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The ground beneath your feet is mostly made of just eight elements, and the top two — oxygen and silicon — account for nearly three-quarters of it by mass. It surprises most people that the most abundant element in solid rock is a gas we breathe.
The top eight
Approximate percentages by mass in the Earth’s continental crust. Different studies give slightly different figures, but the order is consistent.
| Rank | Element | Approximate % by mass |
|---|---|---|
| 1 | Oxygen | 46% |
| 2 | Silicon | 28% |
| 3 | Aluminium | 8% |
| 4 | Iron | 5–6% |
| 5 | Calcium | 4% |
| 6 | Sodium | 2–3% |
| 7 | Magnesium | 2% |
| 8 | Potassium | 2% |
Together they make up more than 98% of the crust. Everything else — titanium, hydrogen, phosphorus, manganese, and all the familiar metals like copper, zinc, lead, silver and gold — shares the remaining 1–2%.
Why so much oxygen?
The oxygen in rock isn’t gas. It’s chemically bound, mainly in silicate minerals — compounds of silicon and oxygen with metals — and in oxides and carbonates. Quartz (SiO₂), feldspars (aluminium silicates of sodium, potassium and calcium) and micas are the most common minerals in continental rock, and all of them are built around silicon–oxygen units.
Oxygen atoms are also relatively large, so by volume the crust is even more dominated by oxygen: roughly 90% of the crust’s volume is taken up by oxygen ions, with the smaller metal and silicon ions packed into the gaps.
Aluminium: abundant but once precious
Aluminium is the most abundant metal in the crust, yet it was one of the last common metals to be isolated (in 1825). It’s bound so tightly to oxygen that heating its ore with carbon can’t free it. In the 1850s aluminium was more expensive than gold — Napoleon III reportedly served his most honoured guests with aluminium cutlery. The Hall–Héroult electrolysis process, invented in 1886, made it cheap, and today it’s everywhere.
The crust is not the Earth
The crust is a thin skin — on average around 35 km thick under the continents and only about 7 km under the oceans, compared with the Earth’s 6,371 km radius. The whole Earth has a very different composition, because heavy iron and nickel sank to the core when the planet was young and molten.
Approximate composition of the whole Earth by mass:
| Element | Whole Earth |
|---|---|
| Iron | ~32% |
| Oxygen | ~30% |
| Silicon | ~15% |
| Magnesium | ~14% |
| Sulfur | ~3% |
| Nickel | ~2% |
| Calcium | ~1.5% |
| Aluminium | ~1.4% |
So iron is the most abundant element in the Earth as a whole, but most of it is thousands of kilometres below your feet.
The oceans and atmosphere
- Oceans: by mass, seawater is mostly oxygen (about 86%) and hydrogen (about 11%) — it’s water, after all — followed by chlorine and sodium from dissolved salt.
- Atmosphere: nitrogen (about 78% by volume), oxygen (21%) and argon (0.9%). Carbon dioxide is only about 0.04%.
Why abundance and availability differ
Some elements are fairly abundant but hard to use because they’re spread thinly (like the rare earth elements or titanium, the ninth most abundant element). Others are rare overall but concentrated by geological processes into rich ore deposits — copper, lead, tin and gold, for example — which is why humans used them first.
Quick answers
What is the most abundant element in the Earth’s crust? Oxygen, about 46% by mass.
What is the most abundant metal in the crust? Aluminium, about 8%.
What is the most abundant element in the whole Earth? Iron, because of the core.
Related
For the universe as a whole, see the most abundant elements in the universe; for the scarce end, see the rarest element on Earth.
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