Question

How far do the continents move?

and how have continents always been moving?

Answer 1

The continents move only a few milimeters a century. They move because they're on tectonic plates.

Check these sites for more information.

http://www.geocraft.com/WVFossils/continents.html

http://science.nasa.gov/headlines/y2000/ast06oct_1.htm