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The Structure of the Earth

Updated on August 12, 2012
Digging a hole to the center of the Earth isn't the only way to find out what's down there.
Digging a hole to the center of the Earth isn't the only way to find out what's down there. | Source

Despite more than a century of research into the structure and composition of the Earth's interior, a dedicated group of Hollow Earth enthusiasts and other assorted science deniers continue to assert that we do not know what lies beneath our feet. The structure of the inner Earth is a mystery, they claim, because we have never actually seen it.

This is, of course, a ridiculously flawed argument. We don't need to tunnel to the Earth's core to confirm its existence. We can see quite clearly into the depths of the Earth using seismic waves generated by earthquakes. In much the same way that reflecting ultrasound waves in the womb can be constructed into a picture that tells us the gender of a baby, seismologists can use seismic waves to construct a picture of the interior of the Earth.

Illustration of the Earth's interior
Illustration of the Earth's interior | Source

The Earth's outermost layer, the crust, is composed primarily of silicate rocks - minerals containing silicon and oxygen. The oceanic crust, 5-10 km thick, is mainly made of magnesium silicates such as basalt. The much thicker continental crust is composed mostly of aluminum silicates such as granite. The continental crust can be as many as 70 km thick.

The crust is broken into tectonic plates which move very slowly, spreading apart at mid-ocean ridges and overlapping at subduction zones. Earthquakes are usually caused by these movements as the boundaries of the plates bind and release. This gradual motion of the crust is due to convection currents in the next layer, the mantle.

The mantle is the thickest of Earth's layers, extending from the crust to 2890 km below the surface. The boundary between the crust and mantle is known as the Mohorovičić discontinuity or Moho. This boundary is marked by a change in the velocity of earthquake waves as they pass through it.

Composed of iron and magnesium-rich silicate rocks, the mantle itself is divided into an upper and lower layer. Though both layers are solid rock, the temperature and pressure of the upper mantle makes it just viscous enough to produce very slow convection currents that drive plate tectonics in the crust above.

The Earth's core is composed of two iron layers - a 2,200-kilometer thick liquid outer core and a solid inner core 2,400 km in diameter. The solid inner core has been found to be rotating slightly faster than the rest of the planet, generating circular currents in the liquid outer core that produce our planet's magnetic field. The temperature of the inner core is estimated at 5,700K or 9,800 degrees Fahrenheit.

Illustration of the seismic wave shadow zone of the Earth. P-waves do not arrive directly between 105 and 143 degrees from the epicenter. S-waves do not arrive beyond 105 degrees.
Illustration of the seismic wave shadow zone of the Earth. P-waves do not arrive directly between 105 and 143 degrees from the epicenter. S-waves do not arrive beyond 105 degrees. | Source

How Do We Know What's Inside the Earth?

Knowing what's beneath our feet would seem like a lot guesswork. Nobody has actually been to the core, and the deepest holes we've drilled into the crust have only gone a few kilometers down.

However, we are able to see into the Earth's interior using the seismic waves generated by earthquakes. The speed of these waves varies depending on the type of materials they pass through. Thus, by measuring the time it takes for different types of seismic waves to arrive at monitoring stations around the world, seismologists can construct a picture of the different layers of the Earth and their composition.

There are four main types of seismic waves, and these are divided into body and surface waves. The two types of body waves, which travel deep into the Earth, are:

  • P-waves: These Primary (or Pressure) waves travel via compression of the molecules of the substance they are passing through. This is the same way in which sound waves travel. P-waves are the fastest waves, and can pass through solids, liquids, and gases - albeit at very different speeds. P-waves pass through rock at about 5 kilometers per second, liquid at 1 km per second, and air at 330 meters per second.
  • S-waves: Secondary (or Shear) waves travel in a side-to-side motion, much like the way a wave travels in a whipped rope. These waves only travel through solids, not liquids and gasses. S-waves travel about 60% the speed of P-waves, so seismologists can use the difference in arrival time to estimate the distance from the monitoring station an earthquake occurred.

The other two types of earthquake waves are surface waves. While these are not useful for mapping Earth's interior, they are worth noting as they cause most of the damage from an earthquake:

  • Love waves: Named for mathematician Augustus E. H. Love, these are a side-to-side elastic motion of the Earth's surface following an earthquake. These waves dissipate with depth into the crust.
  • Rayleigh waves: These waves are named for John William Strutt, the same Lord Rayleigh who discovered the principle of atmospheric scattering that makes the sky blue. Rayleigh waves have a rolling circular motion, a combination of compression and shear similar to the way water waves travel.

Knowing how these body waves travel through different materials allows seismologists to understand the Earth's internal structure. It is due to the way P and S waves refract that scientists discovered that Earth's outer core is liquid.

When P-waves encounter the boundary between a solid and liquid, they are bent, or refracted, and slow down considerably. When S-waves encounter a solid-liquid boundary, they either die out or convert to P-waves, depending on the incoming angle. These refractions produce a shadow zone on the side of the Earth opposite the earthquake.

Initial S-waves disappear entirely more than 105 degrees away from the quake's epicenter. Initial P-waves disappear at 105 degrees, then reappear 143 degrees from the epicenter, slowed a bit by their passage through the liquid core. The only P-waves that are seen in the shadow zone are those reflected from the core or other boundary regions underground, presenting a distinctive pattern in seismograph readings different from that of primary P-waves.

