E2 Internal Thermal Dynamics

Topic

Earth loses heat from its interior primarily through convection within the mantle and conduction through the overlying lithosphere. This heat loss stems from two distinct sources: primordial heat—retained from the planet's accretion and differentiation and slowly dissipating over geological eras—and the continuous radiogenic heat produced by the decay of uranium-238, uranium-235, thorium-232, and potassium-40.

The Earth's mantle undergoes vigorous convection because the parameter measuring the ease with which a fluid begins to convect reaches values ​​between ten million and one billion within the mantle—far exceeding the critical threshold of just one thousand required for convection to start. Crucially, the mantle convects with such vigor despite behaving like a rigid solid over short timescales.

The efficiency of this convective transport compared to simple conduction can be quantified using another parameter; in the Earth's mantle, this value ranges from ten to forty times higher. This implies that if the mantle did not convect and heat were transported solely by conduction, the Earth's internal temperature would be significantly higher than it is today.

The average heat flow reaching the Earth's surface is around eighty milliwatts per square meter, yet this flow is not uniform. It reaches up to three hundred milliwatts per square meter at mid-ocean ridges—where the mantle rises close to the surface—and drops to a mere thirty milliwatts per square meter over the oldest cratons, where the continental crust has insulated the underlying mantle for billions of years.

The temperature at the mantle-core boundary—ranging between 3,500 and 4,000 Kelvin—keeps the outer core liquid and powers the geodynamo that generates Earth's magnetic field. The slow evolution of that same temperature determines, in turn, the rate at which the inner core continues to solidify and how much longer the magnetic field protecting the planet will persist, thereby closing the link between the heat loss with which this description began and the long-term fate of Earth's magnetic field itself.

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