SIMON FRASER / SCIENCE PHOTO LIBRARY SIMON FRASER / SCIENCE PHOTO LIBRARY
Columnar basalt pavement in south Iceland. The close-packed hexagonal prisms consist of basalt, a fine-grained extrusive rock found in most lava flows. The joints arise when a pattern of tensional forces is set up in a layer of lava as it contracts on cooling. The forces tend to pull open a series of hexagonal fractures. Famous examples of columnar jointing may be seen at Giant's Causeway in Antrim and Fingal's Cave on the island of Staffa in the Hebrides. Iceland's location on the border between the European and North American tectonic plates accounts for the intense seismic and volcanic activity found there.
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