# How do we know the centre of the earth is Iron?

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Like how are scientists 100% sure when we’ve never dug that far down

In: 23

Earthquakes generate seismic waves that pulse through the entire planet.

When measuring these at different points around the globe, we noticed that the waves don’t travel directly through the center, they reflect off of something dense and solid in the middle.

When we measure Earth’s gravity we get a similar conclusion: Earth is much heavier than a solid ball of rock would be.

When we measure Earth’s magnetic field we get yet another similar conclusion: something large and metallic is sloshing around in the center.

All these observations combine to make it pretty indisputable: there’s a huge metallic core in the center of the planet that deflects waves, generates a magnetic dynamo, and makes the planet extremely dense.

It’s assumed to be mostly iron because dying stars produce iron in huge quantities – it’s by far the most common heavy metal in the universe and has the right properties to generate the density and magnetism measurements we see.

Ah cool! Thanks

There are a number of clues:

* The majority of the Earth is hot enough to be molten (i.e. liquid). As you can see [here](https://scx1.b-cdn.net/csz/news/800a/2015/whataretheea.jpg) the crust with the continents on it is actually very thin, comparatively, and the interior is liquid rock and other materials. In a liquid, it makes sense that metals, which are denser/heavier than rocks, will sink to the center, because of gravity.

* Gravity itself; how much gravity we feel depends on the mass of the Earth. So we know the mass of the Earth from its gravity, and we’ve measured the diameter of the Earth (and its volume). So we can calculate the density of the Earth, and the density indicates that there’s something *heavier / denser than rock* at the center.

* Several metals are denser than rock, but the Earth has a magnetic field, and from our current understanding of magnetism, the core has to be iron (and/or with nickel mixed in) and not something like gold or copper or other metals that do NOT produce magnetism.

So basically it’s an “educated” guess.

>Like how are scientists 100% sure when we’ve never dug that far down

Scientists are not 100% sure because they have never dug that far down. What they have is a best guess; a preponderance of evidence. We know the volume and mass of the Earth. We know the seismic echos and what the density of the materials would need to be to produce those echos. A molten iron core is the best fit.