US Geological Survey
Usgs: Unanswered Questions
Discover how convection currents in the mantle are the driving force of continental drift. The reading also briefly addresses the scientists who laid the groundwork for this idea.
National Earth Science Teachers Association
Windows to the Universe: Volcano Formation
Explanation of the factors that result in the formation of volcanoes, some basic but helpful animations and photographs.
Other
University of Delaware: Plate Tectonics
This site is a very simple overview of plate tectonics. It includes a map of all the major plates and a diagram of the layers of the earth.