- http://www.space.com/19175-how-was-earth-formed.html
- http://oceanservice.noaa.gov/facts/why_oceans.html
- http://wonderopolis.org/wonder/how-are-mountains-made/
- http://education.nationalgeographic.org/encyclopedia/landform/
- http://www.eoearth.org/view/article/154151/
- http://www.physics.org/article-questions.asp?id=64