No, Earth is not hollow :) What about ultracondensed iron?

Sometimes we hear that if Earth expands, it must be hollow. For conspiracy fans – unfortunately, it is not hollow.

Have a look at a plot from Anneli Aitta (

Densities as a function of pressure.

We see there densities of Earth’s interior from 0 to 360 GPa (center of Earth). The thick solid line marks the PREM densities calculated by Dziewonski and Anderson (1981). The two upper lines (dotted and thin solid) mark densities for solid and liquid iron. We see that they are +/- related to what we observe in the Earth’s interior.

The question is whether the interior is stable or metastable. In other words, does iron tend to relax?

Have a look at the situation a bit closer. Hydrostatic pressure is created by gravity and depth, thus gravity and depth create in fact the environment in which the iron may exist (at those densities). Once you take some particle from a depth A to a lower depth B, it changes its volume and density – the density decreases. Once the density decreases, the hydrostatic pressure of lower layers decreases as well. And so on and so on.

Earth is cooling via ridges where new basalts are generated – heated material rises, cools and the basalt solidifies. The primordial heat of Earth is releasing via deep systems of cracks and the heated material rises – the particle from depth A moves to lower depth B. As a result, as been already mentioned, the density decreases and thus the volume increases – Earth is expanding.

It seems that Earth’s expansion is driven by heat release and UCM relaxation, but the Pangea breakup was a brittle and rather spontaneous global event. Continental lithosphere has been connected together for a very long time – approximately 4.3 billion years, what had happened there?

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