Layers of The Earth | What are The Three Layers of The Earth explained

Layers of The Earth

There is more to the Earth then what we can see on its surface. We can only see its upper part and that also only the above region of that part. In fact if you are able to hold the Earth in your hand and slice it in half, you will see that it has multiple layers. We are far excited and curious to know about space and get to know about different planets and galaxies. But our Earth alone is the center for many mysteries. Even the deepest canyon is just a scratch in front of the planet. 

          

However, advances in seismology (branch of science which studies earthquake and seismic waves) helps us to get to know many things about the inside of the Earth and we get to know that there are many layers inside our Earth. Each layer has its own properties, composition and characteristics which effects many of the processes of our planet. Starting at the center, Earth's inner layer can be classified into two types of layers' system:

1. Layers based on chemical composition

2. Layers based on physical properties.

(In this article we are going to know about layers based on chemical composition).

Broadly speaking Earth is composed of mainly three layers with their sub-layers too. They are from shallowest to deepest which are crust, mantle and the inner most layer core. Except for the crust, no one has ever explored these layers in person. In fact, the deepest human have ever drilled is just over 12 kilometers (7.6 miles) and that even took 20 years! Let's take a look at them and see there's going...

Like all terrestrial planets Earth's interior is differentiated that means the internal structure consists of layers arranged like the skin of an onion. You peel one and you find another distinguished from the last with different geological and chemical properties, as well as vast differences in temperature and pressure.

               

Our modern scientific understanding about interior of the Earth is based upon the knowledge and references given by the seismic monitoring. It measures sound waves causes by the earthquake and measures how passing through different layers causes them to slow down. The change is seismic velocity causes refraction which is calculated (with Snell's law) to determine difference in densities of the different layers of the Earth.

                                                  

They are used along with the measurement of gravitational force and magnetic force inside the Earth. And also by experimenting in laboratory with crystalline solids at same temperature and pressure as it is inside the Earth. In addition, it is understood that the differences in temperature and pressure are due to leftover heat from the planet's initial formation, the decay of radioactive elements, and the freezing of the inner core due to its intense pressure.

History of the study of the layers of the Earth:-

Since ancient times people are eager to know about their planet and its working. Ancient theories were unscientific in nature. Their theories were about creation, myths, miracles or religious fables of Gods and Goddess. But during medieval period various scientific theories started to emerge. They were not properly correct at that time but started to giving real examples and scientific concepts.

At initial stage all the theories are related to flat Earth concept. This was the class of the Mesopotamian culture where the world was portrayed as flat disk floating above the ocean. Mayans believed that the Earth was flat and to its four corner four jaguars known as (bacabs) is going toward sky. The ancient Persians have belief that the Earth was a seven-layered ziggurat (or cosmic mountain), while the Chinese viewed it as a four-side cube.

At 6th century BCE Greek philosophers and scientists started to understand about the shape of the Earth and by 3rd century BCE the idea started to become articulated as scientific matter. But it took many centuries. At 16th and 17th century the study of the structure of the Earth is truly became to advance. At 1692 Edmond Halley gave a new concept about hollow Earth. Between this and an inner sphere, he reasoned there was an air gap of the same distance. To avoid collision, he claimed that the inner sphere was held in place by the force of gravity. The model included two inner concentric shells around an innermost core, corresponding to the diameters of the planets Mercury, Venus, and Mars respectively. Later Newton also published his result by further calculations of his gravity laws and different concepts and publish it in his book principia. But later this all appears to be wrong. But at least they gave a brief insight and helps to grow geology.

By the end of the 17th century Geology became a vital subject and chemistry also started to play a vital role in explaining some branches and concepts of geology at mineral basis and others. At this time layers of the Earth and other theories and discovery also started in the field of Geology. At this time people believed that the Earth have inner layer and that inner layer is only composed of water as Bible said.

At the early 19th century a new thesis came that instead of water the layers of the Earth are composed of molten rock. At that time science and technology has become comparatively very advanced. So the geologists started to study about the fossils have researched, many theories came. Many scientists have chance to research on islands. And one of that was Charles Darwin who has done the discovery of the giant fossil which help the geologists in many ways and also the Darwin for his theory of evolution

At late 19s the government also started to raise their funding's in researches. And also geological maps were started to published. There was also a great curiosity for knowing the exact age of the Earth. By the early 20s with the help of radiometric dating (which is used to determine the age of minerals and rocks) the scientists had concluded the true age of the earth. now the scientists know that the Earth is 2 billion years old, which opened doors for the theories of continental movement during the vast amount of time.

