Earth’s crust is made primarily of oxygen and silicon, which together account for about 74% of its weight and combine to form the silicate minerals that make up most rocks on the planet’s surface. Despite being the layer we live on, the crust is remarkably thin, representing just 1% of Earth’s total volume, with a depth ranging from roughly 5 kilometers beneath the oceans to 70 kilometers under mountain ranges on continents.
The Eight Elements That Make Up Nearly Everything
Just eight elements account for about 98% of the crust’s mass. Oxygen leads at 46.6% by weight, followed by silicon at 27.7%. That might seem surprising since we think of oxygen as a gas, but in the crust it’s locked into solid mineral structures, bonded tightly with silicon and metals. Aluminum comes third at 8.1%, followed by iron at 5.0%, calcium at 3.6%, sodium at 2.8%, potassium at 2.6%, and magnesium at 2.1%.
The top four elements alone (oxygen, silicon, iron, and aluminum) make up 88.1% of the crust’s mass. The remaining 11.9% is spread across roughly 90 other elements, many of them present only in trace amounts. Rare earth elements, for instance, range from about 60 parts per million for cerium down to just 0.5 parts per million for the scarcest members of that group. Gold, platinum, and other precious metals are rarer still, which is exactly why they’re valuable.
Silicate Minerals: The Building Blocks
Because oxygen and silicon dominate the crust, the most common minerals are silicates, compounds built around a basic unit of one silicon atom surrounded by four oxygen atoms. These units link together in different patterns (chains, sheets, rings, frameworks) to create a huge variety of minerals with distinct properties.
Feldspars are the single most abundant mineral group in the crust. They form when silicon-oxygen frameworks incorporate aluminum, potassium, sodium, or calcium. Quartz, pure silicon dioxide, is the second most common. Together, feldspars and quartz make up the bulk of most rocks you’d pick up on a hike. Other important silicate families include micas (sheet-like minerals that flake apart in thin layers), pyroxenes and amphiboles (chain-structured minerals common in darker rocks), and olivine (a dense green mineral more common deeper in the Earth).
Continental Crust vs. Oceanic Crust
The crust comes in two fundamentally different varieties. Continental crust, the kind beneath the land masses, is made of lighter-colored rocks like granite and andesite. These rocks are rich in silicon, aluminum, potassium, and sodium, giving them a relatively low density. Continental crust averages 30 to 50 kilometers thick and can reach 70 kilometers under major mountain belts like the Himalayas.
Oceanic crust is a different story. It’s composed of darker, denser rocks called basalt and gabbro, which contain more iron, magnesium, and calcium. At only about 5 to 10 kilometers thick, oceanic crust is far thinner than its continental counterpart. The color difference between the two types comes down to mineral proportions: at higher pressures deep in the crust, less of the light-colored mineral plagioclase forms relative to darker minerals. This is why oceanic rocks, which crystallize from magma at mid-ocean ridges, tend to be dark gray to black.
This density difference matters. Because continental crust is lighter, it floats higher on the mantle below, which is why continents stand above sea level. Oceanic crust, being denser, sits lower and is covered by water. When the two collide at tectonic boundaries, the heavier oceanic plate slides beneath the continental one.
How the Crust Differs From What’s Below
The crust sits on top of the mantle, and the boundary between them is called the Moho (short for Mohorovičić discontinuity, named after the Croatian scientist who discovered it in 1910). This boundary was first identified because seismic waves from earthquakes suddenly speed up when they cross it, jumping from about 6.5 km/s in the lower crust to around 8 km/s in the upper mantle. That velocity jump reflects a real change in rock chemistry: the crust is made of lighter, silicon-rich rocks, while the mantle is dominated by denser, olivine-rich ultramafic rock.
There’s also an important distinction between the crust and the lithosphere, two terms that are sometimes confused. The crust is defined by its chemical composition. The lithosphere is defined by mechanical behavior: it includes the crust plus the rigid uppermost portion of the mantle, and the whole thing moves as a single solid unit. Tectonic plates are pieces of lithosphere, not just crust. Beneath the lithosphere sits the asthenosphere, where rock is hot enough to flow slowly, allowing the plates above to drift.
Temperature and Pressure Inside the Crust
Temperatures inside the crust increase with depth at a rate of roughly 15° to 30°C per kilometer. At the surface you might experience 15°C on a mild day, but just 3 kilometers down, temperatures can reach 100°C or higher. This geothermal gradient is why deep mines require heavy cooling systems and why geothermal energy is accessible by drilling just a few kilometers in volcanically active regions.
Pressure also rises steadily with depth as the weight of overlying rock accumulates. These increasing temperatures and pressures change how minerals behave. Near the surface, rocks are brittle and crack under stress, producing earthquakes. Deeper in the crust, the same types of rock become more pliable and can deform slowly without fracturing. This transition from brittle to ductile behavior typically happens around 10 to 15 kilometers down, depending on the local temperature and rock type.
What Makes Crustal Composition Vary
The crust isn’t uniform. Its makeup shifts depending on tectonic setting and geologic history. Volcanic island arcs, like Japan and Indonesia, have crust that’s intermediate between oceanic and continental in composition. Rift zones, where continents are pulling apart, expose mantle-derived rocks at the surface. Ancient continental shields, like the Canadian Shield or the core of Australia, contain some of the oldest rocks on Earth (over 4 billion years old in places) and are rich in heavily metamorphosed granite and gneiss.
Sedimentary rocks, though they cover about 75% of the land surface, are really just a thin veneer. Limestone, sandstone, and shale blanket the top of the crust in many areas but make up a small fraction of its total volume. They form from the weathering and redeposition of the igneous and metamorphic rocks that constitute the crust’s deeper bulk. So while you encounter sedimentary rock constantly at the surface, the crust as a whole is overwhelmingly igneous and metamorphic silicate rock, built from those same eight dominant elements combining and recombining over billions of years of geologic activity.