The Earth’s interior is a complex system of nested spheres, divided into four major layers based on distinct differences in chemical composition and physical state. Scientists are unable to sample these deep regions directly, so our understanding comes primarily from analyzing how seismic waves from earthquakes travel through the planet. These waves act like an ultrasound, revealing abrupt boundaries and the varying density and rigidity of the material within each layer. The composition and physical properties of these four layers—the crust, mantle, outer core, and inner core—dictate all the geological processes observed on the surface.
The Earth’s Surface Layer: The Crust
The crust is the outermost, solid, and rocky shell, representing a mere fraction of the planet’s total volume. It is highly variable in thickness, with two distinct types differentiated by composition and density. Continental crust, which forms the landmasses, is relatively thicker, often ranging from 20 to 70 kilometers, and is primarily composed of less dense, granitic rock rich in silicon and oxygen. Oceanic crust, found beneath the ocean basins, is significantly thinner, typically only 5 to 10 kilometers thick, and consists of denser, basaltic rock rich in iron and magnesium. This difference in density explains why the continents “float” higher on the underlying layer than the ocean floors. The crust is the most chemically diverse layer and is where all geological surface activity takes place.
The Largest Layer: The Mantle
Lying beneath the crust, the mantle is a dense, hot layer of silicate rock that accounts for approximately 84 percent of the Earth’s total volume, making it the largest layer. Its composition is rich in iron and magnesium silicates, specifically ultramafic rocks like peridotite. Although described as a solid, the mantle’s material exists in a state of high pressure and temperature that allows it to behave with plasticity, like an extremely viscous fluid over geological timescales. This slow motion is organized into massive convection currents driven by heat escaping from the core below. Hotter, more buoyant material rises toward the surface, cools, and then sinks again, completing a cycle that can take millions of years. These internal currents transfer heat throughout the planet and drive the motion of the tectonic plates on the surface above. The mantle extends to a depth of about 2,900 kilometers, where a sharp boundary marks the transition to the core.
The Liquid Metal Layer: The Outer Core
The outer core is a layer of molten metal approximately 2,300 kilometers thick, situated beneath the mantle. It is composed predominantly of liquid iron and nickel, along with trace amounts of lighter elements like sulfur and oxygen. The temperature here ranges from about 4,400°C to 6,000°C, which is sufficient to keep these metals in a liquid state despite the intense pressure. The movement of this electrically conductive liquid metal generates the Earth’s magnetic field, a process known as the geodynamo. Convective currents within the outer core, combined with the planet’s rotation, cause the molten iron to churn and spiral, inducing the global magnetic field. This magnetic field deflects harmful solar radiation and protects life on Earth.
The Solid Center: The Inner Core
At the planet’s center lies the inner core, a solid sphere with a radius of about 1,220 kilometers. Like the outer core, it is primarily composed of an iron-nickel alloy. The inner core is the hottest layer, with temperatures estimated to be near 6,000°C, similar to the surface of the sun. However, the immense pressure exerted by all the overlying layers—reaching approximately 3.6 million times that of the surface—prevents the iron atoms from freely moving. This extreme compression forces the metal into a crystalline, solid structure, even at such scorching temperatures. The inner core is slowly growing as the surrounding liquid outer core cools and solidifies onto its surface, a process that continues to power the geodynamo.