What Is Earth Made Of? Layers Explained from Crust to Core

The Crust: Earth’s Outer Shell

The outermost layer of Earth is the crust—a thin, brittle shell that forms the surface of the planet. Though it may seem thick from our everyday perspective, in geological terms, the crust is astonishingly thin. It ranges from just 3 miles thick under the oceans to about 40 miles beneath some of Earth’s tallest mountain ranges. There are two types of crust: continental and oceanic. Continental crust, which forms the landmasses, is thicker and composed primarily of granite-like rocks. Oceanic crust, on the other hand, is thinner and denser, made mostly of basalt.

The crust is where all life exists, and it’s also the site of all weathering, erosion, and plate tectonic activity. Continents ride atop massive tectonic plates, which float on the more pliable upper mantle. These plates move at roughly the rate that fingernails grow, but over millions of years, this movement reshapes entire continents and oceans. Earthquakes occur along fault lines where these plates grind past or crash into each other, while volcanoes often mark locations where molten rock pushes through weaknesses in the crust. Despite its thinness, the crust plays a massive role in Earth’s story. It holds the fossil record of life, preserves ancient geological processes, and is the source of most of our natural resources—from soil and fresh water to metals and fossil fuels. It may be just the outermost layer, but it’s also our home and anchor in the vastness of the universe.

The Mantle: The Planet’s Molten Powerhouse

Beneath the crust lies the mantle, a vast, rocky layer that extends to a depth of about 1,800 miles. It makes up roughly 84% of Earth’s total volume and is composed mainly of silicate rocks rich in magnesium and iron. The mantle is not entirely solid, nor is it liquid; it behaves more like a very slow-moving plastic over geological time. This semi-fluid characteristic allows it to flow gradually, a process known as mantle convection. Mantle convection is a critical driver of plate tectonics. Heat from the deeper parts of the mantle and the core causes material to rise toward the crust, where it cools and sinks again. This circular movement helps shift the tectonic plates on Earth’s surface. It’s also responsible for upwelling zones known as mantle plumes, which can form volcanic hotspots like those that created the Hawaiian Islands.

The mantle is divided into several sub-layers based on temperature and mechanical behavior. The uppermost portion of the mantle, combined with the crust, forms the rigid lithosphere. Beneath this is the asthenosphere, a softer, more ductile layer that allows tectonic plates to move. The lower mantle is hotter and more pressurized, and although it remains solid, it too flows over time. One of the most extraordinary aspects of the mantle is its role in the planet’s thermal evolution. It acts as a slow-release battery of primordial and radiogenic heat, much of which has been stored since the formation of Earth 4.5 billion years ago. Even though we’ve never drilled more than a few miles into the crust, seismic waves from earthquakes and laboratory experiments have allowed scientists to infer the composition and behavior of this colossal layer.

The Outer Core: A Liquid Sea of Metal

Descending deeper, we encounter the outer core, a turbulent sea of molten iron and nickel extending from about 1,800 to 3,200 miles beneath the surface. Unlike the solid mantle above it and the solid inner core below, the outer core is entirely liquid. This unique layer exists in a high-pressure environment where temperatures range from 7,200°F to 9,000°F—hot enough to melt metal, but not quite enough to force the atoms into a solid state given the pressure.

The outer core plays an essential role in Earth’s magnetic field. As molten metal in this layer churns and flows, it generates electric currents. This phenomenon, known as the geodynamo, creates the geomagnetic field that surrounds the planet. This field protects life on Earth by deflecting solar wind and cosmic radiation, preserving the atmosphere and helping to guide migratory animals.

Seismic waves provide the clearest evidence for the outer core’s liquid nature. When an earthquake occurs, it releases energy in the form of primary (P) and secondary (S) waves. P-waves can travel through both solids and liquids, but S-waves cannot pass through liquid. The absence of S-waves on the opposite side of the globe from an earthquake indicates that they’ve been blocked by a fluid layer—namely, the outer core. Despite being inaccessible to direct observation, the outer core is a dynamic and life-sustaining engine at the heart of the planet. Its constant motion not only fuels the magnetic field but may also influence mantle convection above it, tying it deeply to the movement of the surface plates.

