M6SCI-3.1

Earth's Layered Structure

Learn the four layers of Earth—crust, mantle, outer core, and inner core—their positions, thicknesses, and whether they are solid or molten.

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A voice-first session with the Crimsora tutor on Earth's Layered Structure, then targeted practice and FRQs — with the tutor adapting to where you get stuck.

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What this lesson covers

Have you ever wondered what's deep inside Earth, far below your feet? Earth is not a solid ball all the way through. Instead, it has distinct layers, each with different properties and materials. Understanding Earth's layered structure helps us explain volcanoes, earthquakes, and how our planet works. In this lesson, you will learn about the crust, mantle, outer core, and inner core—where they are located, how thick they are, and what they are made of.

The Crust: Earth's Outermost Layer

The crust is the thin, solid outer layer of Earth that you live on. It is the coolest and most rigid layer. The crust is made of rock and includes continents and ocean floor. Even though it feels huge to us, the crust is actually very thin compared to the rest of Earth—only about 5 to 70 kilometers thick. The continental crust (under land) is thicker and less dense than the oceanic crust (under oceans). The crust is the layer where you find soil, mountains, and the rocks you study in geology class. Because it is solid and breaks apart into large pieces called tectonic plates, the crust is the layer involved in earthquakes and volcanic activity. When we drill boreholes or dig mines, we are only scratching the crust—we never reach Earth's interior layers. The crust is also the only layer we can observe and sample directly.

The Mantle: The Largest Layer

Below the crust lies the mantle, which is Earth's largest layer by volume. It extends from the base of the crust down to about 2,900 kilometers below the surface. The mantle is made of dense, hot rock, but it is not completely solid or liquid—it behaves like a very thick, slow-flowing plastic or putty. This means the rock in the mantle moves, but extremely slowly, over millions of years. The temperature in the mantle increases with depth, ranging from about 1,000 degrees Celsius near the crust to over 3,500 degrees Celsius at the bottom. Despite these extreme temperatures, the mantle stays mostly solid because of the enormous pressure from all the layers above it. The mantle is responsible for convection currents—slow circulation of hot material rising and cooler material sinking—which drive the movement of tectonic plates and cause geological changes on Earth's surface.

The Outer Core: Molten Iron and Nickel

Below the mantle is the outer core, a layer of liquid (molten) iron and nickel. It extends from about 2,900 kilometers to about 5,150 kilometers below Earth's surface. Even though temperatures here are extremely high—between 4,000 and 9,000 degrees Celsius—the outer core is liquid rather than solid. This is because the pressure here, while still enormous, is less intense than deeper inside Earth, so the iron and nickel cannot solidify despite the heat. The liquid outer core is in constant motion, churning and swirling. This motion generates Earth's magnetic field, which protects us from harmful radiation from the sun. The liquid nature of the outer core is one of the key differences between this layer and the inner core below it. Understanding the outer core helps explain why Earth has a magnetic field and compass needles point north.

The Inner Core: A Solid Center

At the very center of Earth is the inner core, a solid sphere of iron and nickel. It extends from about 5,150 kilometers to 6,371 kilometers below Earth's surface (Earth's center). The inner core is solid despite having temperatures as hot as or hotter than the outer core—reaching about 9,000 to 10,000 degrees Celsius. The reason it is solid is the immense pressure from all the layers above it crushing down on it. This pressure is so great that it forces the iron and nickel to remain in a solid state even at these extreme temperatures. The inner core is the smallest layer and the least understood, since we cannot reach it or sample it directly. Its solid nature contrasts sharply with the liquid outer core just above it, making the boundary between these two layers a major structural feature of Earth.

Comparing Earth's Layers

Earth's four layers differ dramatically in position, thickness, composition, and physical state. The table below summarizes the key properties:
LayerPositionThickness (km)CompositionPhysical StateTemperature
CrustOutermost5–70Rock (silicates)Solid, rigid0–1,000°C
MantleBelow crust~2,800Dense rock (silicates)Plastic (slow-flowing solid)1,000–3,500°C
Outer CoreBelow mantle~2,250Iron and nickelLiquid (molten)4,000–9,000°C
Inner CoreCenter~1,220Iron and nickelSolid9,000–10,000°C
Notice that thickness and temperature do not always increase together, and physical state does not depend on temperature alone—pressure is equally important. The crust is the thinnest but the coolest and most rigid. The mantle is the thickest and behaves plastically. The outer core is liquid, while the inner core is solid despite being hotter. This shows that planetary layers depend on both temperature and pressure working together.

Key terms

Crust.
The thin, solid, outermost layer of Earth made of rock, ranging from 5 to 70 kilometers thick; the only layer we can observe and sample directly.
Mantle.
Earth's largest layer by volume, extending from the base of the crust to about 2,900 kilometers deep; made of dense, hot rock that behaves as a slow-flowing plastic material.
Outer Core.
A layer of liquid (molten) iron and nickel located below the mantle, extending to about 5,150 kilometers deep; responsible for generating Earth's magnetic field.
Inner Core.
Earth's solid, innermost layer composed of iron and nickel, centered at about 6,371 kilometers depth; solid despite extreme heat due to immense pressure from above.
Plastic (in geology).
A material that is solid but capable of flowing very slowly under pressure over long periods of time, like the mantle rock.
Molten.
Melted or in a liquid state; describes the outer core, which is liquid iron and nickel.
Convection Currents.
Slow circulation of material in the mantle where hot material rises toward the crust and cooler material sinks deeper, driving tectonic plate movement.
Tectonic Plates.
Large, rigid pieces of Earth's crust that move slowly over the mantle, causing earthquakes, volcanic activity, and continental drift.

