The layers of the Earth are the crust, mantle, outer core, and inner core. These concentric layers differ in composition, density, temperature, pressure, and physical state. The familiar four-layer model is the simplest description of Earth’s interior. Geologists also classify the interior by its physical behavior, using layers such as the lithosphere and asthenosphere.
Key Takeaways: Layers of the Earth
- The four main layers of the Earth, from the surface inward, are the crust, mantle, outer core, and inner core.
- The mantle is the thickest layer and makes up about 84% of Earth’s volume.
- Most of the mantle is solid rock, but it slowly deforms and flows over geological time.
- The outer core is liquid, while the inner core is solid because of the enormous pressure at Earth’s center.
- Motion of electrically conducting liquid in the outer core produces Earth’s magnetic field through the geodynamo.
- The lithosphere and asthenosphere describe how parts of the crust and upper mantle behave mechanically rather than representing completely separate compositional layers.
Layers of the Earth at a Glance
| Layer | Approximate Depth | Physical State | Main Composition | Key Feature |
|---|---|---|---|---|
| Crust | Surface to about 5–10 km beneath oceans and 30–70 km beneath continents | Solid | Silicate rock | Earth’s thin outer layer |
| Mantle | Base of crust to about 2,900 km | Solid, but flows slowly | Iron- and magnesium-rich silicate rock | Thickest layer |
| Outer core | About 2,900–5,150 km | Liquid | Iron-rich alloy with nickel and lighter elements | Generates Earth’s magnetic field |
| Inner core | About 5,150–6,371 km | Solid | Iron-rich alloy | Solid despite extremely high temperature |
The 4 Basic Layers of the Earth
The Crust
The crust is the Earth’s outermost layer and it’s where we live. It has an irregular thickness, varying from about 5 km beneath the oceans (oceanic crust) to about 30 km beneath the continents (continental crust). The crust mainly consists of lighter rocks, such as basalt in the oceanic crust and granite in the continental crust.
The Mohorovičić discontinuity, often referred to as the Moho, is the boundary between the Earth’s crust and the mantle. Named after the Croatian seismologist Andrija Mohorovičić who discovered it in 1909, the Moho occurs from 5 to 10 kilometers beneath the ocean floor to about 20 to 70 kilometers beneath continental interiors.
The significance of the Moho discontinuity lies in the change in seismic wave velocities that it represents. Seismic waves from earthquakes travel at different speeds depending on the material they move through. Mohorovičić noted that seismic waves speed up abruptly below certain depths. This observation led him to conclude that Earth has a layered structure. The Moho represents the transition from the relatively low-density crust to the higher-density mantle.
The Mantle
Beneath the crust lies the mantle, extending to a depth of about 2,900 km. It contains silicate rocks that are rich in iron and magnesium. The mantle is divided into the upper mantle and lower mantle, separated by a transition zone. Most mantle rock is solid, but it deforms and flows slowly over geological time. The uppermost mantle is relatively rigid, while the asthenosphere within the upper mantle is weaker and more ductile.
The Outer Core
The outer core extends from 2,900 km to about 5,150 km beneath the Earth’s surface. It mainly consists of liquid iron and nickel. The motion within this layer generates the Earth’s magnetic field.
The Inner Core
The inner core is the central part of the Earth. It extends from a depth of about 5,150 km to the Earth’s center at about 6,371 km. Although it is very hot, the inner core is solid due to the immense pressure at this depth. It’s composed primarily of iron, with minor amounts of nickel and other lighter elements.
Detailed Layer Model of the Earth
For a more intricate understanding of the Earth’s structure, geologists divide the layers of the Earth a bit differently, based on their physical and chemical properties.
1. The Lithosphere
The lithosphere, about 10 to 200 km thick, includes the uppermost mantle and the crust. It’s rigid and breaks under stress, which is why it’s broken up into tectonic plates. The lithosphere varies in thickness, being thinner at oceanic ridges and thicker beneath older oceanic and continental regions.
2. The Asthenosphere
Beneath the lithosphere, from about 100 to 350 km, lies the asthenosphere. The asthenosphere is the part of the upper mantle that exhibits plastic (or ductile) behavior. The tectonic plates slide around on top of this layer. It’s composed of similar material to the rest of the upper mantle – mainly peridotite, a rock rich in silicate minerals.
