What Is Earth Made Of?

Question posed by Vipul.

Our home planet, the Earth, is composed of a wide variety of substances that can be boiled down to one word: rock. It belongs to the category of planets known as the 'terrestrials' (named after Earth's Latin name, Terra), and these are all largely composed of silicate rocks and metals. It also has an atmosphere which can be considered to be part of the planet.

The Rocky Bit

The solid planet beneath our feet is dominated by four elements: Iron (32.1%), oxygen (30.1%), silicon (15.1%) and magnesium (13.9%), with the figures being percentage of the Earth's mass accounted for by that element. The other 8.8% is composed sulphur (2.9%), nickel (1.8%), calcium (1.5%), aluminium (1.4%), and trace amounts of other elements. Much of the iron is to be found within the magnetic core.

The Gassy Bit

The composition of Earth's atmosphere is fairly stable (over short timescales). The element that comprises most of its mass? No, it's not oxygen- that only accounts for just under 21% of the Earth's atmosphere. Instead, Nitrogen accounts for a whopping 78% of the gasses swirling around above our heads. Around 1% of the atmosphere is made up of water vapour, just less than 1% is argon, and a tiny 0.04% of the Earth's atmosphere is carbon dioxide*.

* Yes, I know that these percentages total more than 100, but I have, I believe, been sufficiently vague with my roundings. The actual percentages are more specific and do, I assure you, add up to 100%.


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