If you really want to understand Earth, you need to travel 6,400 kilometers (3,977 miles). Starting at the center of earth, Earth is composed of 4 distinct layers. They are as, from deepest to shallowest, the inner core, the outer core, the mantle & the crust. Except for the crust, no one has ever explored these layers in person. However, the deepest humans have ever drilled is just over twelve kilometers (7.6 miles). And even that took 20 years!
Still, scientists know great things about Earth’s inner structure. They have plumbed it by studying how earthquake waves travel through the planet. The speed & behavior of these waves change as they find out different layers of different densities. Scientists including Isaac Newton, 3 centuries ago they have also learned about the core & mantle from calculations of Earth’s total density, gravitational pull & magnetic field.
The Composition and Structure of Earth
Core, mantle, & crust are divisions based on composition of earth. The crust is just made up of less than 1 % of Earth by its mass. The mantle is extremely hot and represents about 68 % of Earth’s mass. Finally, the core is mostly made of iron metal. The core makes up about 31 % of the Earth. Lithosphere & asthenosphere are divisions based on their mechanical properties. The lithosphere is composed of both the crust & the portion of the upper mantle which behaves as a brittle and a rigid solid. However the asthenosphere partially molten its upper mantle material that behaves plastically and can flow.
1. Crust
The Earth's crust is the area best known by scientists, and certainly one of the most familiar to ordinary people, because it is the place where we live. Like all other known organic life, human life exists on the Earth's crust. The crust is the thinnest of the total four layers on Earth, and it is only about 1 % of the entire Earth. The thickness of the crust is measured from only 5 to 70 km depending on the location.
The crust can be further divided into 2 categories - continental crust & sea crust. The continental crust is usually much thicker, less dense and composed of rock, and this 'dry land' is a crust that covers the earth above sea level. The other type of crust is known as marine crust, it is composed of thin, solid & rock basalt. This is a thing below sea level, and these thin layers cover the oceans, seas & bays.
The Earth's crust is also divided into some other different pieces, known as tectonic plates, which fit with each other like puzzles and these are collectively called crusts. These plates, which create the bulk of the crust, are free-floating on the lower layer of fluid called mantle. A tectonic plate is existing in both maritime and also in continental areas, and cross country & continental borders.
2. The Mantle
At close to 3000 km (1865 miles) thick, this is Earth’s thickest layer then all other layers. The Mantle is made mostly of iron, magnesium and silicon; it is very dense, hot and it is semi-solid (think caramel candy). Like the layer below it, this layer also circulates. It just does so far too slowly.
Here is an explanation about how heat moves.
Near upper edges of the mantle, somewhere between about 100 and 200 kilometers (62 to 124 miles) under the ground, the mantle’s temperature reaches the melting point of rock specimens. However, it forms a layer of partially melted rock which is known as the asthenosphere. Geologists believe this weak, also hot & slippery part of the mantle is what Earth’s tectonic plates ride upon and slide across it.
Although, Diamonds are small and tiny pieces of the mantle we can actually touch it easily. Most form at depths above 200 kilometers (around 124 miles). But in rare cases “super-deep” diamonds may have formed as far down as 700 kilometers (about 435 miles) below the surface. These diamond crystals are then brought to the surface in volcanic rock known as kimberlitic.
Most earthquakes are observed on the surface, in the crust; as the plates will ebb & tow tension creates at a time, and when that tension decreases or releases or when something breaks then the earthquake takes place. Although, earthquakes can also happen in the mantle and at that pressure you cannot possibly talk about faulting & breaking. In a sub diction area, where one plate goes beneath another, earthquakes have been observed at depths of up to 670 kilometers. However, the mechanism around this earthquake is still not clearly understood, but the theory is that some minerals shift from one state to another state, at a time, changing their volume in the process. This change in its volume can lead to earthquakes.
However, we are getting closer to understanding the mantle even without getting there. In recent years, researchers have gone close to replicating the high temperature & pressure in the mantle, and modern level computer models are also revealing some of its secrets.
3. Core
Sometimes we refer to the core as just one thing, although the inner core & the outer core are fundamentally different then each other, it is not layers of the same thing. The inner core has a radius of 1220 km, while the outer core extends up to a radius of 3400 km. We can say that the inner core is Solid and the Outer Core is Liquid.
If we could not go to the mantle, how could we say that one is solid and one isn’t? As before, the answer for this is the same: seismic waves (as we’re almost there).
The Inner Core
The Inner Core temperatures and pressures are absolutely extremely high, at approximately 5400 °C (9800 °F) & 330 to 360 gigapascals (3,300,000 to 3,600,000 Atm).
However, it is generally believed that the inner core is growing very slowly, as the core cools down, the outer core solidifies and it becomes a part of the inner core. The cooling rate of this is very low though, at about 100 °C per billions of years. However, this slow growth of cooling rate is thought to have a significant impact in the generation of Earth’s magnetic field by its dynamo action in the liquid state outer core.
The inner core seems to be asymmetric on the East-West line of the earth. There is one model that explains this asymmetry with melting on one side of it and crystallization on the other side.
The Outer Core
The outer core is a low viscosity fluid which is about ten times the viscosity of liquid metals at the surface, while “liquid” is a rather improper term. Because it has a very low viscous property so it has low viscosity, it is deformed easily and it can be malleable. We can say that it is the site of violent convection. It is also thought to suffer very violent convection currents from it. The churning of the outer core & its relative movement is mainly responsible for the Earth’s magnetic field.
However, the hottest part of the outer core is actually hotter than the inner core. Temperatures can reach up to 6000° C (10,800° F) as hot as compared to the surface of the sun.
