Planet Earth: An Illustrated History

Formation
Ireland's Giant's Causeway is constructed of 40,000 basalt columns. Most of them are hexagons, but there are also columns with four, five, seven and eight sides, clustering in a honeycomb pattern and thrusting as high as 36 ft. into the air. They were formed following volcanic eruptions around 50-60 million years ago, when molten basaltic lava cooled, then fractured along strict mathematical lines as it hardened, forming straight edges as sharp as those seen in quartz formations. Giant's Causeway was recognized as a World Heritage Site by UNESCO in 1986.

• The Book, "TIME Planet Earth: An Illustrated History"