In 1912, Alfred Wegener proposed the theory of Continental Drift, which suggested that the continents were joined together at a certain time in the past and formed a single landmass known as Pangaea. Plate tectonics another famous theory also emerges at that time which provided the mechanism for Continental Drift. Geophysical evidence suggested lateral motion of continents and that oceanic crust is younger than continental crust. This geophysical evidence also spurred the hypothesis of paleo magnetism, the record of the orientation of the Earth's magnetic field recorded in magnetic minerals.

Then there was the birth of seismology (the branch of science which studies about the earthquakes, and the propagation of elastic waves through the earth or any other planet). By measuring the time travel of reflected and refracted wave, scientists were able to discover how the Earth was layered and more about the layers of the Earth.

After that number of scientists had gave number of theories about the layers of the Earth, earthquacks, and interior structures of the Earth. And from all of that we have a better understanding and concept of all the layers of the Earth and its interior structure.

Layers of the Earth

As already discussed we can classify layers of the Earth into two main parts which are layer based on physical or mechanical properties which can be further divided into the lithosphere, asthenosphere, mesospheric mantle, outer core, and the inner core.

And the other and more popular one is dividing by its chemical compositions which can be further divided into crust, mantle (which can be sub-divided into outer- mantle and inner- mantle) and core which is also sub-divided into outer-core and inner-core.

The crust is the upper most layer of the Earth. It is the layer through which we are living and most associates with. On its physical properties it is of two types lithosphere and asthenosphere. Lithosphere is only the plates according to plate tectonics theory. 

The inner-core is solid and the outer core is liquid and mantle is solid but little viscous in nature. This is due to relative melting points of different layers and also the temperature and pressure increases as depth increases.

The lower mantle is under tremendous pressure and therefore has lower viscosity as that of upper layer. The metallic nickel-iron is liquid in outer core because of intense temperature. But however it becomes solid in inner core because of very high pressure. 

The differentiation in these layers is due to processes that took place at the beginning of the formation of the Earth (billions of years ago). The melting at that time had cause the denser substance to sink and the less denser substance to rise up and form the upper layer of the Earth. And because of this only the core is largely composed of iron and nickel. And crust is light part with silicate rocks and others.

Crust:

It is the upper-most layer of the Earth. Whatever we see on the earth surface is the crust only. The crust measures 40km on average, ranging from 5 to 70 km (3 to 44 miles) in depth. But still it is the thinnest layer of the Earth comparatively thinner than the skin of the apple. This layer makes up only 1% of the entire volume of the Earth.

There are two types of crust: continental crust and oceanic crust. The oceanic crust is found at the basins of the oceans which are thinner and measure 5 to 10 km (3 to 6 miles). The oceanic crust is composed of dense materials such as iron magnesium silicate, igneous rock (basalts). The oceanic crust is denser and harder than the continental crust. 

While the thicker layer that is continental crust is found above then the oceanic crust. This layer is less dense and composed of granite type rocks and sediments like sodium, potassium, silicate etc.

The upper-most layer of the mantle (see below) together with the crust, constitutes the lithosphere (which is of layers of the Earth with its physical properties). It is an irregular layer with maximum thickness of something 200 km or 120 miles. The oldest known mineral grain was approximately 4.4 billion years old indicating that Earth may be had a solid crust at that long.

The Earth crust is made up of different types of rocks which are igneous rocks (rocks formed from magma), sedimentary rocks (these are form by chemical sediments and debris), basaltic layer (these rocks are in the oceanic crust and these are rocks that are from deep sea bed floor).

The crust is not the rigid thing, but it’s split into several tectonic plates. These tectonic plates are not stationary, but are in relative motion to one another. Depending on the relationship and geologic setting, there are three types of tectonic plate boundaries: convergent (moving one toward the other), divergent (moving away from the other) and transformant (moving laterally).

Formation of The Crust:

Earth's mantle and crust are formed about 100 million years after the formation of the planet, that means about 4.6 billion years ago. At first the crust was very thin. and was probably changed often as the tectonic plates shifted around a lot more than they do now. The crust also has been destroyed many times by the asteroids.

