How do scientists know each layer of the earth?

Question

how do scientists determine each layer of the earth

Answer 1

they have handy dandy posters they look at and say "oh! thats what it looks like"
http://cache.eb.com/eb/image?id=73583&rendTypeId=35

Answer 2

When there's a earthquake somewhere in the world, they can study measurements of the vibrations it produced on recording instruments in other places around the world. From this data, they can determine roughly how dense was the material deep in the earth that the shock waves passed through to reach the distant detection devices. This gives them an indication of the earth's characteristics as a function of depth.

They can also study rocks that have been expelled by volcanism, exposed by erosion, or thrust up by continental collisions and the formation of mountains to see layers not normally exposed elsewhere.

Answer 3

They go to volcanos, lava flows, etc and take specific measurements on the lava... they then gauge the chemistry of the material and compare/contrast all over the world to pretty much theorize the composition of each layer depending on temperature and chemistry...