Planet Earth is much more than she appears. We walk around on our planet’s rocky outer surface, confident that the ground beneath our feet is solid and stable. But that couldn’t be further from the truth. On the Earth’s crust, everything biological life lives upon is suspended over a white-hot core and a flowing ocean of magma.
Advances in understanding the composition of the earth were made very recently in the scientific record. It wasn’t until the 1950s that Alfred Wegener’s plate tectonics theory was confirmed. This theory gave rise to modern scientific understanding of earthquakes.
In this article, we’ll unwrap each layer of the earth, from the outermost layer called the crust, to the innermost layer, the inner core. Our discussion topics will include an analysis of the earth’s layers from two perspectives: the chemical layers and the physical layers. Both are accurate; they rely on different ideas in the physical sciences to support their claims.
Keep reading to discover details about the earth’s crust, mantle and core. You’ll learn the depths at which they start and end, their composition, the history of their discovery, and fun facts. Let’s get started.
The 4 Layers of the Earth
Before diving into a discussion about the earth’s layers, there are some things you need to know. First, the earth’s layers are divided into three main parts: crust, mantle, and core. These layers are considered the ‘chemical layers’ because they are identified based on their chemical composition.
The physical layers match in line with the chemical layers, albeit with a little more detail. For example, there are two parts of the core, mantle, and crust. These parts have different physical traits and compositions.
The earth’s crust is the planet’s outermost layer. It contains all biological life as yet known, yet it is the shallowest layer of all. Some scientists compare the earth’s crust to the top crust on an apple pie – delicate, thin, and covering a piping hot interior.
The crust has two parts which we’ll discuss in more detail later in this article. For now, all you need to know is that there are two kinds of crust, oceanic and continental, and they move around atop a molten bed of rock called the mantle.
It’s not just one solid piece of crust that covers the entire globe. Instead, it’s multiple sections, called plates, that float on the mantle. When they collide with or scrape by one another, this builds mountain ranges and causes earthquakes.
Earth’s crust is not uniformly thick as you travel around the world. Certain places are more or less shallow than others. The crust underneath the oceans is about 10 to 15 km thick, while the crust underneath the continents ranges from 30 to over 70 km thick.
New crust is made in diverging fault zones, where tectonic plates are spreading apart. The magma from the mantle issues up and cools upon contact with ocean water or air.
Another name for the earth’s crust is the lithosphere. The lithosphere gets its name from the word ‘lith,’ meaning ‘stone’ in Greek.
The lithosphere does differ from the chemical definition of ‘crust’ because the lithosphere includes part of the mantle. This part of the mantle is the uppermost level that is solid, not liquified rock.
It is included in this definition because it behaves more like the rock of the crust than it does like the magma in the mantle. There are two divisions in the lithosphere. They focus on the location and composition of the crust. It is either continental or oceanic.
Continental crust is the crust found on dry land. It’s lighter than oceanic crust because it’s made up of granite, which is a lighter rock than that of the oceanic crust.
Oceanic crust is the crust of the earth under the ocean. It is completely submerged underwater. Pound for pound, oceanic crust is heavier than continental crust.
Its composition is basaltic, which is a type of rock created by the cooling of lava. It’s extremely heavy. This means that it will subside under a continental plate when a collision occurs.
The mantle is the second chemical layer of the earth. It is composed of mostly melted rocks, depending on where you are in the depth of the earth. It’s the most abundant of all the layers of the earth – it makes up almost 85% of the volume of the globe.
It’s difficult to study the mantle because there is no way to physically view it. Liquid rock is intensely hot and pressurized far beyond the limits of any human-made probes at this present time.
Even at its most shallow depth, the temperature of the mantle exceeds 1800 degrees F. At its deepest boundary, the mantle is almost 6700 degrees F.
What scientists have done, however, is examine what kind of magma bubbles up from volcanoes and hotspots. This magma, which is called lava after it is exposed to the air, creates rare and unusual rocks that clue geologists in about the characteristics of the mantle.
Other methods that have advanced the study of geology significantly include ground penetrating radar, seismic wave measuring, and ocean floor sonar mapping.
The mesosphere is the largest part of the mantle and the largest section of the layers of Earth. It is located under the asthenosphere and above the outer liquid ring of the earth’s core. Geologists estimate it starts at around 250 – 410 miles below the surface of the earth.
Scientists have no way to reach the mesosphere, since it is solid and rocks from this region don’t erupt out of the earth. This didn’t stop them from determining the composition of the mesosphere. They used methods other than direct observation, namely seismic waves.
Geologists measured the way the rocks responded to vibrations and compared that ‘signature’ with how other rocks would react. Their conclusion: the mesosphere is probably made of perovskite.
The asthenosphere is also part of the mantle. It’s the third layer, if you count the crust and the solid upper mantle, which both make up the lithosphere.
The asthenosphere is the liquid layer of mantle upon which the crust’s tectonic plates float. It is the vehicle for the colliding movement and diverting movement of the earth’s plates.
The depth of the asthenosphere starts between 50 and 120 miles below the surface of the earth, and may extend as deep down as 430 miles. It’s unclear at this time in scientific discovery where exactly the asthenosphere ends and the mesosphere begins.
Scientists believe the rocks of the asthenosphere are molten peridotite. They come to these conclusions by comparing wave imagery of rocks in the mantle with data about molten rocks at sea level, as well as the role of water and carbon dioxide molecules in the melting process.
3. Liquid Outer Core
The core of the earth is composed mainly of nickel and iron, which originated from meteor strikes when the world was first forming. The two parts of the core – the outer solid and the inner liquid – are subject to intense pressure and heat. The reason they are divided into two sections is because of the metals’ reactions to the environmental conditions there.
At 11,000 degrees F, the outer core is actually hotter than the inner core, but there is less pressure here. The liquified iron and nickel actually spins at a very slow rate, while the earth orbits the sun. Scientists believe the earth’s magnetic field originates from the spinning of these compounds.
The inner core, which is the same temperature as the sun, is being pressurized because of the weight of the earth pressing down on it from every direction. It also experiences extreme heat, up to 9,800 degrees F.
The outer core of the earth is bounded by the mesosphere above and the solid inner core below. It is estimated to measure about 1,400 miles thick and it starts about 1,800 miles below the surface of Earth.
Unlike other layers of earth, the liquid outer core is completely liquid, through and through. Other layers have liquid sections, but are partially solid.
The liquid outer core is getting bigger at the rate of 1 mm of thickness per year. This will slowly make the outer core larger and the inner core smaller. However, it’s negligible in terms of the human lifespan.
Geologists are confident that the liquid state of the outer core is what prevents life on earth from dying off due to lack of water and air. The magnetic field that the core generates keeps solar particles from invading the atmosphere. Without that protective field, the sun would destroy the atmosphere, killing off all life on Earth.
4. Solid Inner Core
The solid inner core of the earth is about 70% of the size of the moon. Its radius, the distance from its center to its outer edge, is estimated to be about 759 miles.
Temperatures are almost 10,000 degrees F! Here, the heat is intense enough to liquify any metal, but the pressure at the center of the earth prohibits the core from acting like a liquid.
Scientists theorize that the earth’s core is composed of iron and nickel. They came to this conclusion after studying meteorites, which contain more iron than most crust rocks.
The theory about the formation of the earth’s core posits that when earth was forming, meteorites pelted the earth. Over millions of years, the meteorites, which contained high quantities of iron and nickel, sank deep into the earth and formed the core.