Directly beneath Earth’s crust lies the mantle, a 2,900-kilometer-thick layer of dense, slowly flowing rock that makes up about 84% of the planet’s total volume. Below the mantle sits a liquid outer core of iron and nickel, and at the very center, a solid inner core. Each layer has distinct properties that shape everything from volcanic eruptions to the magnetic field that shields life on the surface.
The Boundary Where the Crust Ends
The crust doesn’t gradually fade into the layer beneath it. Instead, there’s a relatively sharp boundary called the Moho (short for Mohorovičić discontinuity), where seismic waves suddenly speed up as they pass from crustal rock into denser mantle rock. At this boundary, the speed of pressure waves jumps to around 8 kilometers per second, a noticeable increase from the slower speeds in crustal material above.
Under the oceans, the Moho sits only about 5 to 10 kilometers below the seafloor. Under continents, it’s much deeper, typically 30 to 50 kilometers down, and it can reach 70 kilometers or more beneath mountain ranges like the Himalayas. No drill has ever reached it. Everything we know about the layers below comes from studying how earthquake waves travel through the planet.
The Mantle: Earth’s Thickest Layer
The mantle stretches from the base of the crust down to about 2,900 kilometers. It’s made primarily of a rock called peridotite, rich in magnesium, silicon, iron, and oxygen. In the lower mantle, the dominant mineral is bridgmanite, a magnesium silicate that forms under the crushing pressures found at those depths. Despite what many people picture, the mantle is not a sea of magma. It’s solid rock, but at the temperatures and pressures involved, it behaves a bit like very stiff putty over millions of years.
The upper portion of the mantle, from roughly 100 to 300 kilometers deep, contains a particularly important zone called the asthenosphere. Here, temperatures are high enough that rock softens and can flow extremely slowly. The asthenosphere has a consistency sometimes compared to Silly Putty: solid if you hit it quickly, but it deforms and flows under sustained pressure. This slow flow is what allows the rigid plates of Earth’s surface to move.
How Mantle Convection Drives the Surface
Heat escaping from Earth’s deep interior creates convection currents in the asthenosphere. Hot material rises from near the core-mantle boundary, spreads out beneath the rigid plates above, then cools and sinks back down. Where these currents diverge near the surface, they pull plates apart, creating mid-ocean ridges where new seafloor forms. Where cooled, dense oceanic plate sinks back into the mantle, subduction zones form, often marked by deep ocean trenches and chains of volcanoes.
The sinking of cold, heavy plate edges back into the mantle is actually the primary force driving plate movement. The pull of this sinking slab drags the rest of the plate behind it, like a heavy tablecloth sliding off a table. Convection currents assist this process, but the “slab pull” of subducting plates does most of the work.
The Liquid Outer Core
At 2,900 kilometers down, another sharp boundary marks the transition from solid mantle rock to the liquid outer core. This boundary, identified by the geophysicist Beno Gutenberg, is where a dramatic change happens to earthquake waves. Secondary waves (S-waves), which can only travel through solid material, stop completely. Primary waves (P-waves) slow down and bend sharply. This creates a “shadow zone” on the opposite side of the Earth from an earthquake, between about 104 and 140 degrees away, where no direct P-waves arrive. That shadow zone was the key evidence that the outer core is liquid.
The outer core is roughly 2,200 kilometers thick and composed mainly of liquid iron and nickel. Its density is about 8% lower than pure liquid iron would be at those pressures, which tells scientists that lighter elements are mixed in. The best current estimates suggest roughly 6% silicon, 2% sulfur, and 1 to 2.5% oxygen by weight. These light elements matter because they affect how the core cools, how it convects, and how vigorously it generates Earth’s magnetic field.
That magnetic field exists because the liquid iron in the outer core is in constant motion. As heat escapes from the inner core and lighter elements are released during inner core crystallization, convection currents churn through the liquid metal. Because iron conducts electricity, these flowing currents generate and sustain the magnetic field that extends far into space, deflecting solar wind and protecting the atmosphere.
The Solid Inner Core
At the very center of the Earth, starting about 5,150 kilometers below the surface, sits a solid ball of iron and nickel roughly 1,220 kilometers in radius, a little smaller than the Moon. Temperatures here likely fall between 5,000 and 7,000 Kelvin, hotter than the surface of the Sun. The inner core stays solid despite this heat because the pressure at Earth’s center is so immense, over 3.5 million times atmospheric pressure, that iron atoms are forced into a crystalline structure regardless of the temperature.
The inner core wasn’t always there. Early in Earth’s history, the entire core was liquid. As the planet slowly cooled, the center eventually dropped below iron’s melting point at that pressure, and the inner core began to crystallize. This process required the liquid iron to “supercool” significantly below its melting temperature before the first solid crystals could form, because creating a new solid surface within a liquid is energetically unfavorable until enough cooling has occurred. Once nucleation began, the inner core grew outward and continues to grow today, though extremely slowly.
The existence of the inner core was proposed in 1936 by Danish seismologist Inge Lehmann, who noticed faint P-waves arriving in the shadow zone where none were expected. She realized these waves were being reflected off a solid body within the liquid core. Later work confirmed her hypothesis by showing that seismic velocities in this innermost region matched what you’d expect from solid iron under extreme pressure.
How Scientists Map What They Cannot See
No one has ever directly sampled anything below the crust. The deepest borehole ever drilled, the Kola Superdeep Borehole in Russia, reached only about 12 kilometers. Everything known about Earth’s interior comes from seismic tomography, a technique that works on the same principle as a medical CT scan but uses earthquake waves instead of X-rays.
When an earthquake occurs, it sends pressure waves (P-waves) and shear waves (S-waves) radiating through the planet. Thousands of seismograph stations around the world record when these waves arrive and how strong they are. P-waves travel through both solids and liquids. S-waves travel only through solids. By comparing the arrival times at different stations, scientists can calculate how fast waves traveled along each path through the Earth. Waves that arrive earlier than expected passed through faster, denser, or colder material. Waves that arrive late were slowed by hotter or partially molten regions.
This technique has revealed not just the broad layers but also finer details: plumes of hot rock rising from the core-mantle boundary, slabs of cold subducted ocean floor sinking deep into the mantle, and variations in the inner core’s crystal structure that suggest it may have distinct eastern and western hemispheres. The picture is far more complex than a simple set of concentric shells, though those shells remain the essential framework for understanding what lies beneath your feet.