Earth's Geothermal Engine: Sources and Heat Transfer

Sources of Earth's Internal Heat

The Earth's internal heat, the engine driving many geological processes, originates from several key sources:

• Primordial Heat: This is the leftover heat from the planet's initial formation and accretion.
• Core Formation: Significant heat was released as heavy elements, primarily iron and nickel, sank to form the Earth's core.
• Radioactive Decay: The continuous decay of radioactive isotopes within the crust and mantle, such as uranium-238, thorium-232, and potassium-40, is a major source of heat.
• Latent Heat: Heat is released from the crystallization and solidification of the liquid outer core to form the solid inner core.
• Tidal Heating: Frictional heat is generated by the gravitational forces exerted by the Moon and the Sun, which cause tidal flexing of the planet.

Mechanisms of Heat Transfer

Heat from the Earth's interior moves towards the surface through three primary mechanisms: conduction, convection, and radiation.

Heat Conduction

Heat conduction in the Earth transfers thermal energy from hotter to cooler regions without any movement of material, following the temperature gradient. Governed by Fourier's law, this process is most significant in the rigid lithosphere and at boundaries like the core-mantle interface. While less efficient than convection in fluid regions, conduction plays a key role in transferring heat from the Earth's interior to the surface, especially in areas with steep temperature gradients.

Radiative Heat Transfer

Heat radiation is significant only at the core-mantle boundary, where extremely high temperatures and the semi-fluid state of materials allow for limited radiative heat transfer. Elsewhere in the Earth's interior, radiation is negligible due to the high density and opacity of the materials, which absorb or scatter thermal radiation. In the lithosphere, conduction dominates due to rigid rocks, while in the mantle and outer core, convection is far more efficient, making radiation an insignificant heat transfer mechanism in those layers.

Earth's Heat Loss

The Earth continuously loses heat from its interior and radiates energy into space.

Heat Loss from the Interior

The planet loses heat from its core and mantle through several processes:

• Mechanisms: Mantle convection, conduction through the lithosphere, and volcanic activity.
• Rate: The rate of heat loss from the interior is approximately 47 terawatts, driven by the combined effects of radioactive decay, primordial heat, and core crystallization.

Heat Loss to Space

The total heat radiated from the planet into space is much larger than the heat lost from the interior.

• Mechanism: The Earth absorbs energy from the sun and radiates it back into space as infrared radiation.
• Rate: This process radiates around 173,000 terawatts, with the vast majority originating from solar input.

Key Point: The heat flowing from Earth's interior accounts for only about 0.03% of the total heat radiated to space. However, this internal heat is vital for driving crucial geological processes like plate tectonics and volcanism.

Dating the Earth's Formation

Scientists determine the age of the Earth by dating meteorites, which formed at the same time as our solar system.

The Canyon Diablo Meteorite

• Specimen Used: Canyon Diablo Meteorite
• Location: Found near Meteor Crater in Arizona, USA.

The Canyon Diablo meteorite contains high amounts of uranium and lead, making it ideal for uranium-lead radiometric dating. This method helps determine the age of the meteorite, which reflects the time when the solar system and Earth formed, approximately 4.54 billion years ago.