Did you know that the Earth has layers, just like an onion (or an ogre, to paraphrase a famous quote from Shrek)? It’s true! Our planet is made of four distinct layers, and each layer has unique chemical components and structural features. Even though we only live on the outermost layer (and see only that layer in our regular lives), the fact that our layer sits on top of three additional layers has real effects on the world that we experience. Let’s go into a little more detail about the structure and components of each of Earth’s layers. The outermost layer, which is where we live, is called the crust. Underneath that is the mantle, followed by the outer core, with the innermost layer called, appropriately, the inner core.
Layer 1: The crust
The crust of the Earth is very thin relative to the size of the Earth, although it has significant variability in its thickness: whereas the crust that is underneath continents is approximately 70 kilometers thick, the crust under oceans has a thickness of only about 5 kilometers. This means that if you can get all the way down to the bottom of the ocean (an average depth of 3,897 meters), you wouldn’t have to drill so far to reach the next layer, the mantle.
What else is important about the crust? It is very light (in terms of mass density) and cold, especially when compared to the Earth’s other layers. It is light partially because it is made of lightweight elements, including silicon and oxygen. The fact that the crust is so light is important, because it means that the crust can effectively stay on top of the layers underneath, without having to worry about sinking and/or mixing between the two layers.
Even though the crust doesn’t sink into the mantle (i.e., it doesn’t move vertically), it still moves laterally, or from side to side. More specifically, large pieces of the Earth’s crust, called tectonic plates, move approximately 1-2 inches each year. This is almost too slow to be noticeable, so most of the time, scientists don’t pay much attention to the movement of the tectonic plates. However, occasionally, tectonic plates will interact with each other – by sliding past each other, coming together, or breaking apart – in a way that causes earthquakes or volcanoes to occur. That’s when people start paying attention!
Layer 2: The mantle
Moving inward from the crust to the next layer, called the mantle, reveals a second layer of Earth with markedly different properties. The mantle layer comprises approximately 84% (!) of the Earth’s volume and 67% of its mass, which means that it is both the largest and heaviest of the four layers. The mantle is also hot, with temperatures that range from 1000 oC to 3700 oC. These high temperatures mean that the elements found in the mantle (mostly oxygen, silicon, and aluminum) are near their melting points, which causes the mantle to act like a viscous liquid.
Because the mantle is so big, it makes sense to divide it into different sub-layers, each of which has somewhat different characteristics and compositions. The top layer of the mantle, called the “upper mantle,” has a depth of about 410 kilometers and is mainly composed of solid silicates (rocks/minerals that contain silicon and oxygen). This part of the mantle has occasionally been seen by humans (see below for more details).
Below the upper mantle is the “transition zone.” This zone consists of extremely densely packed rocks, which prevents the upper mantle from mixing with the lower layers, including the outer core and inner core. Moreover, these densely packed rocks effectively “trap” a lot of hydroxide ions, which are chemical species that contain two of the three atoms found in water. Underneath the transition zone is the “lower mantle,” which is even hotter and denser than the transition zone.
Can humans see the mantle of the Earth?
Interestingly, the majority of the mantle has never been seen by humans, because that would require us to dig (or drill) very deep, which is not generally practical. Nonetheless, there are a few places on Earth where you can see the upper mantle. In Gros Morne National Park in Newfoundland, Canada, for example, and in a forest outside of Baltimore, Maryland, rocks from the mantle have been “pushed” through the crust and are completely exposed! Scientists have studied these rock formations extensively, both to prove that they actually came from the mantle, and to understand more about the chemical composition and structure of the mantle.
In other parts of the Earth (mostly under oceans), scientists have drilled almost a mile into the Earth in order to reach the mantle. They study the rocks that they find in the mantle to try to understand more about the geology of Earth and what lies beneath us.
So if we can’t see the mantle, how do we know so much information about its structure and chemical composition? Scientists use three main methods to study the mantle: (1) measuring how the energy from earthquakes travels outward from the earthquake’s center; (2) studying heat flow through various parts of Earth; and (3) investigating meteorites, which are assumed to be made from the same materials as Earth’s interior.
Layer 3: The outer core
Underneath the mantle is the third layer of Earth, called the “outer core.” This core, which is approximately 1,300 miles thick, is made of liquid metal, mostly iron and nickel. We are very familiar with these metals from their use in our daily lives, including in machinery, buildings, and jewelry, but we are used to seeing these metals as solids. Why are they liquids in the outer core? Because the temperature in this layer is so hot (4500-6000 oC), and the pressure is so high (1.3-3.3 million atmospheres of pressure), that it causes these metallic elements to liquefy!
Because the outer core is a viscous liquid, it is able to move in a way that the more solid layers above and below it cannot. It is also made almost entirely of magnetic elements, which means that it creates a magnetic field through a process called a “geodynamo.” (Yes, that’s the technical name for this process, even though it sounds kind of like science fiction). The magnetic field of earth is important to us, since it protects us from harmful, high-energy rays from the atmosphere. Moreover, compasses, which are critical tools for navigation (at least in the days before everyone used cellphones), use the earth’s magnetic field to achieve accurate orientation and enable effective navigation.
Layer 4: The inner core
The last and final of Earth’s layers is the inner core, which is found at the very center of the Earth. Unlike the outer core, the inner core is solid, consisting of a sphere with an approximate radius of 1220 kilometers (less than 1% of the total volume of Earth). Interestingly, the inner core and outer core are composed of mostly the same metals (a mixture of iron and nickel, although the inner core has substantially more iron than nickel), which may lead you to wonder why these metals are solid in the inner core and liquid in the outer core. The answer has to do with pressure! More specifically, the inner core is under extremely high pressures (approximately 3.6 million atmospheres of pressure). Under this extreme pressure, even under the high temperatures found in the inner core (approximately 5200 oC), iron and nickel are both solid.
Interestingly, the high temperature of the inner core (approximately the same temperature as the sun) effects its magnetization. More specifically, even though the inner core is mostly iron, and even though iron is generally magnetic, the temperature of the inner core of earth is too hot to create and sustain magnetic fields. Luckily, we have the outer core, whose slightly cooler temperatures enable it to form a persistent magnetic field.
Earth’s Layers, In Conclusion
Now that we have finished talking about the Earth’s four layers, you may be wondering: how did this situation come about that Earth has so many layers? In brief, when Earth was formed 4.6 billion years ago, it didn’t have any layers. Rather, it was an amorphous collection of dust-like particles that slowly cooled and condensed into a planet. As that process happened, the denser elements “sunk” to the center of the earth, and the lighter elements remained on the outside. This explains why the inner and outer core of Earth are composed of nickel and iron (i.e., heavier atoms), whereas the outer layers contain oxygen, silicon, and aluminum (i.e., lighter atoms).
Interestingly, scientists think that Earth is still evolving, or changing, due to the movement of the tectonic plates, due to changes in the average planetary temperature, and due to a number of other geologically-relevant factors. We don’t see that kind of change on a daily, weekly, or even annual basis, but if we look over a period of a hundred thousand years, the changes will become much more obvious. Too bad that most of us won’t be alive for that long!
Author: Dr. Mindy Levine