Figure 2: The Layers of the Earth 9
The Earth can be divided into four layers - crust, mantle, outer core, and inner core. Many geologists believe that after the Earth initially cooled, over four billion years ago, the heavier, denser materials sank to the center and the lighter materials rose to the top. Because of this, the crust is made of the lightest materials (rock- basalts and granites) and the core consists of heavy metals (nickel and iron). The crust is what humans live on, and it consists of only one percent of the Earth's mass. The center of the Earth, a solid ball of nickel and iron, is roughly 70% the size of the moon.10
The Crust is the most widely studied and understood. The Mantle is much hotter and has the ability to flow. The Outer and Inner Cores are hotter still with tremendous pressures.
Geologists have come a long way in terms of the collective knowledge of the Earth and our solar system. Though it is impossible to see directly more than about 10 km deep into the planet's center, various scientific tests and predictions such as geological samples and seismic analysis have helped create a picture of what the Earth (and other planets) look like below the surface. In this way, the Earth has been separated into its distinct layers.11
Why the Earth Has Different Layers
The Earth's layers are caused by what it is made of and how it formed. The core is made of metal (mostly iron), while the rest of the Earth is made of rock. Metal and rock don't easily mix (kind of like oil and water). The metal and rock separated when the Earth first formed. Metal sank to the middle because it was heavier. The crust is made of different kinds of rocks than the mantle. The crust is formed by volcanism, and it's made of lighter rocks than the mantle, so those rocks tend to stay at the surface and form a separate layer after they erupt. The subdivisions within the crust, mantle, and core are often due to phase transitions; for example, the outer core is a liquid while the inner core is a solid, and the layers within the mantle are made of different combinations of minerals that have their atoms arranged in different ways, giving them different properties.12
Earth's layers can be assigned according to chemical composition (what they're made of) or mechanical properties (rock strength and elasticity). Layers based on chemical composition are the core, mantle, and crust. According to mechanical properties, Earth's layers are the lithosphere, asthenosphere, lower mantle (also known as mesospheric mantle), outer core and inner core.
Scientists know the layers are there. Seismic waves can tell them about Earth's interior, including where the lithosphere and asthenosphere are located. During an earthquake, primary (P) and secondary (S) waves spread out through the Earth's interior. Special stations situated around the world detect these waves and record their velocities, as well as the direction of wave travel and whether they have been refracted (bent). Seismic waves travel faster through dense material like solid rocks and will slow down in liquids.
Relative differences in arrival times of waves at several recording stations reveal their velocities and subsequently the density of the material they have traveled through. S waves, for example, cannot travel through liquids and do not travel through Earth's outer core, implying that this layer is liquid.13
The Crust
The crust of the Earth is the area that is arguably best known by scientists, and certainly, the one the general public is the most familiar with, as it is where we live. Human life all exists on the crust of the Earth, as does the rest of known organic life. The crust is the thinnest of the four layers on Earth and is only 1 percent of the whole Earth. The crust’s thickness ranges in measurement from only 5 to 70 km thick, depending on location.
The crust can be further divided into two categories - the continental crust, and the oceanic crust. The continental crust is generally much thicker, less dense, and is composed mainly of rock, and this is the ‘dry land’ crust which includes all earth above sea level. The other type of crust is known as the oceanic crust, is considerably thinner, denser, and made up of rock basalt. This is anything below sea level, and the thinner layers hold the oceans, seas, and gulfs. 14
The Earth’s crust is also broken up into various pieces, known as tectonic plates, which fit together in a puzzle-like manner to form what is collectively called the crust or lithosphere. These plates, which contain large chunks of the crust, are free-floating in/on the deformable solid lower level known as the mantle. Tectonic plates exist in both oceanic and continental areas, and traverse political and continental borders. There are seven major plates: the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American plates, and at least 10 minor plates: Somali, Nazca, Philippine Sea, Arabian, Caribbean, Cocos, Caroline, Scotia, Burma, and the New Hebrides plates.15
The Mantle
The mantle makes up 84 percent of the Earth’s volume and is entirely solid except for very small places beneath volcanoes where the rock has melted into magma. When the Earth was young, the majority of the mantle would have been viscous melted rock (the so-called “magma ocean”), but this has cooled and solidified over millions of years to form the mantle we know today. The mantle is much thicker than the crust, measures some 2,900 km in depth, and is mainly composed of silicate minerals such as olivine, garnet, pyroxene, or magnesium oxide. Several other elements are common in the mantle layer, including iron, minor calcium, and traces of aluminum, sodium, and potassium.16
As you go deeper into the Earth, temperature and pressure increase. Within the mantle, there is a range of temperature, which rises depending on depth. Nearest the crust, the mantle registers temperatures around 1000° Celsius (~1800° Fahrenheit). At its deepest, temperatures can read as high as 3700° Celsius (~6700° Fahrenheit) or more.
As mentioned, the tectonic plates which form the mantle, are often described as ‘floating’ on the mantle asthenosphere. The mantle is least viscous at these plate borders and faults, allowing for mobility of the plates over large expanses of time. The mantle itself can be divided into several sub-layers which include the upper mantle, the transition zone, the lower mantle, and D or D double-prime layer. 17
The Core
Below the mantle lies the layer known as the Outer Core. This is a thick layer - some 2,200 km (~1400 miles) thick - that consists of liquid iron and nickel. In order for the nickel and iron to be in liquid form, the core must sustain intensely high heat. The Outer Core is thought to be as hot as 6,100 degrees Celsius (11,000 Fahrenheit) It has been determined that this layer is liquid, based on the extensive study of seismic waves, and the way in which they bounce off the center of the Earth. The waves move differently through solids or liquids, thus distinguishing the outer core from its solid inner counterpart. This layer is also not static. As the Earth rotates on its axis, the liquid metal of the outer core also spins, turning approximately 0.3 to 0.5 degrees per year relative to the rotation of the surface. The outer core is also thought to be the cause of the magnetic field on Earth.18
At the very center of the Earth is what is known as the Inner Core. Protected by the liquid outer core, mantle, and crust, the inner core is a hot solid ball of highly pressurized nickel and iron, with a temperature of approximately 5,400 °C (~9,800 °F), which is roughly the same as that of the surface of the sun. The core is an extremely dense and highly pressurized environment. The inner core is actually expanding very slowly as the outer core layer solidifies. This solidification can be attributed to the high density and pressure found in the Earth’s center, and the very slow loss of Earth’s heat to outer space. In theory, this means the whole core will eventually fully cool and become a purely solid mass over billions of years. 19
