11/25/2002: Etna dig

[juju]

All this talk of volcanoes reminds me of a Geology class I had a couple semesters ago. The teacher touched on something I thought was really cool: How do scientists <b>know</b> that the Earth's core is liquid? After all, they can't get to the center of the Earth. Are they just guessing or something?

Well, of the various kinds of seismic waves, there are two main types that travel outward in all directions from the center of an earthquake: Primary waves and secondary waves. Because of the way they vibrate, primary waves can travel through liquid. Secondary waves cannot. So, scientists set up seismograph stations in several different countries all over the globe, and wait for an earthquake to occur. When one does, they see which stations received which kinds of waves from the 'quake. The results look something like this:



As you can see, the primary waves from an earthquake can be detected on the other side of the earth, but the secondary waves can't. They never get through. Therefore, the earth must have a liquid core.

The above linked site also talks about the distortion that's found in the Primary wave results (called 'P waves' here):
<blockquote> In contrast, the P waves are detected on the opposite side of the Earth as the focus. A shadow zone from 103° to 142° does exist from P waves, though. (Plummer and McGeary 381, 382) Since waves are detected, then not, then reappear again, something inside the Earth must be bending the P waves and bending them towards the normal. From this evidence using waves, we can tell that part of the core is liquid (S wave shadow) and part (the inner part) must be solid with a different density than the rest of the surrounding material (P wave shadow zone due to refraction). In actuality, the inner core is thought to be made of solid iron and nickel. </blockquote>

Isn't that cool?