MARTIN BOND / SCIENCE PHOTO LIBRARY MARTIN BOND / SCIENCE PHOTO LIBRARY
Basalt columns. Columns of basalt by a black sand beach. 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. Photographed at Myrdalur, near Vik, Iceland. Iceland lies on the boundary between two tectonic plates (on which the continents are carried), and as such is very geologically active.
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