An Overview of Earth’s Layers

Summary

This script delves into Earth's layered structure, contrasting the continental and oceanic crusts and their roles in the subduction process. It explains the mantle's convection and its influence on plate tectonics, driven by slab-pull forces. The mysterious D'' layer's potential as a 'slab graveyard' and source of mantle plumes is highlighted. The geodynamo's role in maintaining Earth's magnetic field through the liquid outer core is unveiled, along with the inner core's super-rotation and implications for magnetic field reversals. The script concludes with Earth's formation, differentiation, and the gradual solidification of its inner core, hinting at a future without Earth's magnetic field.

Takeaways

• 🌏 The Earth's crust is divided into continental and oceanic types, with continental crust being felsic and thicker, while oceanic crust is mafic and thinner.
• 🔍 Oceanic crust is denser than continental crust, leading to subduction where oceanic plates slide beneath continental plates.
• 🏔 Cratons are ancient continental crust areas that are 4 billion years old, whereas the oldest oceanic crust is only 340 million years old due to constant subduction and destruction.
• 🔥 The mantle lies beneath the crust, with temperatures reaching 3500 degrees Celsius and pressures a million times greater than the atmosphere, and it convects slowly over millions of years.
• 🌋 Mid-ocean ridges and rift zones are formed by upwelling mantle, while subduction zones are associated with sinking cold mantle.
• 🌀 Slab-pull force, driven by buoyancy, is the primary force behind plate tectonics, initiated when oceanic crust becomes denser than the underlying mantle.
• 💎 The mantle is primarily composed of peridotite, an ultramafic rock, and diamonds form at depths of about 150 kilometers or more within it.
• 🌌 The D'' layer at the bottom of the mantle is a mysterious region with a large thermal gradient and increased heterogeneity, possibly acting as a 'slab graveyard' or the origin of mantle plumes.
• 🌐 The Earth's magnetic field is generated by the geodynamo effect in the liquid outer core, which is mainly composed of iron and nickel, and is influenced by the Earth's rotation.
• 🌀 The inner core, about the size of the moon, is solid and helps stabilize the magnetic field, although it does not generate it itself and rotates faster than the Earth as a whole.
• 🌐 Earth's magnetic field undergoes periodic reversals, the triggers of which are still unknown, but may involve the collapse of subducted slabs into the D'' layer.

Q & A

What are the two types of Earth's crust and how do they differ in composition and thickness?

The two types of Earth's crust are continental and oceanic. Continental crust is felsic, rich in silica, and is approximately 20 to 70 kilometers thick. Oceanic crust, on the other hand, is mafic, rich in iron and magnesium, and is about 5 to 10 kilometers thick.

Why is oceanic crust denser than continental crust, and what is the geological process that occurs when they collide?

Oceanic crust is denser due to its higher content of iron and magnesium. When oceanic crust collides with continental crust, the denser oceanic crust subducts beneath the continental crust in a process known as subduction.

What is the significance of the age difference between the oldest continental and oceanic crusts?

The age difference signifies the recycling of oceanic crust through subduction. Continental crust can be as old as 4 billion years, while the oldest oceanic crust is only 340 million years old, indicating that oceanic crust is constantly being created at mid-ocean ridges and destroyed through subduction.

What are the primary components of Earth's mantle, and how does its behavior contribute to plate tectonics?

The mantle is primarily composed of an ultramafic rock called peridotite, rich in the mineral olivine (MgFeSiO4). It behaves as a solid but is malleable and convects over millions of years, transferring heat to the surface. This convection, along with slab-pull forces, is the main driver of plate tectonics.

What is the role of the D'' layer in the context of Earth's mantle and geological phenomena?

The D'' layer, located at the bottom of the mantle, is marked by a large thermal gradient and increased heterogeneity. It is hypothesized to be the 'slab graveyard' where subducted slabs settle, and possibly the source of mantle plumes, which can lead to the formation of large volcanoes and may have implications for magnetic field reversals.

How does the composition and state of Earth's outer core contribute to the creation of Earth's magnetic field?

The outer core is a liquid composed mainly of iron and nickel. Its convection, facilitated by Earth's rotation, creates a geodynamo effect. The motion of the electrically conductive iron generates an electric field that strengthens the original magnetic field, thus maintaining Earth's magnetic field.

