Interior of Earth: Structure, Composition & Mohr’s Discontinuity
Gajendra Singh Godara
Oct 29, 2025
15
mins read
The interior of Earth is structured into multiple layers distinguished by their chemical makeup and mechanical properties. These layers include the crust, lithosphere, asthenosphere, mantle, outer core, and inner core. Except for the outer core, which is liquid, all other layers are solid, with the asthenosphere being partially molten. Variations in temperature, pressure, and density during Earth's formation and evolution resulted in the distinct chemical and physical characteristics of these layers.
Understanding Earth’s internal structure relies on both direct and indirect evidence, especially the study of seismic waves, which travel at different speeds and paths through each layer depending on their composition and state.
Methods & Sources to Study Interior of Earth
We cannot travel far underground. To study these layers, geoscientists use both direct and indirect methods.
Direct evidence:
Direct evidence comes from rocks and materials that we can access on or near the surface.
For example, surface rocks, mining sites, and deep drilling projects like the Deep Sea Drilling Project. They provide samples that tell us how temperature, pressure, and density increase with depth.
Volcanic eruptions bring molten material from inside the Earth. The volcanic igneous material gives us useful clues about what lies below, although we cannot pinpoint the exact depth it came from.
Indirect evidence :
However, direct sources only take us a few kilometres down, so most of our knowledge comes from indirect evidence.
Earthquakes create prominent vibrations - we call them seismic waves.
Scientists determine whether Earth's layers are solid or fluid by observing how seismic waves (P-waves and S-waves) move through Earth's interior.
Because S-waves can’t get through the Earth’s outer core, scientists know it’s a liquid; however, they do move through the inner core, meaning that one is solid.
P-wave shadow zone: 103° to 142°.
S-wave shadow zone: Beyond 103° on both sides
Variations in gravitational pull and magnetic field strength reveal the distribution of dense materials and magnetically active substances within a planet.
Meteorites which are the fragments from when Earth first formed also hint at what our planet is built from.
Together, these observations let scientists infer the internal layers of the Earth, even though we can’t see them directly.
Structure of the Interior of Earth
Earth is made up of several layers. According to the mechanical properties, Earth's layers are the Lithosphere, Asthenosphere, Lower mantle (also known as mesospheric mantle), Outer core, and Inner core. Chemically, the layers are the Crust, Mantle, and Core.
Chemical layers:
Crust: Outermost layer of solid rock, thin (5–70 km). Oceanic crust (~7–10 km thick) is dense and basaltic. Continental crust (~30–50 km thick) is lighter and granitic.
Mantle: Thick shell (~2,900 km deep) of mostly solid rock rich in silicates (iron-magnesium silicate minerals).
Core: Innermost sphere (~3,500 km radius) made mainly of iron and nickel.
Mechanical layers:
Lithosphere: The rigid outer layer (crust + uppermost mantle), broken into tectonic plates.
Asthenosphere: Soft, flowing upper mantle (below the lithosphere) that allows plates to move.
Mesosphere (Lower Mantle): The solid, deeper mantle (between ~660–2,900 km depth).
Outer Core: Liquid iron-nickel (between ~2,890–5,150 km depth).
Inner Core: Solid iron-nickel (center of the Earth, ~5,150–6,370 km depth).
Crust
The crust is the outer thin skin of the Earth. It is the thinnest crust under the oceans (around 5-10 km) and much thicker under the continents (averaging 35 km, up to 70 km under the mountains).
Continental crust is mostly light, granitic rocks (silicon + aluminum), while oceanic crust is made of heavier basalt (rich in iron and magnesium).
The average density of the crust is 2.7-3.0 g/cm3, and it contains most of the Earth’s land, and all the soil, rocks, and mineral deposits which we utilize.
For instance, it contains the ores of the metals and the sediment layers which are important soil.
There are two types of crust: oceanic crust and continental crust. The term oceanic crust applies to crust located under the oceans. Continental crust is less dense than oceanic crust, and therefore it floats on top of the mantle.
Mantle
Down below the crust is the mantle - a hefty zone, some 2,900 kilometers deep. This layer makes up most of our planet, nearly 84% of its volume, holding around two-thirds of Earth’s weight.
