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Lesson Explainer: Components of Earth Geology

In this explainer, we will learn how to identify the components of Earth and recognize its internal structure, atmosphere, hydrosphere, and biosphere.

Earth is a dense medium-sized planet in our solar system that is about 4.54 billion years old. It can be divided into four main internal layers: the crust, the mantle, the outer core, and the inner core. Each layer has very different properties, and the boundaries between them mark significant compositional and physical changes.

Figure 1: A schematic diagram of the cross section of Earth showing its four internal layers.

Earth’s surface is known as the crust, and it is the thinnest and outermost layer of the planet. The crust only makes up about 1% of Earth’s volume.

Key Term: The Crust

The crust is the outermost layer of Earth. It is the thinnest of all Earth’s layers.

The crust sits directly on top of the mantle and is formed of continental crust and oceanic crust.

Continental crust makes up Earth’s land masses and consists of granitic rocks. This part of the crust is known as sial, which is composed of rocks rich in silica and aluminum. It is older, thicker, and less dense than oceanic crust. It has a thickness of about 60 km.

Oceanic crust is found beneath Earth’s oceans and consists of basaltic rocks. This part of the crust is known as sima and is composed of rocks rich in silica and magnesium. It is younger, thinner, and denser than continental crust. Its thickness varies from 8 to 12 km. Because the different types of the crust have different densities, the crust is always trying to achieve isostatic equilibrium.

Key Term: Isostatic Equilibrium

Isostatic equilibrium is when the buoyancy force pushing the crust up equals the gravitational force pulling it toward Earth’s center.

The mantle lies directly beneath the crust and is the thickest layer of Earth’s internal structure. It extends from the base of the crust to a depth of approximately 2‎ ‎900 km and makes up 80% of the planet’s volume.

The mantle is mainly composed of silicates and iron and magnesium oxides.

Figure 2: A schematic diagram showing the internal structure of Earth with the mantle labeled.

Key Term: The Mantle

The mantle is the thickest internal layer of Earth. It lies underneath the crust and is approximately 2‎ ‎900 km thick.

Toward the top of the mantle, the rock is solid. This uppermost solid part of the mantle combined with the crust is known as the lithosphere.

The lithosphere is broken up into tectonic plates that move around on top of the underlying mantle.

Key Term: The Lithosphere

The lithosphere is the solid outer part of Earth that is composed of the crust and the uppermost part of the mantle.

Figure 3: A schematic diagram showing the lithosphere, which includes the crust and uppermost solid mantle.

The underlying mantle is subject to higher temperatures so it is partially molten. This partially molten upper part of the mantle is known as the asthenosphere and is approximately 350 km thick.

Partial melting occurs deeper in the mantle due to the higher temperatures toward the center of the planet, giving rise to convection currents.

Convection is a type of heat transfer that occurs in fluids. Heat transfer occurs in the mantle when there is a significant temperature difference between two areas in the melt. Convection currents arise when hot material at the base of the mantle rises due to a decrease in its density. It then travels along the base of the lithosphere and cools. As it cools, it becomes denser and sinks back down into the mantle. This process repeats, creating a circular motion of molten rock known as a convection current. These convection currents are thought to be the driving force behind the movement of the overlying tectonic plates.

Key Term: The Asthenosphere

The asthenosphere is the molten part of the upper mantle that lies directly beneath the lithosphere.

At the base of the mantle, there is a boundary marking a compositional change. The silicate minerals of the mantle are adjacent to metallic iron–nickel material, which makes up Earth’s core.

Earth’s core is very hot, with temperatures exceeding 5‎ ‎000 degrees Celsius. Although the core only makes up approximately one-sixth of Earth’s volume, it contributes for approximately one-third of Earth’s weight. This is because the core is very dense. It has a radius of approximately 3‎ ‎486 km and is split up into two spheres: the outer core and the inner core.

Figure 4: A schematic diagram showing Earth’s internal structure with the inner and outer cores labeled.

Key Term: The Outer Core

The outer core is a liquid iron and nickel layer within Earth’s internal structure that lies beneath the mantle.

Key Term: The Inner Core

The inner core is the most central layer of Earth’s internal structure and is composed of solid iron and nickel.

The outer core is approximately 2‎ ‎100 km thick and is composed of liquid iron and nickel, with a density of approximately 10 g/cm3. It is liquid due to the high temperatures at the center of Earth. The movement of these liquid metals generates electricity, which induces magnetism, which is thought to create Earth’s magnetic field. It is subjected to approximately 3 million atmospheric pressure.

The inner core has a radius of 1‎ ‎386 km and is a mixture of solid iron and nickel, with a density of approximately 14 g/cm3. It is solid due to the extremely high pressures it is subjected to at the very center of Earth, approximately 3.6 million atmospheric pressure.

