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Earth's Internal Heat Budget: Unveiling the Mysteries of Our Planet's Thermal History

CEO Khai Intela
Global map of the flux of heat, in mW/m2, from Earth's interior to the surface. The largest values of heat flux coincide with mid-ocean ridges, and the smallest values of heat flux occur in stable...

Global map of the flux of heat, in mW/m2, from Earth's interior to the surface Global map of the flux of heat, in mW/m2, from Earth's interior to the surface. The largest values of heat flux coincide with mid-ocean ridges, and the smallest values of heat flux occur in stable continental interiors.

The Earth's internal heat budget is not only fundamental to our planet's geological processes but also crucial in understanding its thermal history. The flow of heat from the Earth's interior to the surface is estimated at 47±2 terawatts (TW) and is derived from two main sources: radiogenic heat produced by the radioactive decay of isotopes in the mantle and crust, and primordial heat left over from the formation of Earth.

Earth's Internal Heat and its Geological Significance

Earth's internal heat plays a pivotal role in driving various geological processes that shape our planet. It fuels mantle convection, plate tectonics, mountain building, rock metamorphism, and even volcanism. Additionally, it is theorized that convective heat transfer within the planet's high-temperature metallic core sustains a geodynamo, which generates Earth's magnetic field.

Despite its geological importance, Earth's interior heat contributes only 0.03% of the total energy budget at the surface. This is overshadowed by the dominance of the 173,000 TW of incoming solar radiation that powers most of the planet's atmospheric, oceanic, and biologic processes. However, on land and at the ocean floor, the sensible heat absorbed from non-reflected insolation primarily flows inward by means of thermal conduction, penetrating only a few dozen centimeters on the daily cycle and a few dozen meters on the annual cycle. As a result, solar radiation has minimal relevance for processes occurring internally within Earth's crust.

The International Heat Flow Commission of the International Association of Seismology and Physics of the Earth's Interior collects and compiles global data on heat-flow density, providing valuable insights into the distribution and characteristics of Earth's internal heat.

Heat and the Early Estimate of Earth's Age

In 1862, William Thomson, later known as Lord Kelvin, estimated the age of the Earth at 98 million years based on calculations of Earth's cooling rate, assuming constant conductivity in the Earth's interior. This estimate significantly contrasts with the age of 4.5 billion years obtained through radiometric dating in the 20th century. The assumption of constant conductivity was called into question by John Perry in 1895, suggesting that a variable conductivity in the Earth's interior could expand the computed age of the Earth to billions of years. This was later confirmed by radiometric dating. Furthermore, Thomson's assumption of purely conductive cooling was invalidated by the presence of mantle convection, which alters how heat is transported within the Earth.

Global Internal Heat Flow and Its Uncertainties

Estimates of the total heat flow from Earth's interior to the surface range from 43 to 49 terawatts (TW), with the most recent estimate being 47 TW. This corresponds to an average heat flux of 91.6 mW/m2 and is based on over 38,000 measurements. The mean heat flows of continental and oceanic crust are 70.9 and 105.4 mW/m2, respectively.

While the total internal heat flow to the surface is well constrained, the relative contributions of the two main sources of Earth's heat, radiogenic and primordial heat, are highly uncertain. This is due to the difficulty of directly measuring these heat sources. Chemical and physical models provide estimated ranges of 15-41 TW and 12-30 TW for radiogenic heat and primordial heat, respectively.

The structure of the Earth consists of a rigid outer crust, including thicker continental crust and thinner oceanic crust, a solid but plastically flowing mantle, a liquid outer core, and a solid inner core. The solid mantle can still flow on long timescales, primarily driven by Earth's internal heat. Mantle convection, in response to heat escaping from Earth's interior, plays a crucial role in driving the movement of Earth's lithospheric plates. An additional reservoir of heat in the lower mantle, potentially resulting from the enrichment of radioactive elements, is critical for the operation of plate tectonics.

Heat transport within the Earth occurs through various mechanisms, including conduction, mantle convection, hydrothermal convection, and volcanic advection. Mantle convection is estimated to be responsible for 80% of Earth's internal heat flow, with the remaining heat originating mainly in the Earth's crust. Only a small percentage of Earth's internal heat loss at the surface is due to volcanic activity, earthquakes, and mountain building.

Sources of Heat: Radiogenic and Primordial

The evolution of Earth's radiogenic heat flow over time The evolution of Earth's radiogenic heat flow over time.

Radiogenic heat results from the radioactive decay of elements in the Earth's mantle and crust. About 50% of Earth's internal heat originates from this process. Four radioactive isotopes, uranium-238 (238U), uranium-235 (235U), thorium-232 (232Th), and potassium-40 (40K), are primarily responsible for radiogenic heat production due to their enrichment relative to other isotopes. The radiogenic heat production throughout the mantle is still challenging to accurately determine due to limited rock samples from depths below 200 km. However, estimates suggest layered structures or small reservoirs of radioactive elements dispersed throughout the mantle.

On the other hand, primordial heat represents the heat lost by the Earth as it continues to cool from its original formation. The heat flow from the Earth's core into the overlying mantle is thought to be due to primordial heat. Estimates of mantle primordial heat loss range between 7 and 15 TW, calculated as the remaining heat after removing core heat flow and bulk-Earth radiogenic heat production from the observed surface heat flow. The heat flow from the core is essential for maintaining Earth's geodynamo, which generates the planet's magnetic field and helps sustain its atmosphere and liquid water.

Heat Flow and Tectonic Plates

Understanding the relationship between Earth's heat budget and the dynamics of its mantle is challenging due to the ongoing debate regarding mantle convection processes. Evidence suggests that plate tectonics were not active before 3.2 billion years ago, and early Earth's internal heat loss was likely dominated by advection via heat-pipe volcanism. Terrestrial bodies with lower heat flows, such as the Moon and Mars, conduct their internal heat through a single lithospheric plate. In contrast, higher heat flows, such as on Jupiter's moon Io, result in enhanced volcanism through advective heat transport. Earth's active plate tectonics occur with an intermediate heat flow and a convecting mantle.

Conclusion

Earth's internal heat budget reveals the intricate interplay between radiogenic and primordial heat sources, shaping our planet's geological processes and thermal history. Although the total energy budget at the surface is dominated by solar radiation, the internal heat contributes significantly to various geological phenomena. Further research and exploration are necessary to fully understand the complexities of Earth's heat flow and its implications for our planet's evolution.

References: (Note: No external links provided)

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