The mantle consists primarily of silicate rock—specifically, minerals containing magnesium and iron, such as olivine and pyroxene.
Near the crust, the mantle is cooler and rigid, but deeper down it becomes hotter and slowly flows over long timescales.
The mantle is divided into parts:
the upper mantle (to ~410 km) includes the lithosphere and asthenosphere,
the transition zone (410–660 km) has mineral changes,
the lower mantle (660–2,900 km) convects more slowly.
The mantle’s ability to flow drives Plate tectonics:
Hot rock rises from deep regions and cooler rock sinks. This heat engine causes the tectonic plates of the lithosphere to move.
These processes cause mountains to rise, oceans to form, and earthquakes to occur.
For example, the motion of the Indian Plate colliding with Asia (forming the Himalayas) results from mantle-driven plate tectonics. Other examples include plate movements in Ring of Fire.
Core
At the center of Earth’s interior is the core, split into two parts: the outer core and the inner core.
Outer Core:
From roughly 2,890 to 5,150 kilometers down, exists a zone composed of fluid metal - mostly melted iron mixed with nickel.
It gets incredibly hot - between 3500 to 6000 degrees Celsius. S-waves (shear waves) cannot travel through liquids, so they are completely stopped at the liquid outer core."
Earth’s magnetic field comes from movement deep inside - the liquid outer core. As molten iron swirls, influenced by our planet’s spin, electricity forms. Scientists refer to this as the geodynamo.
Inner Core:
At the very center (below ~5,150 km), the core becomes solid despite the extreme heat (over 5,000 °C) due to immense pressure.
The inner core is mostly solid iron-nickel.
It slowly grows over time (as Earth cools, more solid iron crystallizes out of the outer core) and even rotates slightly faster than the rest of the planet.
These features help sustain and influence the magnetic field.
Lithosphere
The Lithosphere is the Earth’s hard, outermost layer. It’s a solid shell comprising both the crust alongside the uppermost portion of the mantle, sitting on top of a softer zone. Compared to everything else within our planet, this outer layer feels quite cool yet remains remarkably rigid.
Typically, it’s around 100km deep; however, beneath major mountains, this layer swells to nearly 300km.
Under the ocean floor, it is much thinner — less than 50 km.
Earth’s outer shell isn’t one solid bit - it’s fractured into huge slabs, these are called tectonic plates.
These plates float and move slowly over the softer asthenosphere beneath them.
Where tectonic plates meet - their edges brushing or colliding - is generally where you’ll find most earthquakes, volcanic eruptions, alongside the rise of great mountain ranges.
Asthenosphere
The Asthenosphere is a hot, soft, and semi-molten layer of the upper mantle located just below the lithosphere. Its semi-fluid nature makes it weak and ductile, allowing the plates above it to move.
It is sometimes called the “Low-Velocity Zone (LVZ)” because seismic waves slow down here due to the soft and partially molten rock.
Its soft, flowing nature helps the lithospheric plates float and move on top.
It is about 180–220 km thick on average.
The asthenosphere extends from approximately 80-200 km depth to about 350-700 km depth, with variable thickness depending on location.
Mostly made of peridotite, a dense rock rich in the minerals olivine and pyroxene.
Discontinuities and Boundaries in Earth’s Interior:
Geologists use seismic “discontinuities” to mark boundaries between layers where properties change suddenly. Key examples are:
Within the continental crust is a subtle boundary called the Conrad Discontinuity. It’s where the top layer gives way to something denser below. Conrad discontinuity is not universally present but absent in oceanic regions and some continental areas.
The Mohorovičić discontinuity - often called the Moho - is where Earth’s crust gives way to its mantle. Seismic waves accelerate when crossing into the more compact mantle material. Beneath oceans, this transition happens around 5–10 kilometers down; under landmasses, it’s typically 30–50 kilometers deep.
About 2,890 kilometers beneath our feet, where the mantle meets the outer core, the Gutenberg Discontinuity is found. It halts S-wave travel because the outer core is molten, while simultaneously slowing P-waves.
