Seismic waves are vibrations in the center of the Earth. In

the Earth we have a lot of layers: the crust, the mantle, the

inner core and the outer core. The core is made of a solid

inner core made mostly of iron and nickel. This is also

surrounded by a liquid outer core made mostly of iron and

nickel. The diameter of the core is estimated to be 7000 km,

compared with a 12,700 km diameter for the center of the

planet. When the Earth was melting, gravity acted to

concentrate more dense material near the center and less

dense material nearer the surface.

The Asthenosphere

The asthenosphere is composed of plastic flowing

rock, it is 200 km thick.

The Mesosphere