What is the geodynamo, and how does it relate to Earth's magnetic field?

The geodynamo is the process by which the Earth's magnetic field is generated and sustained. It involves the convection of the liquid outer core and the motion of electrically conductive materials within Earth's magnetic field, creating an electric field that, in turn, generates an additional magnetic field.

What is the significance of the Bullen discontinuity, and how does it relate to the core's structure?

The Bullen discontinuity marks the boundary between the liquid outer core and the solid inner core. At this depth, the pressure is so great that iron is forced into the solid phase, indicating a change in the physical state of the core materials.

How does the inner core's super-rotation affect Earth's magnetic field, and what causes it?

The inner core's super-rotation, which is faster than Earth's overall rotation, is thought to be caused by rotating magnetic fields in the outer core exerting a magnetic torque on the inner core. This interaction helps lock-in and stabilize the magnetic field lines generated in the outer core.

What are magnetic field reversals, and what might trigger them?

Magnetic field reversals are periodic events where Earth's magnetic field weakens to near zero and the poles switch. Failed reversals, where the original field reestablishes, can also occur. One hypothesis for triggering magnetic reversals is the sudden collapse of large amounts of cold, subducted slabs into the D'' layer, which can reorganize outer core convection and alter the magnetic field.

How did Earth's layers form, and what was the initial state of the planet?

Earth initially formed as an extremely hot, homogeneous planet. Differentiation began as the planet cooled, with heavier materials sinking toward the center and lighter materials rising toward the surface. Over time, the crust and mantle crystallized, while the core remained molten. The inner core started to crystallize between 2 billion to 570 million years ago.

What is the current rate of growth of Earth's inner core, and what would be the long-term consequence if Earth were to exist for an extended period?

The inner core is currently growing at a rate of a millimeter per year as the liquid iron in the outer core solidifies. If Earth were to exist for 91 billion years, it would eventually have an entirely solid iron core, and Earth's magnetic field would disappear. However, the Sun is expected to engulf Earth in its red giant phase before this scenario occurs.

Outlines

00:00

🌏 Earth's Layers and Plate Tectonics

This paragraph delves into the complexities of Earth's structure, focusing on the crust and its two types: continental and oceanic. Continental crust, being felsic and silica-rich, is thicker compared to the mafic, iron and magnesium-rich oceanic crust. The density difference between these crusts is crucial as it drives the process of subduction, where oceanic crust sinks beneath the continental crust. The mantle, a layer with extreme temperatures and pressures, behaves like a massive convection system, influencing the formation of mid-ocean ridges and subduction zones. The paragraph also introduces the concept of slab-pull force as the primary driver of plate tectonics. Additionally, it touches upon the mantle's composition, the presence of xenoliths, and the mysterious D'' layer, which may be the resting place for subducted slabs or the origin of mantle plumes.

05:03

🌌 Earth's Core and Geodynamo

The second paragraph explores the dynamic nature of Earth's core and its role in generating the planet's magnetic field. It describes the outer core as a liquid layer primarily composed of iron and nickel, where convection currents create a geodynamo effect, maintaining Earth's magnetic field. The paragraph explains the role of the inner core, a solid sphere about the size of the moon, which, despite not generating a magnetic field itself, contributes to the stabilization of the field. The concept of super-rotation, where the inner core spins faster than the rest of the planet, is also discussed, drawing an analogy to the workings of an induction motor. Furthermore, the paragraph touches on the phenomenon of magnetic field reversals, their unpredictability, and potential triggers, including the impact of subducted slabs in the D'' layer. It concludes with a brief look at Earth's early formation, the process of differentiation, and the ongoing cooling and solidification of the inner core.

Keywords

💡Crust

The Earth's crust is the outermost layer, which is divided into two types: continental and oceanic. It is defined by its composition, with continental crust being felsic and rich in silica, and oceanic crust being mafic and rich in iron and magnesium. The script emphasizes the importance of crustal types in understanding plate tectonics, as the denser oceanic crust subducts beneath the less dense continental crust, a process crucial to the Earth's geological activity.

