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# How did scientists accurately work out exactly what the Earth's internal structure is like, without ever (I'm guessing) drilling into Earth's core?

Question #91283. Asked by --simone--. (Jan 18 08 11:15 PM)

MonkeyOnALeash

There is no "accuracy". It is all theoretical with a lean toward "High Probability".

http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=0521849594

 Jan 19 08, 12:11 AM
BRY2K

Recordings of seismic waves from earthquakes gave the first clue. Seismic waves will bend and reflect at the interfaces between different materials, just like the prism below refracts and scatters light waves at its faces.

Seismologists noticed that records from an earthquake made around the world changed radically once the event was more than a certain distance away, about 105 degrees in terms of the angle between the earthquake and the seismograph as measured at the center of the earth. After 105 degrees the direct P- and S- waves disappeared almost completely, but slow surface waves and waves taking other paths would arrive from over the horizon. The area beyond 105 degrees distance forms a shadow zone. At larger distances, some P waves that travel through the liquid core (path K on the figure above) would arrive, but still no S waves. The Earth has to have a molten, fluid core to explain the lack of S waves in the shadow zone, and the bending of P waves to form their shadow zone.

You can get a rough estimate of the size of the Earth's core by simply assuming that the last S wave, before the shadow zone starts at 105 degrees, travels in a straight line. Knowing that the Earth has a radius of about 6370 km, you have a right triangle where the cosine of half of 105 degrees equals the radius of the core divided by the radius of the earth.

Combine this research with the fields of magnetism and geophysical science and theories of plate tectonics and you get a pretty compelling case.

http://www.seismo.unr.edu/ftp/pub/louie/class/100/interior.html

 Jan 19 08, 5:28 AM
star_gazer

Very little of the inner parts of the earth can be directly investigated. The deepest mine in the world (a gold mine in the Witwatersrand area in South Africa) descends 3.5 km into the lithosphere.2 The deepest that humans have ever drilled into the earth is on the Kola Peninsula in Russia, where drill core was retrieved from 12.26 km below the surface.3 From that point on to the centre of the planet, some 6,365 km, is all ‘unknown territory’. All we can do is to infer what could be down there from the limited data we have.

http://www.creationontheweb.com/content/view/5199/

 Jan 19 08, 5:43 AM
queproblema

When I was a kid, Project Mohole was going to reveal the mantle by drilling through the crust. It fizzled.
http://www.nas.edu/history/mohole/

 Jan 19 08, 12:52 PM
--simone--

Thanks people, very interesting. xx

 Jan 20 08, 5:24 AM

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