The layers of Earth provide geologists and geophysicists clues to how Earth formed, the layers that make up other planetary bodies, the source of Earth's resources, and much more. Modern advances have allowed scientists to study what lies beneath our feet in more detail than ever before and yet there still remains significant gaps in our understanding.
I hope this guide will walk you through the layers of the Earth, provide a general sense of our understanding and our current gaps. Keep in mind that this is an area of ongoing research and is likely to become more refined in the coming years and decades.
During my second year at Edinburgh [1826-27] I attended Jameson's lectures on Geology and Zoology, but they were incredible dull. The sole effect they produced on me was the determination never as long as I lived to read a book on Geology. - Charles Darwin
The Layers Of Earth
The Earth has layers not unlike an onion and can be dissected to understand the physical and chemical properties of each layer and its influence on the rest of the Earth. Generally speaking, Earth has 4 layers:
- The outer crust that we live on
- The plastic-like mantle
- The liquid outer core
- The solid inner core
When differentiating the layers, geologists lump subdivisions into two categories, either rheologically or chemically. Rheological differentiation speaks to the liquid state of rocks under tremendous pressure and temperature. For instance, rock will respond very differently to strain under normal atmospheric temperatures and pressures as compared to fewer than thousands of kilometers of rock. If we subdivide the Earth based on rheology, we see the lithosphere, asthenosphere, mesosphere, outer core, and inner core. However, if we differentiate the layers based on chemical variations, we lump the layers into crust, mantle, outer core, and inner core.
To understand the difference in various portions of the mantle or outer versus inner core you must understand phase diagrams, which I will speak on below.
Earth's Crust
The crust is what you and I live on and is by far the thinnest of the layers of earth. The thickness varies depending on where you are on earth, with oceanic crust being 5-10 km and continental mountain ranges being up to 30-45 km thick. Thin oceanic crust is denser than the thicker continental crust and therefore 'floats' lower in the mantle as compared to continental crust. You will find some of the thinnest oceanic crust along mid ocean ridges where new crust is actively being formed. In comparison, when two continents collide as in the case of the India Plate and Eurasia Plate, you get some of the thickest sections of crust as it is crumpled together.
The temperatures within Earth's crust will vary from air temperatures at the surface to approximately 870 degrees Celsius in deeper sections. At this temperature, you begin to melt rock and form the below-lying mantle. Geologists subdivide Earth's crust into different plates that move about in relation to one another.
Given that Earth's surface is mostly constant in area, you cannot make crust without destroying a comparable amount of crust. With convection of the underlying mantle, we see insertion of mantle magma along mid ocean ridges, constantly forming new oceanic crust. However, to make room for this, oceanic crust must subduct (sink below) continental crust. Geologists have studied extensively the history of this plate movement, but we are sorely lacking in determining why and how these plates move the way they do.
Earth's crust "floats" on top of the soft plastic-like mantle below. In some instances mantle clearly drives changes in the crust, as in the Hawaiian Islands. However, there is ongoing debate whether oceanic crust subduction and mid ocean ridge spreading is driven by a push or pull mechanism.
In very broad terms, oceanic crust is made up of basalt and continental crust is made up of rocks similar to granite. Below the crust is a solid relatively cooler portion of the upper mantle that is combined with the crust to make the lithosphere layer. The lithosphere is physically distinct from the below-lying layers due to its cool temperatures and typically extends 70-100 km in depth.
Below the lithosphere is the asthenosphere layer, a much hotter and malleable portion of the upper mantle. The asthenosphere begins at the bottom of the lithosphere and extends approximately 700 km into the Earth. The asthenosphere acts as the lubricating layer below the lithosphere that allows the lithosphere to move over the Earth's surface.
Earth's Mantle
The mantle is the layer of the earth that lies below the crust and is by far the largest layer making up 84% of Earth's volume. The mantle starts at the Mohorovicic Discontinuity, also known as the Moho. The Moho is defined as the density contrast from less dense crust to denser mantle and where seismic wave velocities increase. The mantle acts similar to plastic and at very high temperatures and pressures the rock is deformable at geologic timescales. This deformation causes a convection like process in the mantle where you have larige-scale upwelling and downwelling zones.
The mantle extends down to 2,890 km into the Earth's surface Temperatures that range from 500 to 900 degrees Celsius in the upper portion to over 4,000 degrees Celsius near the core boundary. Earth's mantle is believed to be composed of bulk mineralogy similar to peridotite. Gem quality peridotite is called peridot, so next time you're in a jewelry shop take a look at the peridot and you'll be looking at something similar to 84% of Earth!
The video above provides a glimpse into the global circulation of mantle magma around the Earth. Of course, this is greatly simplified but provides a schematic of the process creating mid ocean ridges, volcanoes, and mountains.
Earth's Outer Core
The outer core is the liquid largely iron layer of the earth that lies below the mantle. Geologists have confirmed that the outer core is liquid due to seismic surveys of Earth's interior. The outer core is 2,300 km thick and goes down to approximately 3,400 km into the earth. No one has ever seen the outer core but based on a number of indicators, geologists believe the outer core is 80% iron, some nickel and a number of different lighter elements. When Earth was just beginning to cool billions of years ago, heavier elements sunk down into the center of the Earth, while less dense elements rose to the surface. Therefore, we see a general increase in density, as you get closer to the center of the Earth.
The outer core is hot enough to be melted but not under quite enough pressure to make the iron solid again, as seen in the inner core. The temperature of the outer core ranges from about 4,030 to 5,730 degrees Celsius. Amazingly, the outer core is fluid enough and low enough in viscosity that it may spins faster than the entire Earth. This differential velocity of spinning, along with convection and turbulent flow of the iron outer core, creates Earth's magnetic field.
Earth's Inner Core
The inner core is the centermost layer of Earth and is in many ways similar to the outer core. It is also primarily iron and nickel and has a radius of about 1,220 km. The differentiation between the outer core and inner core is density driven. The pressures become high enough that despite very high temperatures, the inner core is solid. It is also enriched in unusual heavy elements including gold, silver, platinum, palladium, and tungsten.
Temperatures reach up to 5,400 degrees Celsius and pressures up to 360 gigapascal. The inner core is about 70% of the Moon's radius and is approximately the same temperature as the surface of the Sun! Now, let's answer some commonly asked questions in case you're looking for quick answers.
Frequently Asked Questions About Earth's Layers
- What is the outer layer of the Earth?
- The outer layer of the Earth is the crust, a hard thin layer that is composed of continental and oceanic crust.
- What are the different parts of the Earth?
- The different parts of the Earth are the crust, mantle, outer core, and inner core.
- How many layers are there in the Earth?
- There are 4 layers in Earth generally speaking. However, it depends how you measure each layer, based on physical or chemical properties.
- What is the depth of the inner core of the Earth?
- The inner core of the earth starts at 5,150 km from the Earth's surface and extends to the center of the Earth.
- What materials make up the inner core?
- The inner core is made up of primarily iron at 80% and nickel, along with trace amounts of heavy metals.
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How deep is the Earth's crust?
- Earth's crust ranges from 5 to 60 kilometers depending on oceanic versus continental crust
- What are the two types of Earth's crust?
- The two types of crust are the dense and thin oceanic crust and less dense and thicker continental crust.
I hope you enjoyed this guide to the layers of the Earth and it sparked a newfound interest in what lies beneath our feet!
