From: cleoabram
The Earth is composed of distinct geological layers, from the thin outer crust to the dense, hot core. While humans have only managed to dig a tiny fraction into the Earth, scientists piece together indirect evidence to understand what lies beneath our feet [00:00:37].
Attempts to Dig Deep
One of the deepest human endeavors was the Kola Superdeep Borehole project by the USSR [00:00:04]. This hole, started in the 1970s, was dug for 20 years and reached a depth of 12.2 km [00:00:22] [01:58:20]. It is deeper than the deepest part of the ocean and taller than Mount Everest [00:00:06]. The equipment was operated remotely, and scientists had to invent new methods to pierce through the thick, hot, pressurized rock [02:10:04]. During the project, they discovered fossilized organisms dating back 2 billion years and found water much deeper than previously thought [02:19:00]. Despite multiple attempts to dig deeper, the new holes tended to collapse [02:33:00]. This “moonshot” project, which reached only 0.2% of the way to the Earth’s center, is unlikely to be repeated or surpassed [02:38:00] [02:44:00].
The Crust
The outermost layer of the Earth is the crust [01:17:00]. On land, it is typically less than 40 km deep [01:20:00]. As one descends, the temperature increases, with every kilometer through the crust becoming approximately 25°C hotter [01:13:00]. Within the crust, one passes through soil, layers of hard rock, precious metals, and fossils [01:25:00]. Various deep human-built structures exist within the crust, including the Kamioka and Sanford research labs for dark matter and neutrino physics, China’s nuclear command bunker, the deepest discovered cave, and some of the world’s deepest mines [01:29:00].
The Mantle
Below the crust lies the mantle, which begins at about 30 to 50 km deep [02:53:00]. Initially, the rock appears brittle like the crust but is denser [03:02:00]. As the temperature rises above 1300°C, the rock starts to behave like hot plastic [03:07:00]. Although the temperature is higher than the rock’s melting point, the immense pressure keeps it solid, giving it a gooey consistency [03:20:00]. Hot material slowly moves from the bottom of the mantle to the top in giant convection currents over millions of years [03:27:00].
The Earth’s heat is partly due to radiation and partly leftover heat from its formation when meteorites repeatedly collided [03:38:00]. This process of cooling is very slow, taking billions of years [03:49:00]. The stirring of this hot, gooey material brings enormous amounts of heat from the center toward the crust [03:53:00].
How We Understand Earth’s Interior
Since direct access to the deep Earth is impossible, understanding its interior is like being a detective, piecing together indirect evidence [04:15:00].
Historical Theories
Ancient scientists had various theories about Earth’s interior [04:26:00]:
- Some believed the Earth contained a central fire with underground lakes and lava chambers [04:30:00].
- Others thought it was hollow, perhaps a set of concentric shells with life on each ring, a concept that inspired the “Hollow Earth” in Godzilla movies [04:35:00].
- Isaac Newton suggested that the material in the center must be denser than the top based on observable gravity [04:40:00].
It was not until the early 20th century that scientists could prove the Earth had a central core and several different layers above it [04:47:00].
Role of Seismic Waves
Breakthroughs in understanding Earth’s interior largely came from studying earthquakes [04:54:00]. Earthquakes send seismic waves deep into the Earth, which behave like sound waves [04:59:00]:
- P waves (Primary waves): Can travel through both liquids and solids [05:05:00].
- S waves (Secondary waves): Can only travel through solids [05:11:00].
Both wave types behave differently depending on the density of the rock they move through [05:15:00]. By measuring which waves arrive at different detectors on the surface, scientists can infer the composition and state of the material deep inside [05:20:00]. This method helped determine how the mantle differs from the crust [05:25:00].
The Outer Core
The existence of a liquid layer deep within the Earth was revealed by an “S wave shadow” on the opposite side of the planet during earthquakes [05:29:00]. This liquid layer is known as the outer core [05:42:00]. It is a liquid soup of metals, cooking at around 4,400°C [05:46:00]. Here, temperature overcomes pressure, making the material liquid [05:56:00].
The constant churning of this hot metal soup generates enormous electric currents, which create Earth’s magnetic field [06:01:00]. Without this magnetic field, cosmic radiation would endanger all life on the surface [06:05:00]. Earth’s magnetic field also reverses, with the North and South poles slowly swapping places [06:16:00]. Evidence of these reversals can be found recorded in the ocean floor’s basalt, which acts like a barcode of past magnetic fields [06:24:00]. The last reversal occurred around 780,000 years ago [06:53:00].
The Inner Core
At the very center of our planet, there is a solid metal ball, almost as hot as the surface of the Sun [08:45:00]. For a long time, it was assumed the entire core was molten [08:55:00]. However, two scientists studying seismic waves noticed that waves acted strangely at the center, bending and reflecting [09:05:00]. The math supported the theory of a solid core within the larger liquid one, a theory later confirmed by more precise recordings and better computational models [09:15:00]. The inner core is solid because the pressure is so incredibly high that iron atoms literally cannot move [09:25:00].
More recently, scientists have found slight discrepancies in data suggesting that Earth’s inner core rotates at a different rate than the surface, speeding up and slowing down on a roughly 70-year cycle [09:31:00] [09:53:00]. The center of the Earth is approximately 6,400 km from the surface, with pressure around 3.6 million times that on the surface [10:22:00].
Formation of Layers
The Earth formed as a “magma ocean,” an entirely molten liquid [07:04:00]. During this time, the densest materials, primarily iron metal, sank to form the core [07:11:00]. The lightest minerals floated to the top, and denser minerals settled to the bottom, resulting in the segregation of layers by density [07:19:00].
Ongoing Research and Future Discoveries
Despite significant progress in exploration and discoveries about Earth’s core, much about the deepest parts of our planet remains a mystery [10:38:00]. Some scientists hypothesize an “inner inner core” based on new seismic data suggesting different atomic packing at the very center [10:44:00]. Current cutting-edge research involves:
- Finding new ways to analyze seismic waves with better computational models [11:01:00].
- Building experiments that mimic inner Earth conditions here on the surface to learn more and potentially predict Earth’s magnetic field and its reversals [10:57:00] [11:05:00].
As a curious species, humans continue to find new ways to “peek inside” our own home [11:10:00].