Layers of the Earth Diagram

The Earth is structured in several layers, each with unique physical and chemical properties. These layers are divided based on their composition and behavior under temperature and pressure. From the outermost to the innermost, these layers include the Crust, Mantle, Outer Core, and Inner Core. Understanding the Earth's layers helps us comprehend phenomena like earthquakes, volcanic activity, and plate tectonics.

Layers of the Earth including the solid inner core, liquid outer core, mantle, and crust

The Earth is made up of several layers, each with its own distinct properties. Here are the primary layers from the outside in:

The Crust

The crust is the Earth's outermost layer, where all life exists. It’s relatively thin, ranging from about 5 kilometers under the oceans to 70 kilometers under continents. There are two types of crust:

Continental Crust: Thickness: 30-50 km thick generally; up to 70-80 km under major mountain ranges and plateaus like Tibet and the Altiplano. Composition: Mainly granite, less dense, felsic rocks. It forms the continents.
Oceanic Crust: Thinner, about 5-10 km thick, primarily composed of Denser mafic rocks like basalt, diabase, and gabbro. It forms the ocean floors.

The Mantle

The mantle is the thickest layer of Earth, making up about 84% of the planet’s volume. It extends from the bottom of the crust to a depth of about 2,900 km. It is composed of silicate rocks rich in iron and magnesium. The mantle is semi-fluid and contains convection currents that move heat, driving the movement of tectonic plates. It is divided into two sections: the upper and lower mantle.

The Upper Mantle

Thickness: Around 660 km.

Composition: Composed mainly of silicate minerals rich in magnesium and iron, such as olivine and pyroxene.

Sub-layers:

Lithosphere: Includes both the crust and the uppermost, rigid portion of the mantle (up to ~100 km deep). The lithosphere is divided into tectonic plates. It's broken into several large and small tectonic plates that move and interact with each other, causing earthquakes, volcanoes, and mountain building. The lithosphere's rigidity is due to its relatively low temperature and pressure.
Asthenosphere: Located beneath the lithosphere, from about 100-350 km deep. It is partially molten, allowing for the slow flow of rock. The asthenosphere is important because it allows tectonic plates to move over it.

Earth’s layered structure diagram Earth's internal structure, showing the core, mantle, and crust

Earth’s layered structure. The right side of the large cross section shows that Earth’s interior is divided into three different layers based on compositional differences—the crust, mantle, and core. The left side of the large cross section depicts layers of Earth’s interior based on physical properties—the lithosphere, asthenosphere, lower mantle, outer core, and inner core. The block diagram to the left of the large cross section shows an enlarged view of the upper portion of Earth’s interior.

The Lower Mantle (Mesosphere)

Thickness: About 2,300 km, From 670 km to about 2,900 km.

Composition: Rich in silicates, but under greater pressure, causing rocks to behave plastically.

Characteristics: Higher temperatures and pressures than the upper mantle. It remains solid but can flow slowly over geological time.

The mantle's convection currents, where heat causes material to rise, cool, and sink, are essential for driving plate tectonics and the movement of Earth's lithospheric plates.

The Outer Core

Beneath the mantle, starting at about 2,900 kilometers and extending to 5,150 kilometers deep, is the liquid outer core. Composed mostly of iron and nickel, with some lighter elements.

State: Unlike the mantle, the outer core is entirely liquid because the temperatures here (between 4,000°C and 6,000°C) are high enough to melt iron and nickel.

Magnetic Field: The movement of the liquid iron in the outer core generates Earth's magnetic field through a process known as the geodynamo. This magnetic field is essential for protecting the planet from harmful solar radiation.

The Inner Core

At the very center of the Earth is the solid inner core, a sphere with a radius of about 1,220 kilometers. It is composed primarily of iron and nickel. Despite the extremely high temperatures, estimated to be around 5,200°C (9,392°F)—similar to the surface of the Sun—the immense pressure at this depth keeps the inner core solid.

Rotation: The inner core rotates slightly faster than the rest of the Earth, and its solid nature plays a role in stabilizing the magnetic field.

Discontinuities Inside the Earth

Discontinuities Inside the Earth refer to the sudden changes in the physical properties of the Earth's interior, such as seismic velocity, density, and composition. These discontinuities are typically observed at specific depths within the Earth and are used to define the boundaries between different layers of the Earth's interior.

