What’s Inside the Earth? A Look at Crust, Mantle, and Core Composition

Every time you stand on solid ground, you’re resting on just the outermost shell of a planet that extends nearly 6,400 kilometres to its centre. That thin outer shell – the crust – is where all life exists, yet it makes up less than one percent of Earth’s total volume. Beneath it lie two massive, chemically distinct layers: the mantle and the core. Together, these three layers tell the story of how Earth formed, how it generates heat, and why the ground beneath our feet is never truly still.

Continental and oceanic crust: not all crust is alike 🔗

Earth’s crust comes in two fundamentally different types, and the distinction matters far beyond geology textbooks. Continental crust – the rock that forms the landmasses – is thick, averaging around 35 kilometres in depth, and can reach up to 70 kilometres beneath major mountain ranges. It is predominantly felsic in composition, meaning it is rich in silica and aluminium, forming light-coloured rocks such as granite. Because of this mineral makeup, continental crust is comparatively low in density, which is why continents ride high on the surface rather than sinking into the mantle.

Oceanic crust, by contrast, is thin – typically just 5 to 10 kilometres deep – and is made up largely of basaltic rock, which contains more iron and magnesium and significantly less silica. As the California Academy of Sciences notes, this denser, darker rock is what underlies the ocean floors. Because oceanic crust is denser than continental crust, it sinks beneath continents at subduction zones – a key driver of plate tectonics. In broad geochemical terms, continental crust is classified as sial (silica-aluminium), while oceanic crust is classified as sima (silica-magnesium).

The Moho: where crust ends and mantle begins 🔗

The boundary between the crust and the mantle is called the Mohorovičić discontinuity, named after Croatian seismologist Andrija Mohorovičić who first identified it in 1909. Studying seismic records from an earthquake near Zagreb, he noticed that seismic waves arrived in two distinct sets – one travelling through the shallower crust, and one refracted through a denser layer below. The speed jump he observed was sharp and unmistakable: P-waves move at around 6.7-7.2 km/s through basaltic crust, and accelerate to 7.6-8.6 km/s once they pass below the Moho into the denser mantle rock.

Today this boundary – commonly called the Moho – sits at roughly 35 kilometres depth beneath continents and around 7 kilometres below the ocean floor, as confirmed by Britannica. No drill has ever penetrated it. The Soviet Kola Superdeep Borehole reached a depth of 12.26 kilometres – the deepest human-made hole on Earth – before the project was abandoned in 1992, still far short of the Moho beneath continental crust. The Moho remains a key marker in geophysics, separating the lighter, silica-rich crust from the denser, iron-magnesium-rich mantle beneath.

Mantle composition and mineral content 🔗

The mantle is Earth’s thickest layer, extending from the base of the crust to a depth of approximately 2,890 kilometres. It accounts for roughly 84% of Earth’s total volume, making it by far the planet’s largest layer. Despite being solid rock, it behaves plastically over geological timescales – flowing slowly in convective currents that drive tectonic plate movement at the surface.

In terms of bulk composition, the mantle is described as ultramafic – even richer in iron and magnesium than the basaltic oceanic crust, and very low in silica. The dominant rock type in the upper mantle is peridotite, a coarse-grained rock composed primarily of two key minerals: olivine and pyroxene. As Physical Geology (LibreTexts) explains, pressure increases dramatically with depth, forcing minerals to reorganise their crystal structures. Deeper in the lower mantle, extreme pressures transform these minerals into denser forms, and rocks like eclogite – containing garnets – become more common.

The mantle as a heat-transfer engine 🔗

The mantle does more than just sit between the crust and the core – it is the engine that redistributes Earth’s internal heat. Hotter material deep in the mantle rises slowly, cools as it approaches the crust, and then sinks again. This process, known as mantle convection, is what moves tectonic plates. According to Britannica, the mantle also generates periodic superplumes – massive jets of partially molten rock rising from deep near the core-mantle boundary – which have influenced large-scale volcanic events throughout Earth’s history. Temperatures in the upper mantle begin at around 700°C near the crust and climb to approximately 4,000°C near the core boundary.

