MadSci Network: Earth Sciences

Re: Why is the Earth's inner core solid?

Date: Tue Oct 9 17:03:01 2001
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 1002057216.Es

Dear Pat,

[Corrected first paragraph, 7/28/2005, submitted by Ion Simbotin]
Even though the inner core is extremely hot (as hot as the outer core, or hoter), it is solid because of the imense pressure that it is under. The presure is of course due to the weight of the earth above it, and it increases as we go deeper towards the center of the Earth. At a certain depth, the presure increases above a certain value, and iron can no longer be liquid (depsite being hotter than the melting point at atmospheric pressure). That certain depth marks the transition between the outer (liquid) and inner core (solid).

Eventually the entire core will solidify at some time in the distant geological future. The reason it has not done so as yet is that the Earth generates heat in its interior. There are two main sources of heat that contribute to the Earth’s heat budget. One is called the heat of accretion, and this is heat released by the conversion of gravitational energy as the materials that formed the Earth from the original cloud of dust and debris in the solar nebula collapsed together under gravity to form the planet. The heat of accretion was heat trapped in the new planet as it formed. The heat of accretion has been diminishing in importance as a component of the Earth’s heat budget throughout geological time as heat has been lost to space, but today it is estimated that some 20% of the Earth’s heat flow is still derived from this source. The other source of heat is the decay of radioactive elements such as uranium, thorium and potassium in the Earth’s core, mantle and crust. Radioactive decay continually renews the Earth’s inner heat, and although it has also diminished over geological time as the quantity of radioactive elements left has diminished, it is still more than sufficient to drive convection in the Earth’s outer core and in the mantle, keeping the Earth geologically active far longer than it would otherwise have been.

So, the cooling of the Earth has not been a simple matter of losing its original heat of accretion to space. If it had been the Earth would be a dead or nearly dead world by now, with a solid core, no volcanism, no plate tectonics and probably no life.

I hope this answers your question.

Best wishes,

David Scarboro

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