3. The Mesosphere
Below the asthenosphere and extending to about 2,900 km is the mesosphere or lower mantle. The mesosphere is a region of strong, rigid rocks that deform slowly under the intense heat and pressure. It’s composed of silicate minerals that change in structure with depth due to increasing pressure.
4. The Outer Core
The outer core spans from 2,900 to about 5,150 km deep. The convection currents within this liquid layer create the Earth’s magnetic field through the geodynamo effect. This magnetic field extends into space and forms the Earth’s magnetosphere.
5. The Inner Core
The inner core extends from 5,150 km to the center of the Earth at about 6,371 km. In recent years, it has been suggested that the inner core itself may have an inner-inner core with distinct physical properties, but this remains an area of active research.
Physical Properties of the Earth’s Layers
Each of these layers has unique physical properties, including temperature, pressure, density, and composition. The crust and uppermost mantle (lithosphere) are cool and rigid, while the asthenosphere is hotter and more ductile. Despite its ability to flow, the asthenosphere is mostly solid rock rather than a layer of molten magma. Deeper in the Earth, temperatures and pressures rise dramatically. The core, for example, has temperatures similar to the Sun’s surface and pressures more than 3 million times atmospheric pressure.
The Earth’s density also increases with depth, from around 2.7 g/cm³ in the continental crust to over 13 g/cm³ near the center of the Earth.. This density gradient is due to both increasing pressure and changes in composition.
In terms of composition, the crust is mostly silicate rocks and oxygen, while the core is largely iron and nickel. The mantle, which comprises the majority of Earth’s volume, is predominantly composed of silicate minerals rich in iron and magnesium.
10 Facts About the Layers of the Earth
- The mantle is the largest layer by volume. It is about 2,900 km thick and accounts for roughly 84% of Earth’s volume.
- Pressure at Earth’s center is enormous. It reaches about 360 GPa, or approximately 3.6 million times atmospheric pressure at sea level.
- Earth’s center is about as hot as the surface of the Sun. Estimates place temperatures near the center at roughly 5,000–6,000°C.
- The outer core produces Earth’s magnetic field. Motion of electrically conducting, iron-rich liquid powers the geodynamo.
- Oceanic and continental crust are different. Oceanic crust is generally thinner and denser, while continental crust is thicker and less dense.
- The two types of crust also differ in composition. Oceanic crust is mainly basaltic, while continental crust is generally more silica-rich and commonly includes granitic rocks.
- Tectonic plates are pieces of lithosphere. Their movement is responsible for most earthquakes, volcanic activity, mountain building, and the creation and destruction of oceanic crust.
- The asthenosphere is mostly solid. Its rock is hot and weak enough to deform and flow slowly over geological time, allowing the overlying tectonic plates to move.
- The outer core is Earth’s only completely liquid major layer. The mantle is mostly solid rock, despite sometimes being illustrated as molten material.
- The inner core is more complicated than a simple solid ball. Seismic studies indicate variations in structure and anisotropy, including evidence for a distinct innermost region within the inner core.
Layers of the Earth Worksheet
Quiz yourself!
Download the free Layers of the Earth worksheet to label the crust, mantle, outer core, and inner core.
[Google Apps worksheet][worksheet PDF][worksheet PNG][answers PNG]
References
- Engdahl, E.R.; Flinn, E.A.; Massé, R.P. (1974). “Differential PKiKP travel times and the radius of the inner core”. Geophysical Journal International. 39 (3): 457–463. doi:10.1111/j.1365-246x.1974.tb05467.x
- Harris, P. (1972). “The Composition of the Earth“. In Gass, I. G.; et al. (eds.). Understanding the Earth: A Reader in the Earth Sciences. Horsham: Artemis Press for the Open University Press. ISBN 978-0-85141-308-2.
- Haynes, William M.; David R., Lide; Bruno, Thomas J., eds. (2017). CRC Handbook of Chemistry and Physics (97th ed.). Boca Raton, Florida: CRC Press. ISBN 978-1-4987-5429-3.
- O’Reilly, Suzanne Y.; Griffin, W.L. (December 2013). “Moho vs crust–mantle boundary: Evolution of an idea”. Tectonophysics. 609: 535–546. doi:10.1016/j.tecto.2012.12.031
- Rogers, N., ed. (2008). An Introduction to Our Dynamic Planet. Cambridge University Press and The Open University. ISBN 978-0-521-49424-3.