Predicted P-Wave travel times for the August 23, 2011 earthquake in central Virginia.
Predicted P-Wave travel times for the August 23, 2011 earthquake in central Virginia. | Source

Testing the Theory

At 1:51 pm local time on August 23, 2011, central Virginia was rocked by a 5.8 magnitude earthquake, centered about 40 miles northwest of Richmond. The quake, which was felt up and down the eastern seaboard of the United States, occurred near an inactive fault line - the site of an ancient collision of continents dating back to the formation of Pangaea around 300 million years ago.

The seismic waves from this earthquake quickly spread around the world at many times the speed of sound. From the moment the earthquake struck, the models of modern seismology predicted not only when the waves would reach monitoring stations across the globe, but how they would get there - directly through the upper mantle, diffracting deep in the mantle, or bouncing off the core. One by one, seismometers around the world recorded the earthquake at or within a few seconds of the time predicted.

Seismometers near the epicenter recorded Pn waves, pressure waves bottoming out in the upper mantle, in the first four minutes after the quake. Over the following nine minutes, seismometers up to 96 degrees away recorded P waves traveling through the mantle. Then, five minutes later, PKPdf waves - seismic waves reflecting off the inner core - were recorded in and around the shadow zone for the quake.

On average, four earthquakes the magnitude 5 and larger strike somewhere in the world every day. Each one represents a test of the model of the inner Earth geologists and seismologists have constructed over the past century. So far, the prevailing model - with a solid mantle, liquid outer core, and solid inner core - has passed every time.

Magnitude 5.8 - VIRGINIA, August 23, 2011

show route and directions
A markerEpicenter -
get directions

Time: 17:51:04 UTC

B markerBRAL - Brewton, AL, USA -
get directions

Time: 17:53:28.76 UTC

C markerANMO - Albuquerque, NM, USA -
get directions

Time: 17:56:11.17 UTC

D markerBORG - Borgarnes, Iceland -
get directions

Time: 17:58:59.81

E markerAGG - Agios Georgios, Greece -
get directions

Time: 18:02:41.64

F markerASAJ - Asahikawa, Kamikawa,Japan -
get directions

Time: 18:04:06.54 UTC

G markerKULM - Kulim, Malaysia -
get directions

Time: 18:10:27.46 UTC

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    • Teresa Coppens profile image

      Teresa Coppens 5 years ago from Ontario, Canada

      Fantastic hub Scott. I learned a lot from this. I am aware of the structure of inner earth but was not aware about exactly how the earth's layers were determined. Fantastic job and so glad I can share your reality!!!

    • watergeek profile image

      watergeek 5 years ago from Pasadena CA

      Well, this is interesting information Scott. I'd always thought the inner core was liquid metal. How can it be solid when it's at such a high temperature? Is that because it's compressed?

    • artblack01 profile image

      artblack01 5 years ago from New Mexico

      Talk about a bat shit crazy person.

    • Lady Guinevere profile image

      Debra Allen 5 years ago from West By God

      Oh I know very well what a "Theory" is dear heart. You have just proved tome that you won't or can't read anything the will prove you wrong. So be it and live in YOUR fantasy world. How about reading some Science Daily.

      So you aren't in MY reality if I don't "pretend" that you exist.

    • scottcgruber profile image
      Author

      scottcgruber 5 years ago from USA

      "How about worm holes and other such scientific theories that are just that theories. I have never seen anything like those but they say that they exist straight out and want everyone to believe them just because science says so."

      I don't know of any scientists who are insisting that parallel universes and wormholes exist. They have been proposed as hypothetically and mathematically possible given the known laws of physics, but no sane physicists are saying that these are definitely real.

      You seem to be having some difficulty with the word "theory." A theory in science is a model that explains observed facts that has been demonstrated valid through repeated experimentation. A hypothesis is an untested idea. These two words are not interchangeable.

    • Lady Guinevere profile image

      Debra Allen 5 years ago from West By God

      art how nice of you to say such a thing. Now do you hold true to the theory of folding space and time and parallel universes. How about worm holes and other such scientific theories that are just that theories. I have never seen anything like those but they say that they exist straight out and want everyone to believe them just because science says so.

      Now what did Gallileo supposed about the earth that almost got him killed? Was that fantasy in his time?

    • artblack01 profile image

      artblack01 5 years ago from New Mexico

      Lady Guinevere, you are in such a fantasy world I am not even gonna justify any sort of response.

    • scottcgruber profile image
      Author

      scottcgruber 5 years ago from USA

      What you are describing is not separate realities. Merely separate perspectives on the one reality we all share. Your experiences are part of your perspective. Not a separate reality.

      The proverbial bear does its proverbial thing in the woods whether we're watching it or not.

    • Lady Guinevere profile image

      Debra Allen 5 years ago from West By God

      Thank you Scott. Do you see what I see where I am and does a bear poop in the woods if you don't see the bear in the first place? You can't experience all the I experience and you can take that to the bank. Just as I am en entitled to my opinion you are not entitled to my experiences. If I leave you and never speak to you then you do not exist in my world--my reality and not yours and they are different.

    • scottcgruber profile image
      Author

      scottcgruber 5 years ago from USA

      No, sorry. Reality doesn't work that way. There is one reality, by definition. Individuals perceive it differently, interpret it differently, and have different opinions about aspects of it, but there is still one objective reality. Alternate reality only exists in fiction.

      You're entitled to your own opinions, but not your own facts.

    • Lady Guinevere profile image

      Debra Allen 5 years ago from West By God

      Explain the Earth's Black Hole then. It just gives us more food to throw back at you art. LOL Reality is subjective and it is individual. I don't know that you exist because I can't see you, hear you or speak to you in reality. You are just a product of the etherial space on some machine.

    • artblack01 profile image

      artblack01 5 years ago from New Mexico

      Awesome hub. Now perhaps those Hollow Earth idiots will shut up.