Worked example

A geologist is studying the interior of Earth and wants to identify which layer is located 3,000 kilometers below the surface and describe whether it is solid or liquid. Use what you know about Earth's layers to identify the layer and explain why it has this physical state.
First, we need to locate the 3,000-kilometer depth on Earth's interior structure. Let's check each layer:

The crust extends from 0 to 5–70 km, so 3,000 km is too deep.

The mantle extends from about 70 km to about 2,900 km, so 3,000 km is slightly below the mantle.

The outer core extends from about 2,900 km to about 5,150 km, so 3,000 km is within the outer core.

The layer at 3,000 kilometers is the outer core.

Next, we determine the physical state. The outer core is composed of iron and nickel and is liquid (molten). Why is it liquid? Even though temperatures in the outer core are extremely hot—between 4,000 and 9,000 degrees Celsius—the pressure at this depth is lower than in the inner core below. This lower pressure allows the iron and nickel to melt and remain in a liquid state. If we moved deeper to the inner core, the pressure would become so great that the iron and nickel would solidify back into a solid, even though the temperature is just as hot or hotter.

Answer: The outer core is located 3,000 kilometers below the surface. It is liquid (molten) because the combination of high temperature and moderate pressure allows iron and nickel to exist in a molten state.

Practice questions

Which of the following correctly lists Earth's layers from the surface down to the center?
  1. Crust, mantle, inner core, outer core
  2. Crust, mantle, outer core, inner core
  3. Mantle, crust, outer core, inner core
  4. Outer core, inner core, mantle, crust

Answer: Crust, mantle, outer core, inner core

Earth's layers in order from the surface inward are: crust (outermost), mantle, outer core, and inner core (at the center). A common mistake is reversing the core layers—remember that the outer core is liquid and sits above the solid inner core. The other choices disorder the layers incorrectly.
The mantle is much hotter than the crust, yet the crust is solid and rigid while the mantle is plastic and flows slowly. Why does temperature alone not determine whether a layer is solid or liquid?
  1. Pressure has no effect on the state of matter
  2. The mantle is actually liquid, not plastic
  3. Pressure from the weight of layers above affects the melting point and state of material
  4. The crust is actually hotter than the mantle

Answer: Pressure from the weight of layers above affects the melting point and state of material

This question tests understanding that both temperature and pressure together determine physical state. Even though the mantle is hotter than the crust, the enormous pressure from the layers above forces the mantle rock to remain solid (though it flows very slowly like plastic). In the outer core, high temperature combines with lower pressure to create a liquid state. Students often mistakenly think temperature is the only factor controlling whether something melts or solidifies.
Explain why the inner core is solid even though it is as hot as or hotter than the outer core, which is liquid.

Answer: The inner core is solid because of the immense pressure from all the layers above it crushing down on it. Even at temperatures of 9,000 to 10,000 degrees Celsius, this extreme pressure forces iron and nickel to remain in a solid state. The outer core is liquid because although it is also very hot (4,000 to 9,000 degrees Celsius), the pressure there is lower, allowing the iron and nickel to melt. This demonstrates that both temperature and pressure work together to determine the physical state of material.

This open-ended question asks students to reason about the relationship between pressure and melting point. The correct answer must explain that pressure, not just temperature, determines physical state. Students should recognize that the inner core's solid nature contradicts the idea that hotter materials are always more liquid, and should attribute this to the crushing pressure of Earth's weight. This is a place where many students go wrong—they focus only on temperature and are confused by the inner core being hotter but still solid.

FAQ

If the outer core is liquid, why doesn't it flow up and out of Earth like lava?
The outer core does not flow out because it is held in place by the enormous pressure of all the layers above it—the entire weight of the crust and mantle pressing down. Additionally, the outer core is surrounded by the mantle, which contains and supports it. The liquid outer core does move and flow, but only slowly in circular convection patterns. This motion is what generates Earth's magnetic field. In rare cases, when magma (molten rock from the mantle) reaches the crust, it can erupt as lava at volcanoes, but the outer core itself stays deep inside.
How do we know what Earth's interior is like if we cannot dig down to it?
We learn about Earth's interior using several indirect methods. Scientists study seismic waves from earthquakes, which travel through Earth at different speeds depending on the material they pass through. By analyzing these waves, geologists can determine layer boundaries and whether material is solid or liquid. We also study meteorites (rocks from space) and understand that Earth's composition is similar. Additionally, Earth's magnetic field tells us the outer core is liquid and moving. Finally, the heat that escapes from Earth and the way gravity affects objects tell us about the interior structure. These methods combined give us a detailed picture without needing to drill to Earth's center.
Are the numbers for layer thicknesses exact, or do they vary?
Layer thicknesses vary slightly and are not exact numbers. The crust is especially variable—oceanic crust is only about 5 to 10 kilometers thick, while continental crust can be 30 to 70 kilometers thick. The mantle, outer core, and inner core have more consistent thicknesses, but scientists continue to refine these measurements as technology improves. The numbers given in your textbook are averages based on seismic studies and represent our best current understanding. When you see a range (like 5–70 km for crust), that reflects real variation in Earth, not uncertainty in the measurements.
Does Earth's magnetic field come from the crust or the outer core?
Earth's magnetic field is generated by the outer core, not the crust. The liquid iron and nickel in the outer core are constantly moving and swirling due to convection. This motion of liquid metal acts like a giant electromagnet, creating the magnetic field that surrounds Earth. This field is what makes compass needles point north and protects us from harmful charged particles from the sun. The crust does not generate the field, though some rocks in the crust contain minerals that are slightly magnetic. Understanding that the outer core creates the field helps explain why it is scientifically important to study Earth's interior.

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