💡Subduction

Subduction is the geological process where one tectonic plate moves under another and sinks into the mantle. It is a key concept in the script as it explains the interaction between oceanic and continental plates, leading to the formation of deep-sea trenches and volcanic arcs. The script mentions subduction in the context of the oceanic crust sliding beneath the continental crust, which is a driving force behind plate tectonics.

💡Mantle

The mantle is the layer beneath the Earth's crust, extending from 70 to 2900 kilometers below the surface. It is characterized by high temperatures and pressures, and its solid yet ductile nature allows for convection currents that drive plate tectonics. The script describes the mantle as a dynamic layer where the subducted oceanic crust sinks and where mantle plumes initiate, contributing to volcanic activity.

💡Peridotite

Peridotite is an ultramafic rock that makes up the majority of the Earth's mantle. It is primarily composed of the mineral olivine (MgFeSiO4). The script uses peridotite as an example to explain how scientists can study the mantle by analyzing xenoliths brought to the surface during volcanic eruptions, providing insights into the composition and conditions of the deep Earth.

💡D'' Layer

The D'' layer, located at the bottom of the mantle, is a region of increased heterogeneity and a large thermal gradient. The script suggests that it may be the final resting place for subducted slabs, referred to as a 'slab graveyard,' and also discusses the possibility of mantle plumes originating from this layer, which are significant in the formation of large volcanoes.

💡Geodynamo

The geodynamo is the process by which the Earth's magnetic field is generated and maintained. It involves the convective motion of the liquid outer core, composed mainly of iron and nickel, and the interaction with the Earth's existing magnetic field. The script explains that the geodynamo is responsible for the Earth's protective magnetic field, which is crucial for life on Earth.

💡Super-Rotation

Super-rotation refers to the phenomenon where the Earth's inner core rotates faster than the rest of the planet. The script describes this as being caused by the interaction of the rotating magnetic fields in the outer core with the inner core, similar to the principle of an induction motor, highlighting the complex interplay between the Earth's layers.

💡Magnetic Field Reversals

Magnetic field reversals are events where the Earth's magnetic field weakens and then reestablishes with the poles switched. The script discusses these as periodic occurrences and mentions the possibility of failed reversals, where the original field reestablishes after weakening. These phenomena are significant in understanding the dynamics of the Earth's magnetic field.

💡Differentiation

Differentiation in the context of the Earth's formation refers to the process where heavier materials, such as iron and nickel, sank towards the center, while lighter materials, like silicon and oxygen, rose to the surface. The script explains that this process began after the Earth cooled enough following its initial accretion, leading to the formation of distinct layers within the Earth.

💡Bullen Discontinuity

The Bullen discontinuity is the boundary between the liquid outer core and the solid inner core of the Earth. The script describes how the immense pressure at this depth forces iron into a solid state, marking a significant transition in the Earth's internal structure.

💡Seismic Waves

Seismic waves are vibrations that travel through the Earth's layers and are used by geologists to study the Earth's interior. The script mentions seismic waves as a method to detect the physical properties of Earth's layers, providing a non-invasive way to understand the composition and behavior of the Earth's deep interior.

Highlights

Earth's crust is divided into continental and oceanic types, with continental crust being felsic and oceanic crust being mafic.

Continental crust is thicker, ranging from 20 to 70 kilometers, while oceanic crust is thinner at 5 to 10 kilometers.

Oceanic crust is denser than continental crust, leading to subduction where oceanic plates slide beneath continental plates.

Subduction results in the constant destruction of oceanic crust, with the oldest being only 340 million years compared to ancient continental crust at 4 billion years old.

The mantle is a solid but ductile layer with extreme temperatures and pressures, facilitating convection over millions of years.

Mantle convection is responsible for the formation of mid-ocean ridges and rift zones due to upwelling and sinking of material.

Slab-pull force, driven by buoyancy, is the primary force behind plate tectonics, according to scientific consensus.

The mantle is composed mainly of peridotite, an ultramafic rock, which can be analyzed to determine its depth of origin.

Diamonds form at depths of about 150 kilometers or deeper and are associated with kimberlite xenoliths.

The D'' layer at the bottom of the mantle is a geologically significant and mysterious layer with a large thermal gradient.