The pressure in the outer core is less than in the inner core. This allows liquid to exist, which explains the difference in the physical states of the two layers.

Example 1: Determining the Physical States of Earth’s Layers

Which layer of Earth is in a liquid state?

  1. The crust
  2. The mantle
  3. The outer core
  4. The inner core
  5. The lithosphere

Answer

The outer core is Earth’s internal layer that is liquid. This is because Earth is hotter toward its center due to heat left over from the formation of the planet and also the decay of radioactive elements. These high temperatures at the center of the planet cause rocks to be completely melted.

However, the inner core of Earth is solid because even though it is subjected to high temperatures, the extreme pressures at the center of the planet cause the rocks here to be solid. The outer core is subject to relatively lower pressures compared to the inner core because it is not as deep within the planet. This explains how the two layers of the core can exist in different physical states.

The outer core of Earth is in a liquid state, so the correct answer is C.

Trying to determine the compositional and physical properties of the internal components of Earth is challenging. Earth's crust can exceed 60 km in thickness and humans have only been able to drill 12 km into it. This means that most of the internal components of Earth cannot be investigated directly. Instead, geologists must use indirect methods to discover what lies beneath the crust.

The main indirect method used to investigate the deeper internal layers of Earth is to use seismic data.

Seismic waves from earthquakes can be monitored as they travel through Earth. The waves’ behavior can help geologists determine the physical states of the layers that they have traveled through. For example, using seismic waves has helped identify which internal components of Earth are liquid and which are solid.

Example 2: Determining the Most Common Source of Seismic Waves

Geophysicists use seismic waves to determine the internal structure of Earth. What is the most common source of seismic waves?

  1. Earthquakes
  2. Volcanic eruptions
  3. Convection currents
  4. Tsunamis
  5. Landslides

Answer

The behavior of seismic waves as they travel through different mediums is used to help understand the internal structure of Earth. Humans can only collect direct evidence from the crust; therefore, indirect evidence is needed to help determine the characteristics of the mantle, outer core, and inner core.

Earthquakes are the most common source of these seismic waves. These waves are also the most useful when determining the internal structure of Earth because they are powerful. This means that they have the ability to travel long distances through the internal layers of Earth and be redetected at the surface.

Also understanding how these waves are reflected between different mediums helps to locate the boundaries between the internal layers, which helps us to understand the thickness of these internal components.

The other options in this question either do not produce seismic waves or, if they do, are not strong enough to travel the long distances required.

So the correct answer is A, earthquakes.

As well as the four internal components of Earth, there are also three main components that exist on or above Earth’s surface: the atmosphere, the hydrosphere, and the biosphere.

The atmosphere is a layer of gases that surrounds the solid Earth and is held in position by Earth’s gravitational pull. The atmosphere extends to approximately 1‎ ‎000 km above Earth’s surface.

Key Term: The Atmosphere

The atmosphere is the layer of gases that surrounds the solid Earth. This provides protection from harmful solar rays and enables organisms to respire.

Geologists currently estimate that the atmosphere formed over 4.5 billion years ago when Earth first formed.

It is thought that the atmosphere formed from gases from volcanic eruptions. Because of this volcanic activity, the early atmosphere initially contained high quantities of carbon dioxide.

Throughout geological time, the composition of the atmosphere has changed. For example, as oceans formed, they absorbed carbon dioxide, reducing its levels in the atmosphere. When photosynthesis started occurring, this also reduced carbon dioxide and increased the proportion of oxygen in the atmosphere.

Presently, the air in the atmosphere is composed of 78% nitrogen and 21% oxygen. The remaining 1% of the atmosphere is made up of other gases, including: hydrogen, helium, argon, krypton, xenon, carbon dioxide, and water vapor.

Figure 5: A pie chart showing the relative composition of the atmosphere.

Atmospheric pressure is the pressure an object would experience when placed at a particular point in the atmosphere. This varies depending on the distance from Earth’s surface. At the surface of the crust, atmospheric pressure is at its highest.

This is because air particles closer to the surface have more air particles above them, so more pressure is exerted down onto them. Air particles at a higher point in the atmosphere have fewer air particles overlying them, which means less pressure being exerted down onto them and consequently a lower air pressure. Atmospheric pressure decreases by one-half of its value for every 5.5 km up into the atmosphere.

So, at high altitudes (e.g., on top of a mountain), atmospheric pressure is lower, but at low altitudes (e.g., at sea level), atmospheric pressure would be higher.

Figure 6: A diagram showing how atmospheric pressure decreases with altitude. Atmospheric pressure is higher at point B than at point A.

