DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Columnar basalt rock in a quarry. These pentagonal or hexagonal columns of basalt formed when molten rock (magma) from below the Earth's surface rose to the surface during volcanic activity. Rocks of magmatic origin are called igneous rocks. As these igneous rocks formed at the surface, they are called extrusive igneous rocks. Basalt rock has a relatively low silica (silicon dioxide) content of around 50 percent, and a relatively high content of iron and magnesium minerals. Over 90 percent of volcanic rocks are basaltic. The geometric columns form due to contraction on cooling, and make them ideal for use in construction. Photographed in the Eifel region of Germany.
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