JOHN SHAW / SCIENCE PHOTO LIBRARY JOHN SHAW / SCIENCE PHOTO LIBRARY
Basalt columns, 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. The black sand is formed by the erosion of the columns by the sea.
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