DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Slate metamorphic rock. Slate is formed when a fine-grained sedimentary rock, such as mudstone, is subjected to high temperatures and pressures within the Earth's crust. This changes its crystal structure, and it forms a grey rock that can be split into thin, flat sheets. This outcrop, in Brittany, France, measures about 7 metres across.
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