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AP Environmental Science by University of California College Prep (CC BY)
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Earthquakes by OpenLearn (CC BY-NC-SA)
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An Introduction to Geology by Johnson, Affolter, Inkenbrandt, and Mosher (CC BY-NC-SA)
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Plate Tectonics by OpenLearn (CC BY-NC-SA)
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NIH Curriculum Supplement Series by National Institutes of Health (PD)

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