Elements of the Earth's crust

Topic: Minerals, Rocks and the Rock Cycle · NCERT: Beyond NCERT

Meaning

The elements of the Earth's crust are the basic substances that make up its thin, solid outer layer. By weight, oxygen (46.6%) and silicon (27.7%) lead, followed by aluminium (8.1%), iron (5.0%), calcium (3.6%), sodium (2.8%), potassium (2.6%) and magnesium (2.1%). Together these eight make up more than 98% of crustal material [1].

This list matters for two reasons. It explains why most rocks are silicates, and it explains why aluminium, not iron, is the most common metal in the crust. UPSC often tests this exact difference.

Explanation

What "element" and "by weight" mean

• An element is a basic substance that cannot be broken into simpler substances. Oxygen and iron are examples.
• By weight means the share of the crust's mass that each element makes up. It does not mean the number of atoms or the space they fill.
• Standard abundance tables give the same order at the top: oxygen, silicon, aluminium, iron [3].
• Each of the eight main elements is above 1% of the crust, and oxygen alone is close to 50% by weight [2].

Rank	Element	Share by weight (%)	Point to remember
1	Oxygen (O)	46.6	A gas, but here it is locked inside rocks
2	Silicon (Si)	27.7	Joins with oxygen to form silicates
3	Aluminium (Al)	8.1	The most common metal in the crust
4	Iron (Fe)	5.0	The second most common metal
5	Calcium (Ca)	3.6
6	Sodium (Na)	2.8
7	Potassium (K)	2.6
8	Magnesium (Mg)	2.1	The last of the "big eight"
—	The eight together	about 98	All other elements share the last ~2%

• Memory trick: "Oh See All India Can See Pink Mangoes" gives O–Si–Al–Fe–Ca–Na–K–Mg.

Elements are joined together, not free

• Most crustal elements are not found pure. They are chemically joined to other elements and form minerals.
• How silicates form:
• Oxygen and silicon are the two most common elements, and together they make up about 74% of the crust.
• The two bond easily with each other.

• The result is silicate minerals, which are built on a silicon–oxygen unit. Aluminium, iron, calcium, sodium, potassium or magnesium are often added to this unit. Feldspar and quartz are examples.

• Result: the crust is mostly silicate rock. About 6,000 mineral species are known today. Most of them are inorganic, and most are silicates [4].

Why the crust differs from the whole Earth (differentiation)

• Cause: the young Earth was hot and partly molten.
• Process: differentiation (Earth's material sorting itself by density, meaning how heavy a substance is for its size).
• Heavy metals, mainly iron and nickel, sank inward and formed the core. This is why the core is called nife (Nickel + Fe, the symbol for iron).

• Lighter silicates, rich in oxygen, silicon and aluminium, floated up and formed the crust.

• Result:

Feature	Earth's crust	Whole Earth
Most abundant element by weight	Oxygen	Iron
Most abundant metal	Aluminium	Iron
Why	Light elements rose upward	Heavy iron sank to the centre

From elements to minerals, rocks and ores

• Mineral: "a homogenous, naturally occurring substance with a definable internal structure" (NCERT).
• Homogenous means it is the same all the way through.
• Definable internal structure means its atoms sit in a fixed, repeating pattern.

• In the wider scientific definition, a mineral is a natural solid with a definite chemical composition and an ordered, usually crystalline, atomic structure [5].

• Rock: a natural mixture of one or more minerals.

• Ore: a mineral deposit that is concentrated enough to mine at a profit.
• Abundance is not availability:
• Aluminium is 8.1% of the crust.
• But workable deposits are only about 1% of the crust.
• So only ore-grade bauxite, where the aluminium is concentrated, is worth mining.

Where it occurs

• Across the whole crust of the world: the same eight elements lead everywhere. They are locked mainly in silicate minerals such as quartz and feldspar.
• In common rocks:
• Granite contains several minerals: quartz, feldspar and mica. These are built from oxygen, silicon, aluminium, potassium and sodium.

• Limestone is a single-mineral rock. It is mostly calcium carbonate (calcite), so it is rich in calcium.

• As ores:

• Aluminium is mined as bauxite.

• Iron is mined as haematite and magnetite.

• In India:

• India produces as many as 95 minerals: 4 fuel, 10 metallic, 23 non-metallic, 3 atomic and 55 minor [6].
• In 2023-24 (provisional), metallic minerals made up 90.3% of the value of metallic plus non-metallic minerals, about ₹1,27,599 crore. Non-metallic minerals made up 9.7%, about ₹13,640 crore [7].
• India had 795 metallic and 1,241 non-metallic mineral mines [7].
• Limestone, the calcium-rich single-mineral rock, gave 90.5% of the value of non-metallic minerals. Its output was 451 million tonnes, up 11% from the year before [7].

Don't confuse with

• Most abundant element in the whole Earth: for the whole planet this is iron, because the core is nife. In the crust it is oxygen.
• Most abundant metal vs most abundant element in the crust: the most abundant metal is aluminium. The most abundant element is oxygen, which is not a metal. Iron is only the second most common metal.
• Mineral vs rock vs ore: a mineral has a fixed composition and structure. A rock is a mix of one or more minerals. An ore is a mineral deposit rich enough to mine at a profit. Every ore is a mineral deposit, but not every mineral deposit is an ore.
• Abundance vs workable reserve: an element can be common in the crust, like aluminium at 8.1%, and still be hard to mine. Only about 1% of the crust holds workable deposits.

Prelims Hooks

• Order by weight: O (46.6) > Si (27.7) > Al (8.1) > Fe (5.0) > Ca (3.6) > Na (2.8) > K (2.6) > Mg (2.1). These eight make up more than 98% of the crust [1].
• Trap: the most abundant metal in the crust is aluminium, not iron. Iron leads only for the whole Earth.
• Oxygen and silicon together make up about 74% of the crust, so silicates are the main mineral group [4].
• Calcium comes before sodium, potassium and magnesium. Magnesium is the last of the "big eight".
• Oxygen alone is close to 50% of the crust by weight, and each of the big eight is above 1% [2].
• Minerals known: over 2,000 (NCERT). About 6,000 species are recognised today, and the International Mineralogical Association (IMA) names them [4][5].

Mains Points

• Crust vs whole Earth (GS-I):
• Early differentiation by density sent heavy iron and nickel into the core (nife).
• Light silicates of oxygen, silicon and aluminium rose and formed the crust.

• This is why aluminium leads the metals in the crust while iron leads in the whole planet. It is also why silicate rocks make up most landforms.

• Abundance is not availability (GS-III):

• Aluminium is 8.1% of the crust, yet only ore-grade bauxite can be mined.
• Workable deposits are only about 1% of the crust, and minerals are non-renewable because they take millions of years to form.

• So India needs careful exploration by the Geological Survey of India (GSI), recycling and planned use of its reserves.

• India's mineral economy (GS-III):

• India has a wide base of 95 minerals [6].
• Calcium-rich limestone gives over 90% of the value of non-metallic minerals and feeds the cement and steel industries [7].
• Minerals are spread unevenly, so mapping where they are helps explain where industries, transport links and mining towns grow.

Read more

• Elements of the crust and what a mineral is