JOHN SHAW / SCIENCE PHOTO LIBRARY JOHN SHAW / SCIENCE PHOTO LIBRARY
Lichened basalt columns at Stykkisholmur, Iceland. Basalt is an extrusive igneous rock, which means it forms from the cooling of lava on the surface. Contraction of the lava as it cools leads to the geometry of the columns, which have either hexagonal or pentagonal cross-sections.
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