WILLIAM H. MULLINS / SCIENCE PHOTO LIBRARY WILLIAM H. MULLINS / SCIENCE PHOTO LIBRARY
Imnaha basalts, one of the oldest of the Columbia River Basalt Group, 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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