The oldest oceanic basalt crust today is only about 200 million years. Most of the continental crust is much older. The oldest continental crustal rocks on Earth are cratons between 3.7 to 4.28 billion years old. These have been found in the Narryer Gneiss Terrane in Western Australia, in the Acasta Gneiss in the Northwest Territories on the Canadian Shield, and on the Fennoscandian Shield. A few zircons at least 4.3 billion years old have been found in the Narryer Gneiss Terrane.

Facts Related to Earth Crust:

The Earth crust was formed during the process called fractionation while Earth was cooling down and solidifying during its formation. Therefor the composition of crust, mantle and core are different.

• The thickness of crust varies from ocean to continents. Below ocean it is about 5 to 7 km thick whereas below continents it's about 25 to 30 km thick.
• The Earth crust is mainly made up of oxygen, aluminum, silicon, iron and so on. So you are walking on something which is of 47 percent oxygen.
• according to plate tectonics theory Earth is divided into many plates which is moving. And actually Himalaya was also formed by continent to continent collision.

Mantle:

The mantle extends about 2890 km making it the thickest layer in all the layers of the Earth. It makes about 84 percent of the Earth's volume. Everything we about the mental are based upon theories and by measurement only as no human could ever manage to go beyond crust. Many of the data we know about are with the help of seismology only.

Its basically accepted that mantle is not in steady state but its in continuous motion. There is a general convective circulation, with hot material upwelling towards the surface and cooler material going deeper. It is generally thought that this convection actually directs the circulation of the plate tectonics in the crust.

Based upon seismologic studies mantle can be divided into layers: upper mantle and lower mantle.

Upper Mantle:

The upper mantle is not uniform throughout. The topmost, thin layer of it is very similar to the Earth's crust. Together with the crust, we call it the lithosphere. Below the lithosphere is a layer of upper mantle called the asthenosphere. This is made up rock that is fluid and can move. It is this fluidity that powers the movement of the tectonic plates of the Earth's crust. Circular convection cycles in the hot, fluid upper mantle rock move the plates over the surface of the Earth.

The mantle which makes 84 percent of the Earth volume is mainly solid but behaves as viscous fluid in geological time. The upper mantle, which starts at the "Mohorovicic Discontinuity" (aka. the "Moho" – the base of the crust) extends from a depth of 7 to 35 km (4.3 to 21.7 miles) downwards to a depth of 410 km (250 miles). 

Seismic study helps us a lot in finding and researching about the deeper layers of the Earth. Movement in the mantle i.e. convection current is expressed at the surface through the motion of tectonic plates. Scientists measure the time taken of refracted or reflected waves and then measure there densities.

The mantle is also chemically distinct from the crust, in addition to being different in terms of rock types and seismic characteristics. This is due in large part to the fact that the crust is made up of solidified products derived from the mantle, where the mantle material is partially melted and viscous. This causes incompatible elements to separate from the mantle, with less dense material floating upward and solidifying at the surface.

The crystallized melt products near the surface, upon which we live, are typically known to have a lower magnesium to iron ratio and a higher proportion of silicon and aluminum. These changes in mineralogy may influence mantle convection, as they result in density changes and as they may absorb or release latent heat as well.

In the upper mantle, temperatures range between 500 to 900 °C (932 to 1,652 °F). Between the upper and lower mantle, there is also what is known as the transition zone, which ranges in depth from 410-660 km (250-410 miles).

Lower Mantle:

The lower mantle approximately represents 56 percent of the Earth total volume, and is the region from 660 to 2900 km below the earth surface between the transition zone and the outer core. Temperature in this region of the planet can reach about 4000 °C (7,230 °F). The lower mantle is mainly composed of three components i.e. bridgemanite, ferropericlase, and calcium-silicate perovskite. Although we don't know much about this layer apart from that it appears to be relatively seismically homogeneous. 

Formation of The Mantle:

Mantle was formed by the process of differentiation. When the Earth, and the other objects in the Solar System were formed, it was a random mixture of lots of things, and it was very hot. As it cooled, but was still molten, the denser chemicals, such as iron, sank towards the middle of the planet, where the force of gravity is strongest, forming the core. The lighter chemicals, such as silicon and aluminum floated to become the crust, and the middle ones sat in between to form the mantle.

