http://scign.jpl.nasa.gov/learn/plate1.htm
http://en.wikipedia.org/wiki/Structure_of_the_Earth
http://volcano.und.nodak.edu/vwdocs/vwlessons/lessons/Earths_layers/Earths_layers1.html
http://www.mnh.si.edu/earth/text/4_1_4_0.html
http://www.solarviews.com/eng/earthint.htm
http://kids.yahoo.com/reference/encyclopedia/entry?id=earth-pl