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The Structure of the Earth
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.
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.
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.
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
Time: 17:51:04 UTC
Time: 17:53:28.76 UTC
Time: 17:56:11.17 UTC
Time: 18:04:06.54 UTC
Time: 18:10:27.46 UTC
Sources and Further Information
- Seimic Waves and Earth's Interior
When you look at a seismogram the wiggles you see are an indication that the ground is being, or was, vibrated by seismic waves. Seismic waves are propagating vibrations that carry energy from the source of the shaking outward in all directions.
- IASPEI standard phase list
The following list of seismic phases was approved by the IASPEI Commission on Seismological Observation and Interpretation (CoSOI) and adopted by IASPEI on July 9th 2003.
- Interpreting Seismograms - A Tutorial for the AS-1 Seismograph
This tutorial is intended as a resource for the interpretation of seismograms recorded by educational seismographs.
- International Registry of Seismograph Stations: station search
This is the standard search page for the International Registry of Seismograph Stations.
- Magnitude 5.8 - VIRGINIA
USGS Earthquake Hazards Program, responsible for monitoring, reporting, and researching earthquakes and earthquake hazards
- USGS CMG InfoBank
InfoBank, KnowledgeBank's field data catalog, is a structured information storage scheme of databases and software that provide organized access to USGS Coastal and Marine data and metadata.