SIMON FRASER / SCIENCE PHOTO LIBRARY SIMON FRASER / SCIENCE PHOTO LIBRARY
Svartifoss waterfall in Skaftafell National Park, Iceland. Notice that the rock exhibits columnar jointing; those 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. Iceland's location on the border between the European and North American tectonic plates accounts for the intense seismic and volcanic activity found there. One result of this activity is the vast flows of basalt, thousands of metres thick, which the island boasts.
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