About 5,150 kilometers down, where the outer core meets the inner core – that’s the Lehmann Discontinuity. Once past this point, P-waves gain pace because they’re now traveling through solid material.
These seismic boundaries help scientists map the internal structure of the Earth and have taught us about the solid/liquid nature of each region.
Temperature, Pressure & Density Gradients:
1. Temperature Increase with Depth
As we go deeper into the Earth—through mines, deep wells, or by studying volcanic lava—we observe that temperature keeps rising. This proves that heat increases toward the Earth’s center.
The rise in temperature is not uniform.
In the upper 100 km, the temperature increases very rapidly, by about 15°C to 30°C per km.
Deeper into the mantle, the rate of increase slows down, but it picks up again near the base of the mantle, and then continues to rise steadily through the core.
Approximate temperatures at different depths:
Base of the crust: ~1000°C
Base of the mantle: ~3500°C
Center of the Earth: ~6000°C
The extremely high temperature inside the Earth is believed to be due to heat released from radioactive decay and chemical reactions occurring under immense pressure.
2. Pressure Increase with Depth
Pressure also increases as we move deeper because of the heavy layers of rock pressing down from above.
The deeper we go, the greater the pressure becomes—reaching millions of times higher than what we experience at the surface.
Near the Earth’s center, the pressure is estimated to be around 3 to 4 million times the atmospheric pressure at sea level, making the inner layers extremely compressed.
3. Density Increase with Depth
The average density of Earth is about 5.5 g/cm³, but this density is not the same throughout.
As we move inward, the density of layers increases because of two main factors:
Higher pressure compresses materials tightly.
Heavier elements, like iron and nickel, are concentrated toward the Earth’s core.
This means the crust is the lightest layer, while the inner layers, especially the core, are much denser.
The crust’s average density is ~2.7 g/cm³, the mantle’s is ~4.5 g/cm³, and the core’s is ~10–13 g/cm³ (iron is very dense).
Overall, Earth’s average density is about 5.5 g/cm³. These gradients in heat and density drive the convection of the mantle and the geodynamo in the core.
Processes & Dynamics Related to Earth’s Interior
The interior of Earth powers many dynamic processes:
Mantle Convection & Plate Tectonics:
Hot rock rises in the mantle and cooler rock sinks, creating convection currents. This motion moves the lithospheric plates.
Diverging currents at mid-ocean ridges push plates apart, while sinking currents at subduction zones pull plates under.
This is why continents drift and collide.
For example, the collision of India with Asia (continental drift) is the same process that created the Himalayan mountains.
Earthquakes & Volcanoes:
When plates interact, stress builds in the crust. Sudden release of this stress causes earthquakes.
Many major earthquake zones lie around the Pacific Ring of Fire, where plates meet.
Volcanoes form where hot mantle rock melts through the crust (such as at subduction zones or hotspots). The only active volcano in India, Barren Island, is a direct result of such a subduction process.
Tsunamis:
Undersea earthquakes (often at oceanic trenches) can suddenly move the sea floor.
The displaced water sends out tsunami waves. Studying seismic activity in Earth’s interior helps predict where tsunamis can originate.
Earth’s Magnetic Field (Geodynamo):
Flowing liquid iron in the outer core creates electric currents, which in turn produce the magnetic field that surrounds Earth.
This field flips its polarity occasionally (north and south swap) due to changes in core flow.
Resource Distribution:
The distribution of minerals and metals is tied to the internal structure of the Earth.
Many ore deposits form from hydrothermal fluids rising from the mantle.
Geothermal energy is heat from the mantle. It is a renewable resource tapped in volcanic regions.
Over 40% of the world’s geothermal potential lies in the Ring of Fire belt.
The composition of soils and the location of mountains and plains also depend on crustal structure.
The interior of Earth is a dynamic, layered system that we cannot see directly, but science allows us to understand it well. Remember that research in geophysics and seismology is ongoing, so new discoveries (like inner core behavior or better imaging of the mantle) continue to refine our understanding. Staying updated on such developments, along with foundational concepts, will strengthen your grasp of Earth’s interior – a topic that’s both academically important and full of real-world UPSC relevance.
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