Various evidence tells us about the earth's inner structure, although the best evidence comes from the way earthquake waves are transmitted through the planet.
The Earth has a crust at the surface, a mantle below the crust, and a core at the very centre. The crust is less dense than the mantle, and floats on it.
The Earth's crust and the mantle are separated by a division called the Mohorovicic discontinuity, the point where the density of the planet changes.
There is a standard pattern at continental fringes: the continental shelf joins the continental slope that ends in the continental rise near the abyssal plain.
Under very high pressure, many solids such as rock and ice will flow as viscous fluids. They will also bend without breaking under some conditions.
The bending of rocks in the past can be seen in anticlines, synclines monoclines, each structure being formed by compression and other forces.
Isostatic effects in the earth include ice loading in Ice Ages and the later rebound, as the previously loaded rocks are able to rise back up from the mantle.
Mountains are a part of the less dense crust that floats on the more dense mantle. Mountain 'roots' sink deeper into the mantle, a principle called isostasy.
This file is http://members.ozemail.com.au/~macinnis/scifun/splatsisostat.htm, first created on February 16, 2008. Last recorded revision (well I get lazy and forget sometimes!) was on February 16, 2008.