There are a selection of geology tutorials from introductory to advanced levels covering a range of topics which together form a free online textbook.
If you enjoy reading our tutorials, please let other people know about them. If you think you can add something to the tutorial, please feel free to comment on them via the forums (you'll need to register to do this) or even better, you can write one yourself. Simply create an account if you don't have one and click "Create content"!
Below is the abstract of an example tutorial.