The Giant's Causeway
The Giant's Causeway in Northern Ireland is made of 40,000 interlocking basalt columns resulting from an ancient volcanic eruption. Most of the columns are hexagonal, although there are some with four, five, seven and eight sides. The tallest one is about 12 metres (36 feet) high, and the solidified lava in the cliffs is 28 metres thick in places. The columns were created because when the lava cooled, contraction occurred. While contraction in the vertical direction reduced the flow thickness (without fracturing), horizontal contraction could only be accommodated by cracking throughout the flow. The extensive fracture network produced the distinctive columns seen today.