Crust? Mantle? Core?

Move over, adult world, here come the 8^th graders. They are a LOT smarter than WE were in the 8^th grade.

Q: Hello Mr./Mrs. Geologist, I am sending you this email because I am wondering about something. How can you be sure about the dimensions of say: the mantle, or the inner core? I am an 8th grader and I would be very grateful if I was sent a response to this question?

- Oliver McE

A: The short answer is seismology.

The long answer is that when medium to large earthquakes occur around the Earth, they send a substantial amount of wave energy through the body of the Earth. Seismologists have learned how to use a large and sophisticated global network of seismometers to time the arrivals of many different kinds of waves – and analyze their characteristic signatures. Some of these waves (specifically S-waves or shear waves) cannot propagate through liquids. At the same time, P-waves (pressure waves, similar but lower frequency than the sounds you hear with your ears) will refract, or bend, as they cross boundaries with different physical properties. Refraction is what light does as it crosses the air-water boundary in a pool - so fish in a stream look like they are shallower than they actually are.

By carefully timing the arrivals of the different types of waves, seismologists can discern different physical properties at different depths. Among these, the Crust-Mantle, Mantle-Core, and Outer Core-Inner Core boundaries stand out very clearly.

The analysis is much more complicated than this, or course, but it's pretty obvious when S-waves can be seen up to a certain distance away around the curve of the Earth from the earthquake source - but not beyond. This means that a solid-liquid boundary has been crossed.

Here are some helpful links that will let you visualize what I'm talking about:

http://en.wikipedia.org/wiki/Structure_of_the_Earth

This is a good place to start.

http://www.bing.com/images/search?q=earth%27s+structure+layers&qpvt=earth%27s+structure+layers&qpvt=earth%27s+structure+layers&FORM=IGRE

I got this rather complicated link by doing a search for "earth's structure layers".

http://www.bing.com/images/search?q=seismic%20waves%20diagram&qs=IM&form=QBIR&pq=seismic%20waves&sc=8-13&sp=2&sk=IM1

I got this by doing a search for "seismic waves diagram".

There are other ways that we geophysicists can "see" different structures inside the Earth, including

2. torsional oscillations of the Earth in response to very large earthquakes,

3. study of the Earth's complex dipolar and changing secular magnetic field,

4. ground-based and satellite-based gravity and gravity gradient measurements,

5. mathematical modeling, and

6. high-pressure, high-temperature laboratory experiments.

The combination of  all these approaches taken together has allowed geophysicists to actually do something called "tomography" - sort of like a CT-scan or an MRI scan of the human body that doctors use - to actually see complex details beyond just simple layering in the deep Earth. Taken together, we are becoming increasingly confident of things like where the down-going oceanic slabs beneath the continents are, and what the minerals – and metals – at different depth might be.