Structure of Interior of Earth

Let’s embark on a journey deep into the Earth’s interior, peeling away its layers like an explorer venturing into the unknown depths of a vast, uncharted cave. Imagine yourself standing on solid ground, the surface of our planet. It feels stable, firm, and unyielding. But beneath your feet, an entirely different world exists—a dynamic, fiery realm shaping the mountains, oceans, and even life itself.

Why Do We Study Earth’s Interior?

Before we dive in, let’s first understand why this knowledge matters. Think of Earth like a living organism. If you want to understand its behavior—why mountains rise, why earthquakes shake cities, why volcanoes erupt—you must study its internal organs.

• Understanding Natural Disasters – Earthquakes, volcanic eruptions, and tsunamis originate from deep within. Knowing their causes helps us predict and mitigate their impacts.
• Formation of Earth’s Surface – The landscapes we admire—Himalayas, plateaus, ocean trenches—are all shaped by internal forces.
• Mineral Wealth – Ever wondered why India has rich coal reserves in Jharkhand or iron ore in Odisha? The Earth’s interior holds the answer.
• Clues to Other Planets – Studying our own planet’s structure helps us understand how Mars, Venus, and even exoplanets might be composed.

Now, let’s begin our descent into the Earth.

How Do We Know What’s Inside?

You might ask, “If we have never drilled beyond 12 km (Kola Superdeep Borehole, Russia), how do we know what’s beneath?” Great question! Our knowledge comes from three main sources:

1. Indirect Methods – Like a Doctor Using an X-ray

Since we can’t physically go inside, we use indirect clues—just as doctors use X-rays or MRIs to see inside your body.

• Density Clues:
  • The crust’s density is about 2.9–3.3 g/cm³, but the whole Earth’s average density is 5.5 g/cm³.
  • Clearly, the core must be denser—later found to be around 11 g/cm³!
  • So, what is it made of? The answer lies in pressure.
• Pressure vs. Composition:
  • Initially, scientists believed that increasing pressure alone caused high density. But pressure alone cannot explain everything—beyond a point, it stops compressing matter.
  • This means the core must be made of heavy elements like Iron (Fe) and Nickel (Ni).
• Temperature Mystery:
  • The temperature increases as we go deeper. If we calculate based on surface heating rates, the core should be around 25,000°C—which would have melted the entire Earth!
  • But this doesn’t happen because most radioactive elements (Uranium & Thorium) are found in the crust, generating heat at shallower depths rather than in the core.
  • Why are Uranium and Thorium in the crust, even though they are heavy? Their chemical behavior prefers bonding with silicates instead of iron, keeping them near the surface.
• Gravity & Magnetism Clues:
  • Gravity is strongest at the poles and weakest at the equator because of Earth’s shape (oblate spheroid).
  • Differences in gravity reveal where heavier or lighter materials are distributed inside the Earth.
  • Earth’s magnetic field comes from the movement of molten iron in the outer core. Without this, compasses wouldn’t work, and Earth would be vulnerable to harmful solar radiation.
• Meteors – Time Capsules from Space:
  • When meteors crash onto Earth, they give us hints about its composition.
  • Since planets and meteors formed from the same cosmic dust, their composition is similar, helping us infer Earth’s deep structure.

2. Theories of Earth’s Interior – Early Attempts to Explain the Unknown

Think of a time when people believed Earth was flat. Similarly, early scientists had theories about Earth’s interior.

• Edward Suess’ Model (Like a Multi-Layered Cake):
  • SIAL (Silica + Aluminium) – Light outer layer (granite)
  • SIMA (Silica + Magnesium) – Denser middle layer (basalt)
  • NIFE (Nickel + Iron) – The heavy core

His idea of continents floating over SIMA was later refined but laid the foundation for plate tectonics.

• Nebular & Tidal Hypotheses – Suggested that the core should be in a liquid state, aligning with modern seismic studies.

3. Direct Sources

Though we cannot travel deep into Earth, nature occasionally gives us glimpses.

• Volcanoes – Windows into the Deep:
  • Earlier, scientists believed there was a vast underground magma ocean.
  • However, high pressure increases the melting point of rocks, keeping the deep interior solid.
  • Volcanic eruptions occur when pressure is released suddenly, lowering the melting point and causing magma to rise.
• Deep Mining & Drilling:
  • South Africa’s Mponeng gold mine, the deepest mine on Earth (4 km deep), reveals rock composition from deep below.
  • The Kola Superdeep Borehole, reaching 12.3 km, found extreme temperatures (~180°C) but couldn’t go deeper.

References:

• Zharkov, Vladimir N., and V. A. Kalinin. Interior Structure of the Earth and Planets. Taylor & Francis, 1988.
• “The Interior of the Earth.” U.S. Geological Survey, 1997. The Interior of the Earth
• Dziewonski, Adam M., and Don L. Anderson. “Preliminary Reference Earth Model.” Physics of the Earth and Planetary Interiors, vol. 25, no. 4, 1981, pp. 297–356.