How do we know the Earth's core is solid?
I read somewhere that it was discovered that the inner core of the earth was solid because shear waves had passed through it and they can't pass through a liquid. Since the inner core is solid and the outer core is liquid, how did the shear waves pass through the outer liquid core in order to get to the detectors?
Dave - There's two different main kinds of waves:
One is a pressure wave. It's like a sound wave, whereby the movement, the wobble, is in the same direction as the wave is travelling.
A shear wave is like a wave on a string, so it's at a right angle to the way the wave is travelling.
In a liquid you can't get any shear waves except on the surface. So, in the bulk of a liquid - in the body of a liquid - all the waves must be pressure waves. In fact, that's true in a fluid of any kind, which includes gases as well.
But at the surface of a liquid you can get shear waves and you can get shear waves created when a pressure wave hits a surface.
So, basically, the way they found out that the centre of the earth is solid is that when waves created by earthquakes on the surface go down, they hit the liquid outer core and then, when they go from a liquid outer core into the solid inner core, you get a extra wave going off at right angles in a strange direction. This would only happen because if it's a shear wave.
Then you see the results of that coming out through the liquid outer core and bouncing off to an earthquake station on the wrong side of the planet where it shouldn't be.
The only way, you can explain those waves getting to the earthquake station there is if the centre of the earth is solid and there's a shear wave going through it.