The mantle is divided into an upper mantle and a lower mantle, separated by a transition zone at around 660 kilometres depth. The uppermost portion of the mantle, fused to the base of the crust, forms the rigid lithosphere. Just below that is the asthenosphere – a zone where tiny amounts of partial melt make the rock weak and pliable enough for tectonic plates to slide across it.

Core composition: iron, nickel, and extreme conditions 🔗

At Earth’s centre lies the core, beginning at a depth of roughly 2,900 kilometres. The boundary between the mantle and core – the Gutenberg discontinuity – was identified in 1912 when seismologist Beno Gutenberg noticed that S-waves (shear waves) stop entirely at this depth, indicating they had reached a liquid layer. S-waves cannot travel through liquids, and this observation confirmed the outer core is molten.

The outer core is approximately 2,200 kilometres thick and composed primarily of liquid iron and nickel, along with possible lighter elements such as sulfur, oxygen, or silicon. As Earth rotates, this liquid metallic layer flows and generates Earth’s magnetic field – the planetary shield that deflects harmful solar radiation. Without it, life on the surface would be far more vulnerable.

The inner core: solid under pressure 🔗

At the very centre – from about 5,100 kilometres depth to Earth’s centre at 6,371 kilometres – is the inner core. Although temperatures there reach an extraordinary 5,000-7,000 K, comparable to the surface of the Sun as noted by Britannica, the inner core remains solid. The reason is pressure: at these depths, the pressure is so immense that it prevents the iron-nickel alloy from melting despite the extreme heat. Some researchers suggest the inner core may even consist of a single giant iron crystal structure oriented along Earth’s north-south axis, based on how seismic waves behave as they pass through it.

The core as a whole is believed to be composed of roughly 80% iron, with nickel making up most of the remainder. Heavier elements like lead and uranium, while dense, are either too scarce or chemically incompatible with the core’s dominant iron phase, so they remain locked in the crust.

How scientists study Earth’s interior 🔗

No instrument has ever directly sampled the mantle or core. Everything known about these layers comes from indirect methods, primarily seismology. When earthquakes occur, they generate seismic waves that travel through the entire planet. Different wave types – P-waves (compressional) and S-waves (shear) – behave differently depending on the material they pass through. P-waves slow down or speed up at layer boundaries; S-waves stop entirely when they hit liquid. By mapping where these waves arrive and at what speed, scientists can construct a detailed picture of Earth’s interior without ever drilling into it.

Other methods include measuring variations in Earth’s gravitational and magnetic fields, studying meteorites (which share chemical similarities with Earth’s deep interior), and replicating core conditions in laboratory settings using diamond anvil cells that compress tiny material samples to extreme pressures and temperatures. Each of these approaches adds a layer of detail to what remains, in many ways, an ongoing scientific puzzle.

What do you think? Given that no human-made drill has ever reached the mantle, how much confidence should we place in our current models of Earth’s interior – and what kind of technological breakthrough would it take to finally sample the Moho directly? And considering that the liquid outer core generates the magnetic field that protects life on Earth, what might happen to surface conditions if that convection were to slow significantly?

References

1. https://www.calacademy.org/explore-science/from-core-to-crust-defining-earths-layers
2. https://en.wikipedia.org/wiki/Mohorovi%C4%8Di%C4%87_discontinuity
3. https://www.britannica.com/science/Moho
4. https://pubs.usgs.gov/gip/dynamic/inside.html
5. https://geo.libretexts.org/Bookshelves/Geology/Physical_Geology_(Panchuk)/03:_Earths_Interior/3.01:_Earths_Layers-_Crust_Mantle_and_Core
6. https://www.britannica.com/place/Earth/The-interior