Core:

The Earth core is the center of the Earth. It is present in the middle part of the Earth. Its temperature raises from 4,400 km (at outer region) to 6,600 km (at inner region). The core can be sub-divided into two layers which is outer core and inner core. Both are fundamentally different. Outer core is liquid whereas at inner core because of intense pressure it is in solid form.

Outer Core:

The outer core, which has been confirmed to be liquid (based on seismic investigations). It is 2300 km thick, extending to a radius of 3,400 km. In this region, the density is estimated to be much higher than the mantle or crust, ranging between 9,900 and 12,200 kg/m3. The outer core is believed to be composed of 80% iron, along with nickel and some other lighter elements.

Denser elements, like lead and uranium, are either too rare to be significant or tend to bind to lighter elements and thus remain in the crust. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core. The temperature of the outer core ranges from 4,300 K (4,030 °C; 7,280 °F) in the outer regions to 6,000 K (5,730 °C; 10,340 °F) closest to the inner core.

Because of its high temperature, the outer core exists in a low viscosity fluid-state that undergoes turbulent convection and rotates faster than the rest of the planet. This causes eddy currents to form in the fluid core, which in turn creates a dynamo effect that is believed to influence Earth's magnetic field. The average magnetic field strength in Earth's outer core is estimated to be 25 Gauss (2.5 MT), which is 50 times the strength of the magnetic field measured on Earth's surface.

Convection current combine with Coriolis effect in outer core give rise to Earth's magnetic field. The solid inner core is too hot to hold a permanent magnetic field. It may stabilize the magnetic field generated by the liquid outer core.

Without the outer core maybe life would not be possible on Earth. Convection of liquid metal in outer core creates magnetic field. This magnetic field extend outward from the Earth to several thousand kilometers cresting a protective layer called magnetosphere above the Earth which deflects the solar wind from Sun. If outer core was not there and solar wind could enter to the Earth. Then without this field it could directly strike the Earth atmosphere making it lifeless which may have happened to Mars.

Inner Core:

The inner core of the Earth, as detected by seismology, is a solid sphere about 1,216 km (760 mi) in radius, or about 70% that of the Moon. It is believed to be an iron–nickel alloy, and may have a temperature similar to the Sun's surface, about 5778 K (5505 °C). Density in it ranges from 12,600 to 13,000 kg/m3 which suggest that there may be a great deal of heavy metals such as gold, silver, platinum, etc.

The inner core is composed of very high pressure in it and that's why the metals are in solid state in such a high pressure too. Because the inner core is not rigidly connected to the Earth's solid mantle, the possibility that it rotates slightly faster or slower than the rest of Earth has long been considered. By observing changes in seismic waves as they passed through the core over the course of many decades, scientists estimate that the inner core rotates at a rate of one degree faster than the surface. More recent geophysical estimates place the rate of rotation between 0.3 to 0.5 degrees per year relative to the surface.

It’s generally believed that the inner core is growing very slowly – as the core cools down, more of the outer core solidifies and becomes a part of the inner core. The cooling rate is very low thought, at about 100 degrees Celsius per billion years.

Recent discoveries also suggest that the inner core is itself composed of different layers separated by transition zone about 250 to 400 km thick.

The freezing of liquid iron into crystalline form at the inner core boundary produces residual liquid that contains more light elements than the overlying liquid. This in turn is believed to cause the liquid elements to become buoyant, helping to drive convection in the outer core. This play an important role in the generation of Earth's magnetic field by the dynamo action in the liquid outer core.

A scientist from China in 2015 suggested the inner core has two layers. The seismic wave data suggests that crystals in the inner-inner-core are in an east to west direction. Those in the outer-inner-core are lined up north to south. Another scientist commented: "If this is true, it would imply that something very substantial happened to flip the orientation of the core to turn the alignment of crystals in the inner core north-south as is seen today in its outer parts". Other scientists are checking their data. 

It also means that the Earth's inner core, and the processes that drive it, are far more complex than previously thought! We have to know many things still about our own planet. And really our Earth alone is very mysterious to Earth. There are many theories related to its mysteries.                                                                                             

                                             

Also see: Planets nearest to Earth

Also see: Branches of Physics

Also see: What is Physics about