The Earth’s crust is the thin, solid outermost layer of the planet, made primarily of rock. It sits on top of the much thicker mantle and accounts for less than 1% of Earth’s total volume. Despite being the thinnest of Earth’s layers, the crust is the only one we interact with directly, and it comes in two distinct varieties with very different properties.
Two Types of Crust
The crust under your feet isn’t the same everywhere. It exists as continental crust and oceanic crust, and the two differ in thickness, composition, and age.
Continental crust makes up the landmasses and ranges from 20 to 70 kilometers thick, averaging about 30 kilometers. Under major mountain ranges like the Alps or Sierra Nevada, it can extend as deep as 100 kilometers. It’s composed mostly of granite and similar rocks rich in silica and aluminum, which makes it relatively lightweight. Continental crust is also ancient. Some portions date back billions of years, with the oldest known in-place rock being the Acasta Gneiss in the Canadian Shield, dated to about 4.0 billion years old.
Oceanic crust, found beneath the ocean floors, is much thinner. It typically measures only 5 to 10 kilometers thick. It’s made primarily of basalt, a denser rock rich in iron and magnesium. Oceanic crust is also far younger. The oldest oceanic crust on Earth is roughly 200 million years old, a fraction of the age of its continental counterpart. That’s because oceanic crust is constantly being created at mid-ocean ridges and destroyed at subduction zones, where it dives beneath continental crust and melts back into the mantle.
What the Crust Is Made Of
If you broke the entire crust down to its individual elements, just two would account for nearly three-quarters of its weight. Oxygen makes up 46.6% by mass, and silicon accounts for 27.7%. These two elements bond together to form silicate minerals, which dominate virtually every rock type in the crust. The remaining quarter is split among aluminum (8.1%), iron (5.0%), calcium (3.6%), sodium (2.8%), potassium (2.6%), and magnesium (2.1%). Every other element on the periodic table, including gold, copper, and uranium, makes up the small fraction left over.
This composition differs between the two crust types. Continental crust is richer in silica and aluminum, giving its rocks a lighter color and lower density. Oceanic crust contains more iron and magnesium, making it darker and denser. That density difference is exactly why oceanic crust sinks beneath continental crust when the two collide.
How the Crust Moves
The crust isn’t one continuous shell. Together with the rigid upper portion of the mantle, it forms the lithosphere, which is cracked into 15 to 20 tectonic plates. These plates fit against one another like pieces of a broken eggshell and are in constant, slow motion. Heat from radioactive decay deep inside Earth drives convection currents in the mantle, pushing the plates sometimes toward each other, sometimes apart.
Where plates pull apart, new oceanic crust forms as molten rock rises to fill the gap. Where they collide, one plate may slide beneath the other, recycling crust back into the mantle. Where two continental plates crash together, neither sinks easily because of their low density, so the crust buckles upward into mountain ranges. The Himalayas are still growing today because of this process. Earthquakes and volcanic eruptions cluster along plate boundaries, which is why places like the Pacific Ring of Fire are so geologically active.
Temperature and the Lower Boundary
The crust gets hotter the deeper you go. The rate of temperature increase, called the geothermal gradient, averages roughly 25 to 29°C per kilometer of depth. That means for every kilometer you descend, the surrounding rock is about 25 to 29 degrees Celsius warmer. Deep gold mines in South Africa, for instance, reach temperatures that require active cooling systems to keep workers safe.
The bottom boundary of the crust is called the Mohorovičić discontinuity, or simply the Moho. It’s not a visible line you could see in a rock face. Instead, it’s defined by a sudden change in the speed of seismic waves (the vibrations produced by earthquakes). When these waves cross from the crust into the denser mantle rock below, they abruptly speed up. That velocity jump is how geologists first identified where the crust ends and the mantle begins, and it remains the primary way the boundary is mapped today. The Moho sits about 5 kilometers below the ocean floor and around 30 to 70 kilometers beneath continents.
How the Crust Formed
Earth’s earliest crust began forming over 4 billion years ago. In the planet’s first few hundred million years, known as the Hadean eon, the surface was largely molten. As Earth cooled, a primitive crust solidified from an extensive magma ocean. This protocrust formed after the planet’s iron had already sunk to create the core, leaving the lighter silicate material to rise and harden at the surface.
Evidence from tiny, remarkably durable crystals called zircons suggests that some form of continental crust and liquid water oceans existed as early as 4.4 billion years ago. The crust has been evolving ever since, with continental material gradually accumulating over billions of years while oceanic crust continuously recycles on a roughly 200-million-year cycle. The continental crust you stand on is, in a real sense, a slowly growing archive of Earth’s geological history.