The Inner Core: Earth’s Solid Iron Heart

At the very center of the planet lies the inner core—a dense, solid sphere of iron and nickel about 760 miles in radius. Even though it experiences temperatures comparable to the surface of the Sun (up to 10,800°F), the immense pressure from the layers above keeps it in a solid state. This paradoxical combination of extreme heat and pressure defines one of the most fascinating regions of Earth’s interior. The inner core is thought to grow slowly over time as the planet cools and iron in the outer core crystallizes onto its surface. This gradual solidification releases latent heat, contributing to the motion in the outer core and sustaining the magnetic field. In essence, the inner core is a kind of geological dynamo in its own right—a solidifying engine that feeds the surrounding molten metal with the energy needed to power Earth’s defenses.

Recent studies suggest that the inner core might not be a uniform solid. Seismic wave data hints at the presence of distinct zones, possibly indicating different crystal alignments or even a superionic phase of matter—an exotic state between liquid and solid. Some scientists even hypothesize that the inner core may be rotating at a slightly different rate than the rest of the planet, although this remains a topic of active research. The very existence of a solid inner core helps explain the longevity and strength of Earth’s magnetic field, which has persisted for billions of years. Without this dense heart, life on Earth might never have flourished. It serves as a reminder that even the most hidden components of our planet can have profound effects on everything above.

How We Know What’s Beneath Us

Despite the fact that no human has ever drilled beyond the upper crust, scientists have developed ingenious ways to study the inner layers of Earth. The primary tool for this exploration is seismology—the study of vibrations caused by earthquakes and man-made explosions. By observing how seismic waves travel through Earth’s interior, scientists can determine the properties of different layers based on wave speed, direction, and refraction. Mineralogy and high-pressure physics experiments also play a key role. In laboratories, scientists simulate the intense pressures and temperatures found inside the Earth using diamond anvil cells and laser heating. These experiments reveal how minerals behave at depth, helping refine our models of the mantle and core.

Moreover, Earth’s gravitational field, magnetic field, and heat flow patterns offer additional clues. Satellites can detect minute variations in gravitational pull that correspond to differences in rock density, while changes in the geomagnetic field hint at processes deep within the outer core. By combining these indirect observations, geologists have painted an astonishingly detailed picture of the Earth’s interior—a testament to the power of scientific inference and interdisciplinary research.

The Dynamic Earth: Why These Layers Matter

Understanding the Earth’s internal structure is not just academic curiosity—it has real-world implications that affect everything from natural disasters to climate stability. Earthquakes and volcanic eruptions are direct results of activity in the crust and upper mantle. The motion of tectonic plates drives the creation of mountain ranges, ocean basins, and rift valleys, constantly reshaping the planet’s surface. The core, particularly the outer core, is responsible for generating Earth’s magnetic field, which shields us from deadly cosmic radiation. Without this field, solar winds could strip away the atmosphere, much like what occurred on Mars. Additionally, heat from the mantle and core influences ocean circulation, which in turn affects global climate systems.

The slow cooling of the Earth’s interior also plays a role in the carbon cycle. Volcanic eruptions release carbon dioxide from the mantle, balancing the carbon stored in the Earth’s crust and oceans. Over geological time scales, this contributes to long-term climate regulation—a natural thermostat for the planet. Understanding Earth’s internal layers also helps in the search for natural resources. Oil, gas, geothermal energy, and mineral deposits are often associated with specific geological structures created by tectonic and volcanic processes. Thus, geology is not just the study of what’s below our feet—it’s the key to unlocking the materials and energy that support civilization.

A Living, Breathing Planet

Far from being a static rock floating in space, Earth is a living, breathing planet with an intricate internal architecture. From the familiar crust we inhabit to the fiery furnace of the core, each layer tells a story of formation, change, and connection. These layers aren’t just physical boundaries; they are dynamic, interlinked systems that control the very forces that shape life, landscape, and the planet’s fate. Though we may never physically touch the inner core or walk through the mantle, the Earth’s layered composition continues to inspire, protect, and challenge us. It’s a profound reminder that our home is more than just the surface we see—it’s a vast and mysterious world beneath our feet, pulsing with energy and history, waiting to be further understood.