The ck-12 Foundation offers several free digital textbooks – flexbooks – in Earth Science and geology for students in middle school and high school. You can read the textbooks online or download them to your ereader, with the Flexbook app for iPhone/iPad or Android device.
In addition to Flexbooks, cK-!2 has PLIX modules – Play, Learn, Interact, Explore – where you can narrow in on specific concepts, such as mechanical weathering, to explore them more thoroughly. Concepts can be displayed in list format or within a concept map. PLIX modules also have apps for iPhone/iPad or Android device.
Finally, the site has study guides on many of the earth science topics, such as metamorphic rocks. The study guides include interactive quizzes, activities and projects, and summarizations of key points.
There are online forums for students and teachers and study groups. You can participate in these, and save your progress through the learning materials, with free registration on the site. Teachers also have the ability to customize materials for their students.
So hands-down, this is one of the premier web resources for K-12 Earth Sciences.
