Unveiling the Eternal Heat of the Earth’s Core: A Phenomenon Sustaining Billions of Years

The Earth’s mysterious core, brilliantly portrayed in vibrant yellow in the exposed cross-section, unravels the enigma of its scorching temperatures, comparable to the Sun’s scintillating surface. Resembling the layers of an onion, our planet boasts a stratified composition comprising the crust, mantle, outer core, and inner core, each possessing distinct attributes.

How has the Earth’s internal heat persisted at the intensity of the Sun’s surface for countless eons?

The answer lies in the Earth’s layered structure, orchestrated by the perpetual motion of tectonic plates and the residual heat from its primordial origins, coupled with the gradual decay of radioactive isotopes. Geoscientists employ seismic waves to explore these internal formations and movements, as they play a pivotal role in comprehending environmental transformations and the evolution of life on our planet. The intrinsic heat generated drives plate tectonics, culminating in seismic events, volcanic eruptions, the emergence of landmasses and oceans, thereby rendering Earth habitable.

Analogous to the concentric layers of an onion, the Earth’s structure reveals itself in progressive succession.

Ascending from the outermost layer, we encounter the crust—the familiar ground beneath our feet. Descending further, the mantle unfolds, predominantly composed of solid rock. Delving deeper, we encounter the liquid iron constituents of the outer core. Lastly, nestled at the heart of our planet lies the inner core, a solid iron sphere with a radius encompassing 70% of the Moon’s dimensions. As explorers venture into the depths, temperatures rise exponentially, with sections of the core reaching scorching levels akin to the Sun’s surface.

This illustrative representation delineates the four strata concealed beneath the Earth’s surface.

An Expedition to the Center of the Earth

As a scholar specializing in earth and planetary sciences, my research endeavors delve into the hidden realms of our world. Just as medical practitioners employ sonography to visualize internal structures within the human body using ultrasound waves, scientists employ a similar technique to fathom the intricate internal composition of our planet. Instead of ultrasound, geoscientists utilize seismic waves—sound waves generated by seismic events.

At the Earth’s surface, we encounter the familiar terrain of dirt, sand, grass, and concrete. Yet, seismic vibrations divulge the concealed strata beneath—rocks of various sizes constituting the crust, extending to depths of up to 20 miles (30 kilometers), atop the underlying mantle.

The upper mantle region typically moves in unison with the crust, collectively referred to as the lithosphere. On average, the lithosphere measures approximately 60 miles (100 kilometers) in thickness, although variations may occur in certain regions.

The lithosphere manifests as numerous immense sections known as tectonic plates. For instance, the Pacific Plate resides beneath the vast expanse of the Pacific Ocean, while the North American Plate spans across the majority of North America. These plates harmoniously interlock like pieces of a puzzle, covering the Earth’s surface.

However, the plates are far from static; they exhibit motion. Sometimes imperceptible movements occur over the course of several years, while other instances witness more sudden and substantial shifts. Such movements instigate seismic events and volcanic eruptions, fundamentally influencing the Earth’s environment and necessitating adaptation for the sustenance of life.

Beneath Your Feet: A Vibrant Ecosystem Unveiled

The sustenance of plate tectonics necessitates a scorching mantle. Consequently, as one delves deeper into the Earth, temperatures steadily ascend.

At the lowermost region of the plates, approximately 60 miles (100 kilometers) beneath the surface, temperatures soar to around 2,400 degrees Fahrenheit (1,300 degrees Celsius).

Upon reaching the boundary between the mantle and the outer core, approximately 1,800 miles (2,900 kilometers) below the surface, temperatures escalate to nearly 5,000 °F (2,700 °C).

Subsequently, at the junction connecting the outer and inner cores, temperatures escalate twofold, surpassing 10,800 °F (over 6,000 °C). This region exhibits temperatures akin to the Sun’s surface, where nearly all matter—be it metals, diamonds, or even human beings—vaporizes into gaseous form. However, due to the colossal pressures enveloping the core, the iron composing it remains in either solid or liquid state.

The Magnitude of Plate Tectonics: The Essence of Human Existence

What accounts for this substantial heat?

Contrary to popular belief, it does not emanate from the Sun. Although sunlight warms the Earth’s surface, it fails to penetrate the miles-deep interior.

Rather, two heat sources contribute to this phenomenon. Firstly, the Earth inherited a substantial amount of heat during its formation 4.5 billion years ago. Our planet emerged from the solar nebula—a colossal gaseous cloud—through countless collisions and amalgamations of rock fragments and debris known as planetesimals. This transformative process spanned millions of years, generating an immense quantity of heat capable of liquefying the entire Earth. While some of this heat dissipated into space, a significant portion remained locked within the planet, enduring to this day.

The second heat source emerges from the decay of radioactive isotopes dispersed throughout the Earth.

To comprehend this, envision an element as a familial unit, with isotopes serving as its individual members. Although each atom of a specific element contains an identical number of protons, the various isotope counterparts possess distinct quantities of neutrons.

Radioactive isotopes exhibit an inherent instability, releasing a constant stream of energy that materializes as heat. Potassium-40, thorium-232, uranium-235, and uranium-238 are among the radioactive isotopes responsible for maintaining the Earth’s internal warmth.

Some of these names may sound familiar, with uranium-235, for instance, serving as a fuel in nuclear power plants. Fortunately, Earth faces no dearth of these heat-generating sources. While a significant portion of the original uranium-235 and potassium-40 has depleted, ample reserves of thorium-232 and uranium-238 remain, capable of fueling the Earth’s core for billions of additional years.

Together with the scorching core and mantle, these heat-releasing isotopes facilitate plate movements, shaping the Earth’s surface continually.

Absence of Heat, Absence of Plate Movements, Absence of Life

Even in the present era, the perpetual motion of tectonic plates orchestrates ongoing transformations across the Earth’s surface, generating new landmasses and oceans over millions and billions of years. Additionally, plate tectonics exert substantial influence on the atmosphere, albeit over similarly extensive timeframes.

However, without the Earth’s internal heat, plate movements would cease, resulting in the planet’s gradual cooling. In the absence of such heat, our world would likely be devoid of life, rendering it inhospitable for human existence. In such a scenario, the marvel of your existence would remain a distant possibility.

Reflect upon this the next time you feel the Earth’s solid ground beneath your feet.

Authored by Shichun Huang, Associate Professor of Earth and Planetary Sciences, University of Tennessee.

Adapted from an article originally published in The Conversation.

Frequently Asked Questions (FAQs) about Earth’s core heat

More about Earth’s core heat

• Plate Tectonics
• Earth’s Internal Heat
• Radioactive Decay and Earth’s Heat
• Structure of the Earth
• Geological Processes and Plate Movements
• Seismic Waves and Earth’s Interior
• The Sun’s Surface
• Earth Formation and Planetary Evolution