Hypotheses suggest the D'' layer may be a 'slab graveyard' or the origin of mantle plumes, influencing Earth's volcanic activity.

Mantle plumes are thought to be responsible for some of the largest volcanoes in Earth's history, such as those in Hawaii, Iceland, and Yellowstone.

The geodynamo, driven by the liquid outer core's convection and the Earth's rotation, maintains Earth's magnetic field.

The inner core, composed mainly of iron and nickel, is solid due to extreme pressure at the Earth's center.

The inner core's super-rotation is influenced by the magnetic fields in the outer core, similar to the operation of an induction motor.

Earth's magnetic field undergoes periodic reversals, with the last one occurring around 780,000 years ago.

One hypothesis for magnetic field reversals involves the collapse of subducted slabs into the D'' layer, affecting core convection.

Earth's formation involved differentiation of materials, with heavy elements sinking and light elements rising to form the crust and mantle.

The inner core is growing at a rate of a millimeter per year, and if Earth existed long enough, it would eventually have a solid iron core.

The eventual fate of Earth may involve being engulfed by the sun during its red giant phase, affecting its magnetic field and structure.

Transcripts

Now that we’ve discussed the stresses that affect  Earth’s rocks, let’s go into detail about the  

different layers of Earth. On the surface, Earth  may seem like a relatively stable place, with the  

occasional earthquake or volcanic eruption, but  our experiences are limited to the uppermost part  

of Earth, the crust. Earth’s crust comes in two  varieties: continental and oceanic. Continental  

crust is felsic, or rich in silica, and is around  20 to 70 kilometers thick. Oceanic crust is mafic,  

or rich in iron and magnesium, and is around 5 to  10 kilometers thick. However, the most important  

difference between oceanic and continental crust  is their density. Oceanic crust is more dense  

than continental crust, therefore when a piece  of oceanic crust collides with continental crust,  

the oceanic crust slides beneath the continental  crust in a process that we’ve mentioned,  

called subduction. The subducted oceanic crust  will then sink deeper and deeper into the mantle  

until eventually reaching the bottom of  the mantle. Oceanic crust is constantly  

being subducted and therefore destroyed. There  are large areas of ancient, 4-billion-year-old  

continental crust called cratons, but the oldest  oceanic crust is only 340 million years old. 

Below the crust is the mantle, a solid, but  malleable layer with temperatures reaching  

3500 degrees Celsius and pressures up to a million  times greater than atmospheric pressure. Although  

the mantle is solid, over millions of years,  it convects like the world’s largest lava lamp,  

as earth’s primordial and radiogenic heat is  transferred to the surface and radiated to space.  

Mid-ocean ridges and rift zones  form beneath large areas of hot,  

upwelling mantle, while subduction zones  form beneath areas of cold, sinking mantle.  

When subducting oceanic crust sinks into  the mantle, it creates a suction-like force  

in its wake, which pulls the subducting slab  further into the mantle. This slab-pull force  

is driven by buoyancy, due to oceanic crust  becoming more dense with age. Eventually it  

becomes more dense than the mantle beneath it,  which is when subduction can begin. According  

to the current scientific consensus, slab pull  is the main force that causes plate tectonics. 

The mantle extends from 70 to 2900 kilometers  below the surface and is mainly composed of an  

ultramafic rock called peridotite, which is mainly  composed of the mineral olivine, or MgFeSiO4.  

Geologists have found xenoliths, or strange rocks,  of peridotite in massive volcanic eruptions.  

By analyzing the minerals in peridotite,  scientists can determine what depth the peridotite  

came from. Diamonds, for example, are formed  at depths of about 150 kilometers or deeper,  

and are primarily found in another  ultramafic xenolith called kimberlite. 

At the bottom of the mantle lies a very  mysterious, but geologically important  

layer called D’’. This layer is marked by a large  thermal gradient and an increased heterogeneity.  

The exact nature of this layer is unknown, but  there are some hypotheses. Some believe that  

the D’’ layer is the “slab graveyard”, or the  place where subducted slabs eventually settle.  