Example 3: Understanding the Relationship between Atmospheric Pressure, Altitudes, and Oxygen Deficiency

Why do humans suffer from suffocation at high altitudes?

  1. Due to a decrease in atmospheric pressure and a deficiency in nitrogen
  2. Due to a decrease in atmospheric pressure and a deficiency in oxygen
  3. Due to a decrease in atmospheric pressure and a deficiency in hydrogen
  4. Due to an increase in Earth’s gravity and a deficiency in oxygen
  5. Due to an increase in atmospheric pressure and a deficiency in oxygen

Answer

Humans require oxygen to respire and thus to survive. Oxygen is one of the main components of air, making up 21% of the atmosphere.

At high altitudes, atmospheric pressure is lower because air particles here have fewer air particles above them, so less pressure is being exerted down on them. The lower air pressure results in a lower air density. This less dense air at higher altitudes means that humans cannot get enough oxygen for sufficient respiration, which can lead to suffocation.

So the correct answer is B: due to a decrease in atmospheric pressure and a deficiency in oxygen.

The atmosphere is critical to the existence of life on Earth as it provides oxygen for respiration. It also gives protection from harmful solar radiation while trapping heat to keep the planet warm.

Another critical component of Earth is the hydrosphere. The hydrosphere is made up of all the water that exists on Earth on the surface, in the air, and in the ground. This includes oceans, seas, lakes, groundwater, rivers, and even clouds.

Key Term: The Hydrosphere

The hydrosphere is the component of Earth that includes all the water that exists on the planet’s surface, in the ground, and in the air.

The hydrosphere formed at the same time as Earth and the atmosphere. It is thought that during and after Earth’s formation, huge quantities of water vapor were expelled into the atmosphere by ancient volcanoes.

Over time, Earth cooled, and this water vapor condensed and fell as rain. Cooler temperatures provided a more stable environment, meaning water could exist on the surface without being lost as vapor. As the planet cooled, water was also expelled from water-bearing minerals within Earth. All of this water makes up the hydrosphere.

Example 4: Discussing the Formation of the Hydrosphere

Which of the following resulted in the formation of the hydrosphere?

  1. The eruption of ancient volcanoes
  2. The movement of groundwater
  3. The occurrence of earthquakes
  4. The occurrence of high temperatures
  5. The movement of tectonic plates

Answer

The hydrosphere formed approximately 4.5 billion years ago, around the same time the atmosphere was forming. When the planet formed, there were high levels of volcanic activity, which expelled huge amounts of water vapor into the air. Additionally, some minerals from when the planet formed contained water and existed within the deeper layers of Earth.

As Earth cooled, the water vapor in the atmosphere condensed and fell as rain. These cooler conditions also meant that water released by minerals could accumulate on the surface and not be lost to the atmosphere by evaporation.

This marked the formation of the early hydrosphere. Over time, water began to collect in large basins, forming oceans and seas. The water cycle circulates water between the land, oceans, and atmosphere. The process was started by the eruption of volcanoes, so the correct answer is A: the eruption of ancient volcanoes.

The water on Earth is constantly being cycled between Earth’s oceans, atmosphere, and land. This is known as the water cycle. Water evaporates into the atmosphere from Earth’s surface, oceans, seas, and lakes to then condense and fall as precipitation back to Earth.

The hydrosphere is essential to life on Earth as it plays a vital role in ecosystems and in regulating the atmosphere and Earth’s climate.

The biosphere is made up of all the parts of Earth where life exists. This includes all ecosystems, living organisms, and nonliving factors that affect these organisms. It incorporates parts of Earth’s crust, the hydrosphere, and the lower layers of the atmosphere.

Key Term: The Biosphere

The biosphere is the component of Earth that includes all living things.

Figure 7: A schematic diagram of the biosphere, the hydrosphere, the lithosphere, and the atmosphere.

Key Points

  • Earth’s internal structure is split up into 4 main layers: the crust, the mantle, the outer core, and the inner core.
  • The crust is the outermost layer; it is thin and solid and is made up of tectonic plates.
  • The mantle is the thickest layer and is composed of iron, magnesium, and silicon oxides.
  • The outer core is composed of liquid iron and nickel; the flow of these metallic minerals generates Earth’s magnetic field.
  • The inner core is composed of solid iron and nickel because of the extreme pressures it is under.
  • The atmosphere is a layer of gases that surrounds the solid Earth, providing protection from harmful solar rays and enabling organisms to respire.
  • The hydrosphere is the component of Earth that includes all the water that exists on the planet’s surface, in the ground and in the air.
  • The biosphere is the component of Earth that includes all living things.

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