Knowledge of Earth’s interior comes from indirectly exploring this largely inaccessible world. Scientists measure seismic waves generated by earthquakes, as their speed and path change dramatically when encountering different materials. This allows modeling the composition, density, and physical state of the deep interior. Geophysical modeling reveals a planet structured by distinct layers, each controlling Earth’s surface activity.
The Earth’s Thin Outer Shell
The journey begins on the crust, the thin, outermost layer of the planet, which is low in density compared to the layers below. The crust is primarily composed of silicate rocks, rich in oxygen and silicon. Its thickness is highly variable, ranging from 5 to 10 kilometers beneath ocean basins to 20 to 70 kilometers under continental mountain ranges.
The crust separates into two main types: the denser, younger oceanic crust (basaltic rock) and the less dense, older continental crust (granitic material). This low-density shell floats atop the more voluminous layer beneath it. The boundary marking the base of the crust is a distinct compositional change known as the Mohorovičić discontinuity, or Moho.
The Moho is an abrupt interface, not a gradual transition, where the velocity of seismic waves suddenly accelerates. This change indicates a shift from the less dense silicates of the crust to the denser, iron- and magnesium-rich silicates of the mantle. The Moho’s depth marks the beginning of the planet’s deep interior.
The Expanding Extremes of Pressure and Heat
As depth increases past the crust, two fundamental forces—pressure and temperature—rise dramatically, governing the state of all materials. The weight of the overlying rock causes pressure to increase steadily, reaching nearly 4 million atmospheres at the planet’s center. This hydrostatic pressure prevents the deep layers from melting, even at extreme temperatures.
Temperature also rises rapidly along the geothermal gradient, which is the rate of temperature increase with depth. Near the surface, this gradient can be steep (25 to 30 degrees Celsius per kilometer), reflecting radioactive elements and conductive heat transfer in the crust. This rate slows considerably within the deeper layers of the mantle.
The interplay between pressure and temperature dictates whether a material is solid or liquid at depth. Increasing temperature drives material toward a molten state, but the simultaneous increase in pressure raises the rock’s melting point. This competition allows the bulk of the planet to remain largely solid despite the intense internal heat.
The Dynamic and Plastic Mantle
The mantle, immediately beneath the crust, is a vast region making up approximately 84% of Earth’s total volume. Its composition is mainly peridotite, a rock rich in dense silicate minerals containing iron and magnesium. Despite temperatures exceeding 3,500 degrees Celsius near its base, the mantle remains almost entirely solid due to high pressures.
The solid mantle is not rigid but behaves like a highly viscous, pliable material over geologic timescales. This plasticity is most evident in the upper mantle’s asthenosphere, where the rock is near its melting point, allowing it to slowly deform and flow. This slow motion drives plate tectonics at the surface.
Motion within the mantle takes the form of immense convection cells, where warmer, less dense material rises and cooler, denser material sinks. This process transfers heat from the interior toward the surface. These convection currents are the primary engine moving tectonic plates across the globe, causing earthquakes and volcanism.
The mantle is structurally divided into the upper and lower mantle, based on seismic wave velocity changes signifying differences in density and rigidity. The lower mantle extends down to about 2,890 kilometers, is more rigid, and experiences higher pressures. These pressures cause minerals to undergo structural phase transitions, further increasing density. The slow movement of material throughout this plastic layer links the core’s internal heat to dynamic surface processes.
The Iron Core and the Geodynamo
The transition from the silicate-rich mantle to the metallic core occurs at the Gutenberg discontinuity, marking a major shift in composition and density. The core is primarily composed of an iron-nickel alloy, with smaller amounts of lighter elements like sulfur or oxygen. This dense, metallic mass separates into two distinct parts: a liquid outer core and a solid inner core.
The outer core is a layer of molten iron and nickel, approximately 2,200 kilometers thick. It remains liquid because the pressure is insufficient to force the metal into a solid crystal structure, despite the high temperature. Heat flowing from the inner core drives vigorous convective currents within this molten metal.
The constant movement of this electrically conductive, liquid iron generates a self-sustaining magnetic field through the geodynamo process. As the fluid circulates, it generates electric currents that reinforce the magnetic field. This geodynamo creates a protective magnetosphere, shielding Earth’s surface from harmful solar radiation.
At the center of the planet is the inner core, a sphere of solid iron-nickel alloy approximately 1,200 kilometers in radius. The temperature is estimated to be over 5,700 degrees Celsius, comparable to the sun’s surface. The inner core remains solid because immense pressure from the overlying material compresses the iron atoms, raising the melting point above the actual temperature.