WILLIAM H. MULLINS / SCIENCE PHOTO LIBRARY WILLIAM H. MULLINS / SCIENCE PHOTO LIBRARY
Columnar basalt formation near Imnaha, Oregon. 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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