There are a selection of geology tutorials from introductory to advanced levels covering a range of topics which together form a free online textbook.

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Below is the abstract of an example tutorial.

Geological Time

How do we know the age of the Earth? How do we know the age of rocks we find? There are two main ways to date rocks - absolute dating and relative dating. After the rocks have a date assigned to it, they can be put in some sort of order, and the relative dates worked out. This is the geological timescale.

The Ageing Earth

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