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If the Earth is thousands of millions of years old, why is the centre still hot?

If the Earth is thousands of millions of years old, why is the centre still hot?
Mostyn Woodfield from Monmouthshire (Age 55+)

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2 Responses

  1. The centre of the Earth stays hot primarily by means of radioactive decay, rather than by combustion or similar chemical methods.

    In this way it can still pump out a lot of energy over a very long period of time and stay hot.

  2. The Earth is a very good insulator. The radioactive heating takes place in the rocky mantle and uppermost crust, not in the metal core. The electric-blanket effect of the mantle and crust helps keep the core from cooling quickly. Nevertheless, we estimate that the core has cooled by several hundred degrees over the lifespan of the Earth.

    The core does create some of its own heat as the liquid iron freezes onto the solid inner core. This is like the heat that water releases as it freezes. But most of the core’s heat is from its original formation, a time when the whole solar system was much hotter than today because of the abundance of radioactive elements. (More on those early days at http://geology.about.com/od/planets/a/aa_asteroidgeo.htm)

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