plates, where immense heat and pressure concentrate elements that would otherwise be scattered too thinly to mine. Convergent margins, where one plate dives beneath another, are especially productive. The subduction process pulls rock deep into Earth’s interior, where heat and fluids liberate metals and carry them upward into the overlying crust.
The angle at which one plate slides under another matters. Steep subduction zones, like those near the Mariana Trench, tend to produce massive sulfide deposits rich in copper, zinc, and lead. Shallower subduction zones are the domain of large copper and molybdenum deposits, the so-called porphyry systems that supply much of the world’s copper. The Andes mountain chain in South America is a classic example: its long history of shallow subduction created some of the planet’s richest copper and lithium reserves.
Divergent boundaries, where plates pull apart, also generate mineral wealth. Mid-ocean ridges vent superheated, mineral-laden water from Earth’s interior, depositing metals along the seafloor. Transform boundaries, where plates slide past each other, are less productive for ore deposits but can expose older mineral-rich rock through faulting.
Igneous Rocks and Volcanic Systems
Igneous rocks form when molten material cools and solidifies, either deep underground (intrusive) or at the surface through volcanic eruptions (extrusive). As magma cools slowly underground, different minerals crystallize at different temperatures. Heavier, metal-rich minerals often settle to the bottom of the magma chamber, creating concentrated layers of iron, chromium, nickel, and platinum-group metals. South Africa’s Bushveld Complex, which produces most of the world’s platinum, formed exactly this way.
Mafic and intermediate igneous rocks contain significant proportions of iron- and magnesium-bearing minerals like olivine, pyroxene, and amphibole. These darker, denser rocks are the primary source of nickel, chromium, and cobalt. Lighter-colored, silica-rich igneous rocks like granite are more likely to host tin, tungsten, and lithium deposits, along with gemstones like topaz and tourmaline.
Hot fluids escaping from cooling magma also carry dissolved metals into surrounding rock, depositing them in cracks and porous zones. These hydrothermal veins are where you find gold, silver, copper, lead, and zinc in many of the world’s historic mining districts.
Sedimentary Basins
Sedimentary rocks form from layers of material deposited by water, wind, or ice over millions of years. While they’re less famous for metals than igneous formations, sedimentary basins are the primary source of many industrial minerals essential to construction, agriculture, and manufacturing.
The Western Canada Sedimentary Basin illustrates this well. Its biggest products are potash, sulfur, limestone, and sand and gravel. Saskatchewan alone produces enough potash to rank as the world’s second-largest supplier. The basin is also the world’s second-largest sulfur producer and a significant source of magnesite, gypsum, salt, bentonite, and silica sand. These minerals formed as ancient seas evaporated, as organisms deposited calcium carbonate on ocean floors, and as rivers carried sediment into low-lying areas.
Sedimentary environments also concentrate certain metals. Banded iron formations, which formed billions of years ago in ancient oceans, are the world’s primary source of iron ore. Sandstones and shales can host uranium and copper deposits. Shales are particularly interesting because they concentrate trace elements at much higher levels than other sedimentary rocks. Selenium, for instance, averages about 90 micrograms per kilogram in igneous rocks but can reach up to 675,000 micrograms per kilogram in certain shales. Zinc follows a similar pattern: 20 mg/kg in limestone versus 200 mg/kg in bituminous shales.
The Deep Ocean Floor
Vast stretches of the abyssal Pacific Ocean floor are covered in polymetallic nodules, potato-sized lumps rich in manganese, cobalt, copper, nickel, and rare-earth elements. These nodules grow incredibly slowly, accumulating metal from seawater over millions of years. The Clarion-Clipperton Zone between Hawaii and Mexico is the most studied deposit, with nodule concentrations reaching 10 to 15 kilograms per square meter of seafloor.
Hydrothermal vents along mid-ocean ridges create another type of seafloor deposit. Superheated water, sometimes exceeding 350°C, dissolves metals from deep rock and spews them into cold ocean water, where they precipitate into chimney-like structures and surrounding sediments rich in copper, zinc, gold, and silver. No commercial-scale deep-sea mining is currently underway, but the sheer volume of metals on the ocean floor has attracted growing interest from mining companies and governments.
How Geography Shapes Global Mining
The geological processes described above aren’t evenly distributed across the planet, which is why certain countries dominate production of specific metals. Lithium is a clear example. Australia holds the world’s largest reserves at 57 million metric tons, followed by Chile at 9.3 million and Argentina at 4 million. All three owe their lithium wealth to specific geology: Australia’s comes largely from hard-rock pegmatite deposits (a type of igneous rock), while Chile and Argentina extract theirs from brine pools in sedimentary basins beneath salt flats in the Andes.
Copper production is concentrated in Chile, Peru, and the Democratic Republic of Congo, all regions shaped by subduction-related geology or ancient rift systems. Gold deposits cluster in places with extensive hydrothermal activity, past or present. Rare-earth elements, despite their name, are geologically common but rarely concentrated enough to mine economically. China dominates production largely because of a few unusually rich deposits in igneous and sedimentary formations.
From Rock to Soil to Food
The minerals in your food trace back to the same geological processes. Soil minerals come primarily from the weathering of parent rock, a slow breakdown that releases elements like iron, zinc, copper, and selenium into forms that plant roots can absorb. The type of bedrock beneath farmland directly determines which minerals end up in crops.
Elements behave differently depending on their chemical affinities. Iron, zinc, manganese, and boron tend to associate with silicate minerals. Copper, zinc, and nickel prefer sulfide minerals. Iron, copper, molybdenum, and nickel are commonly found with iron oxides in soil. Sandy soils tend to have low mineral concentrations because sand grains don’t hold onto trace elements well. Clay-rich soils retain more minerals, both because clay-forming rocks like shale start with higher concentrations and because clay particles have a greater surface area to bind elements in place.
This connection between geology and nutrition has real health consequences. Regions with selenium-poor bedrock produce selenium-poor crops, which can lead to deficiency in local populations. The same pattern holds for zinc, iodine, and other essential trace elements. Where minerals are found in the ground ultimately shapes what ends up on your plate.