Some believe that mantle plumes, or cylindrical  areas of hot, rising mantle, initiate from within  

D’’ in areas where there is increased heat flux  from the core. Mantle plumes have caused some  

of the largest volcanoes in Earth’s History,  with modern examples being located in Hawaii,  

Iceland, and Yellowstone. The accumulation of  subducted slabs at D’’ also has implications  

for triggering magnetic field reversals; but  before we get there we must discuss the core. 

At 2900 kilometers, the D’’ layer gives way to  a liquid outer core composed mainly of iron and  

nickel. Here, temperatures reach 5500 degrees  Celsius, and pressures exceed 300 million times  

the atmospheric pressure on the surface. Since  this layer is a liquid, it can easily convect,  

carrying heat from the inner core into the  mantle through the D’’ layer. The rotation  

of Earth on its axis further modifies the  flow and creates large, cylindrical helices.  

Since iron is an electrically conductive material,  when it swirls through Earth’s magnetic field it  

creates an electric field, which in turn creates  an additional magnetic field, strengthening the  

original field. This is called the geodynamo,  and it is how earth maintains its magnetic field.  

The geodynamo does require a “seed” magnetic  field to get going, which was likely provided  

during the T-Tauri stage of the early sun. At 5100 kilometers, the pressure becomes so great  

that iron is forced into the solid phase. This  boundary is called the Bullen discontinuity,  

and it marks the division between the liquid  outer core and the dense, solid inner core. The  

inner core is about the size of the moon and has a  temperature equivalent to the surface of the sun.  

Though the inner core does not generate a magnetic  field, it does help lock-in the field lines and  

stabilize the magnetic field generated in  the outer core. The inner core also rotates  

faster than Earth as a whole. This phenomenon is  called super-rotation. It is thought to be caused  

by rotating magnetic fields in the outer core  exerting a magnetic torque on the inner core,  

similar to how an induction motor works. Magnetic  fields induce electric fields in conductors,  

and the created electric field manifests an  additional magnetic field that interacts with  

the original magnetic field. In an electric  motor, the stator windings produce a rotating  

magnetic field, and the rotating magnetic  field induces an electric current in the rotor,  

which induces a secondary magnetic field that  interacts with the original rotating field causing  

the rotor to spin, as it is “dragged” along  by the rotating magnetic field of the stator.  

Earth’s magnetic field undergoes periodic  reversals where the field strength drops to  

near zero, and the poles switch. Failed reversals  also occur, where the field strength drops to  

near zero and the original field eventually  reestablishes. The last reversal was around  

780,000 years ago and we may be due for one soon.  It is not known what triggers magnetic reversals.  

One hypothesis is the sudden collapse of  large amounts of cold, subducted slabs  

into the D’’ layer, which can reorganize outer  core convection, altering the magnetic field. 

Now that we know all about the Earth’s layers,  lets discuss how they formed. When Earth first  

accreted, it was an extremely hot, homogeneous  planet, since the material present was primarily  

chondrite meteorites. At some point early in its  history, the combined heat of radioactive decay  

and kinetic energy of the accretion process caused  Earth to melt. The process of differentiation was  

then able to begin, with the heaviest materials  like iron and nickel, sinking toward the center  

of the Earth, and the lightest materials, like  silicon and oxygen, rising toward the surface.  

Eventually, earth cooled enough to where the  crust and mantle crystallized, with the core  

remaining entirely molten for a long period of  time. The start of crystallization of the inner  

core is debated, with estimates ranging from about  2 billion to 570 million years ago. Earth has been  

cooling, radiating its heat to space, since  its formation. At the moment, the inner core  

is growing at a rate of a millimeter per year  as the liquid iron in the outer core solidifies,  

forming new crystals of solid iron. If  Earth were to exist in 91 billion years,  

the Earth would have an entirely solid iron core,  and Earth’s magnetic field would be gone. However,  

it’s likely that the sun will engulf the Earth  in a red giant phase long before then. A poetic  

end to our hero’s journey, and an amazing sight  for any sentient beings that were able to escape  

the same fate, by venturing into the cosmos. So, with this overview of Earth’s structure  

complete, let’s go into more detail about the  physical properties of each of Earth’s layers,  

and how geologists can detect those  properties with seismic waves.