Discontinuities Inside the Earth

Mohorovičić Discontinuity (Moho)

The Moho is the boundary between the Earth's crust and the mantle. A relatively sharp increase in seismic wave velocity occurs at the Moho, indicating a significant change in density and composition. The change is attributed to the difference in rock types: less dense, silica-rich rocks in the crust versus denser, ultramafic rocks (rich in iron and magnesium) in the mantle. The depth Varies depending on location; shallower under oceans (around 5-10 km) and deeper under continents (around 30-70 km).

Gutenberg Discontinuity

The Gutenberg Discontinuity is the Boundary between the mantle and the outer core, where a significant decrease in seismic wave velocity (particularly P-waves) occurs at the Gutenberg discontinuity. This is because the outer core is liquid, and shear waves (S-waves) cannot travel through liquids. The density also increases significantly. Located Approximately 2,900 km.

Lehmann Discontinuity

Lehmann Discontinuity The boundary between the Earth's outer core and the inner core. It is characterized by An increase in seismic wave velocity occurs at the Lehmann discontinuity, indicating a transition from liquid to solid. This is surprising given the higher temperature in the inner core, but the immense pressure at that depth forces the iron-nickel alloy into a solid state.

Transition Zone (Mantle Discontinuities)

While not a discontinuity in the same sense, The transition zone between the upper and lower mantle is marked by phase changes in minerals, especially olivine. At a depth of about 410 km, olivine transforms into wadsleyite, and at about 660 km, it transforms into ringwoodite. These changes affect the density and seismic velocity of the mantle materials.

Conrad Discontinuity

The Conrad discontinuity is a boundary within the Earth's crust, typically observed at a depth of 10-20 km. It is characterized by a sudden increase in seismic velocity and is thought to represent the change in the composition of the crust from less dense felsic rocks (like granite) in the upper crust to denser mafic rocks (like basalt) in the lower crust.

Repetti Discontinuity

This discontinuity is located in the upper mantle, at a depth of about 410 km. It is characterized by a gradual increase in seismic wave velocities, marking a difference in mineral composition and behavior between the upper and lower parts of the mantle.

These discontinuities help scientists map the Earth's interior and better understand its composition and behavior under various pressures and temperatures.

Additional Earth Structural Concepts

Plate Tectonics

The lithosphere (the crust and the uppermost mantle) is divided into tectonic plates that float on the semi-fluid asthenosphere beneath. These plates move due to mantle convection and interactions at their boundaries, including:

Divergent boundaries: Where plates move apart (e.g., mid-ocean ridges).
Convergent boundaries: Where plates collide, leading to subduction zones or mountain formation.
Transform boundaries: Where plates slide past each other, causing earthquakes.

Map showing Earth's major tectonic plates with arrows depicting the directions of plate movement.

Seismic Activity

Much of our understanding of the Earth’s interior comes from seismology. When earthquakes occur, they send seismic waves (P-waves and S-waves) through the Earth, which change speed and direction when they encounter different materials and boundaries within the Earth. By studying these waves, scientists can infer the structure and composition of the Earth's interior.

P-waves (Primary waves): Compressional waves that can travel through solids, liquids, and gases. They speed up when passing through denser materials.
S-waves (Secondary waves): Shear waves that only travel through solids. The fact that S-waves cannot pass through the outer core (liquid) is key evidence for its liquid nature.

Heat Transfer Mechanisms in Earth's Interior

Heat from the Earth's interior escapes through three mechanisms:

Conduction: Heat transfer through solid materials, such as the crust and solid mantle.
Convection: In the mantle, heat is transferred by the slow movement of semi-fluid material.
Radiation: Primarily occurs at the surface, where heat from the interior is radiated into space.

Geological Features Resulting from Earth's Structure

Mountain Ranges: Formed by tectonic plate collisions (e.g., the Himalayas).
Volcanoes: Often occur at plate boundaries where magma can ascend from the mantle to the surface.
Ocean Trenches: Deepest parts of the ocean, like the Mariana Trench, formed where one tectonic plate is subducted under another.
Mid-Ocean Ridges: Where new oceanic crust is formed as plates move apart.

Layer Summary:

Crust: Thin, solid outer layer; divided into continental and oceanic crust.

Mantle: Thickest layer; semi-solid with convection currents driving plate tectonics.

Outer Core: Liquid metal layer that generates Earth's magnetic field.

Inner Core: Solid metal sphere, extremely hot but kept solid by high pressure.

Earth’s internal structure is a dynamic system, with heat driving the movement of materials, and processes like plate tectonics shaping the surface. Understanding Earth’s structure is key to understanding the planet’s geological processes and its habitability.