Earth's Layers

Core:
-spherical mass composed of metallic iron and nickel
-15% of the Earth
-Densest layer
-Outer core: rotating molten iron that generates a magnetic field (most dense)
-Inner core: solid iron and a little nickel

Mantle:
-Surrounds the core
-84% of the total earth
-Mainly solid or soft intermediate composition
-Lithosphere: includes all of crust and upper region of mantle (solid, rigid, cool)
-Asthenosphere: Upper part of mantle under lithosphere (soft, allows lithosphere to move)
-Mesosphere: lower part of mantle (varies in composition)

Crust:
-surrounds the mantle
-thin rigid layers of lighter rocks
-Oceanic: young rocks that are constantly being formed due to cooling of magma, thin, dense (basalt)
-Continental: Older and thicker rocks, less dense (granite)

link to a diagram of the layer of the Earth: http://pubs.usgs.gov/gip/dynamic/graphics/FigS1-1.gif