Earth is made of four main elements: iron, oxygen, magnesium, and silicon, which together account for roughly 90% to 94% of the planet’s total mass. But those elements are distributed unevenly across distinct layers, from a dense metallic core to a thin gaseous atmosphere, each with its own composition and behavior. Understanding what makes up Earth means looking at both the raw ingredients and how they’re organized.
The Four Elements That Dominate
By weight, iron is the single largest component of Earth at about 31.9%. Most of it is buried deep in the core, far from the surface rocks we interact with daily. Oxygen comes next at 44% by mass, bound up in minerals throughout the mantle and crust rather than floating as a gas. Silicon and magnesium each contribute roughly 15%, rounding out the big four. Everything else on the periodic table, from gold to carbon to hydrogen, fits into the remaining 6% to 10%.
The Core: Iron, Nickel, and Earth’s Magnetic Field
At Earth’s center sits a core that begins about 1,800 miles beneath the surface. It’s split into two parts with very different properties. The outer core is a churning liquid made mostly of iron and nickel, roughly 1,400 miles thick. The inner core, starting about 4,000 miles down and about 800 miles thick, is solid. Temperatures and pressures there are so extreme that the metal atoms can’t flow freely and instead vibrate in place.
The core contains 5% to 15% nickel alongside its iron, plus smaller amounts of lighter elements like sulfur and silicon. That liquid outer core is responsible for something you rely on every day without thinking about it: Earth’s magnetic field. As the planet slowly cools, convection currents circulate through the molten iron. Earth’s rotation twists those currents into spiraling patterns, which generate electric currents that produce the magnetic field. This process, called the dynamo effect, has been running for billions of years. The inner core only crystallized into a solid about 1 billion years ago; before that, the entire core was liquid, and simulations show it still generated a magnetic field even then.
The Mantle: A Slow-Moving Solid
The mantle makes up the bulk of Earth’s volume, stretching from the base of the crust down to the outer core. It’s composed of silicate minerals rich in magnesium, silicon, and oxygen. In the upper mantle, the dominant minerals are forms of magnesium-iron silicate. Deeper down, intense pressure transforms these into denser crystal structures called bridgmanite and ferropericlase, along with smaller amounts of calcium-bearing minerals.
Despite temperatures hot enough to melt rock at the surface, most of the mantle is technically solid. It behaves more like extremely thick putty, deforming and flowing over millions of years. The upper portion of the mantle, called the asthenosphere, is slightly softer and more pliable. This is the zone that allows tectonic plates to drift across Earth’s surface. Above it, the rigid lithosphere (the crust plus the very top of the mantle) rides on the asthenosphere like a raft on slow-moving tar.
The Crust: A Thin, Familiar Skin
The crust is the thinnest layer and the only one humans have ever directly sampled. Its composition is dramatically different from Earth as a whole. Oxygen dominates at 46.6% by weight, followed by silicon at 27.7%. Together they form the silicate minerals that make up most surface rocks. Aluminum is third at 8.1%, then calcium, sodium, potassium, and magnesium, each between 2% and 5%. Iron, which dominates the planet overall, drops to just 2.1% in the crust.
There are two types of crust with distinct personalities. Oceanic crust, found beneath the ocean floor, is thinner and denser, made primarily of dark rocks like basalt and gabbro. Continental crust is thicker and lighter, composed of rocks like granite and andesite. This density difference is why continents float higher on the mantle and why ocean basins sit lower, collecting water.
The Atmosphere: A Blanket of Gas
Earth’s atmosphere is 78% nitrogen and 21% oxygen by volume, with argon making up most of the remaining 0.9%. The gases that get the most attention in climate discussions, carbon dioxide, methane, and nitrous oxide, are trace components totaling about a tenth of one percent. Despite their tiny share, they play an outsized role in trapping heat.
Water vapor is the wild card. Its concentration swings from 0% to 4% depending on location, time of day, and weather conditions. In humid tropical air, water vapor can be the third most abundant gas, outranking argon. In cold, dry polar regions, it’s nearly absent. This variability is why weather exists at all: water vapor carries enormous amounts of energy as it evaporates and condenses.
The Hydrosphere: Where the Water Is
About 71% of Earth’s surface is covered in water, and the oceans hold 96.5% of all of it. Only 2.5% of Earth’s water is fresh, and most of that isn’t accessible. Almost all freshwater is locked in ice caps and glaciers or stored underground as groundwater. The surface freshwater that supports most life on land, the rivers, lakes, and swamps, represents just over 1.2% of all freshwater. Rivers alone account for a mere 0.49% of surface freshwater, which puts into perspective how little of Earth’s water actually flows past you on its way to the sea. Lakes hold about 20.9% of surface freshwater, with ice locking up most of the rest.
What Keeps Earth Hot Inside
Earth’s interior stays hot for two reasons. The first and larger source is leftover heat from the planet’s formation about 4.5 billion years ago. As the early Earth grew by accumulating space debris, the energy from those collisions converted into heat. Gravity compressed the interior further, and as dense iron sank toward the center, friction generated still more heat. All of that primordial energy is still slowly leaking out.
The second source is radioactive decay. Elements like potassium, uranium, and thorium in the crust and mantle continuously break down, releasing energy that replenishes some of the heat Earth loses to space. Without this radioactive contribution, the planet would have cooled much faster, the core might have solidified entirely, and the magnetic field that shields life from solar radiation could have shut down long ago.