Earth’s Crust Facts: Thickness, Makeup and Types

These earths crust facts start with the basics: the crust is the thin, rocky outer layer of our planet, and it makes up only a tiny fraction of Earth’s total volume. Compared with the whole planet it is like the skin on an apple, yet it is where we live, grow food, dig for minerals and feel earthquakes.

All figures below are approximate averages, since the crust varies from place to place and sources round their numbers differently.

What is the Earth’s crust?

Earth has several layers: the crust, the mantle, the outer core and the inner core. The crust is the outermost, solid layer. Below it lies the mantle, a thick region of hot rock that makes up most of the planet’s volume. The boundary between the crust and mantle is called the Mohorovičić discontinuity, usually shortened to the Moho, named after the Croatian seismologist who identified it from changes in the speed of earthquake waves.

The planet’s radius is about 6,371 kilometres, so even the thickest crust is a very thin shell. For a general introduction, see the Wikipedia article on the crust.

How thick is the crust?

Thickness varies a great deal:

• Oceanic crust: typically about 5 to 10 kilometres thick.
• Continental crust: on average roughly 30 to 50 kilometres, though it can be thinner at rifts and thicker beneath mountain ranges.
• Under big mountains: places such as the Himalayas and the Andes have crustal roots that can reach well beyond the continental average.

The deepest holes humans have drilled, such as the Kola Superdeep Borehole in Russia at around 12 kilometres, have barely scratched the surface of the continental crust, let alone the mantle.

What is the crust made of?

The crust is mostly made of silicate rocks, which means they contain silicon and oxygen bonded with other elements. By mass, the most abundant elements are approximately:

Element	Approx. share of crust by mass
Oxygen	46%
Silicon	28%
Aluminium	8%
Iron	5%
Calcium	4%
Sodium, potassium and magnesium	Roughly 2 to 3% each

Figures differ slightly between sources. Importantly, most of that oxygen is not free gas. It is locked inside minerals such as quartz, feldspar and mica. Valuable metals like gold, copper and uranium make up tiny fractions of the crust, which is why mining them requires finding places where natural processes have concentrated them.

Oceanic crust versus continental crust

The two kinds of crust differ in age, density and composition.

Oceanic crust

Oceanic crust is formed at mid-ocean ridges, where magma rises and cools into new rock. It is dense and built mostly of basalt and similar dark volcanic rocks. It is also young: very little of the present ocean floor is older than about 200 million years, because older crust is pulled back into the mantle at subduction zones.

Continental crust

Continental crust is lighter and more complex, containing a wide mix of igneous, metamorphic and sedimentary rocks, with a lot of granite-like material. Because it is less dense, it floats higher on the mantle and is much harder to destroy. Some continental rocks are billions of years old, giving geologists a long record of Earth’s history.

Feature	Oceanic	Continental
Typical thickness	5 to 10 km	30 to 50 km average
Main rock type	Basalt and related rock	Granite-like and mixed rock
Density	Higher	Lower
Age	Mostly younger than about 200 million years	Up to billions of years

Plate tectonics and why the crust moves

The crust and the uppermost, rigid part of the mantle together form the lithosphere, which is broken into large tectonic plates. These slowly move, typically at a few centimetres a year, roughly the rate fingernails grow. Where plates collide, mountains rise and subduction zones form. Where they pull apart, new crust forms. Where they slide past one another, as at the San Andreas Fault, earthquakes occur.

This movement shapes volcanoes, earthquakes, ocean basins and mountain chains. Understanding it helps scientists assess hazards, locate resources and reconstruct the past, such as how continents once joined to form supercontinents.

How scientists study the crust

Because nobody can drill far enough to sample it directly, geologists rely on indirect methods. Seismic waves from earthquakes and controlled explosions travel at different speeds through different materials, so recording them across the planet reveals boundaries such as the Moho. Gravity and magnetic surveys show variations in rock density and composition, while rock samples from mountains, mines and the ocean floor give direct clues about what the crust contains and how old it is. Radiometric dating of minerals then pins down ages.

Why the crust matters to everyday life

• Soil and food: weathered crustal rock becomes the soil that grows crops.
• Resources: metals, stone, sand, coal, oil, gas and groundwater all come from the crust.
• Hazards: earthquakes, tsunamis and volcanic eruptions arise from crustal processes.
• Building and engineering: the geology beneath a site affects foundations, tunnels and dams.

If you are curious about other measurements in our solar neighbourhood, our guide to how far the Moon is from Earth gives distances in miles and kilometres. And as with many science topics, the most reliable way to check numbers is to look at the primary source, an approach we also recommend in our piece on finding reliable facts about a public figure’s age.

Frequently asked questions

Conclusion

These earths crust facts show a thin but dynamic shell that shapes our world, from the ocean floors to the highest mountains. It is thin, varied and always moving, and its minerals, soils and hazards touch everyday life more than most people realise.