Located on the Northern coast of North Ireland, in County Antrim, the Giant’s Causeway looks like a huge walkway of very tall and hexagonal steppingstones. According to legend, giants used this strange geological feature to cross over from Scotland. (By the way, a “causeway” is a road that runs across or alongside a body of water.)
But what is this strange place, and how did it form? It’s a highly unusual basalt formation that developed over three major periods of activity, according to geologists. The first started during the Tertiary Period about 60 million years ago, when this piece of land was located farther south and much closer to the land mass we now call North America — before plate tectonics moved it up to the North Sea.
As the Earth’s crust shifted and changed, magma flowed up from deep within. Magma is a mix of molten rock, solids, and volatile compounds when it’s below the earth’s surface. When it hits air — like when a volcano erupts — we call it lava, and when this magma turned into lava it cooled and formed basalt.
A period of calm followed, lasting hundreds of thousands of years. The next time lava spewed forth on this spot, it formed a big pool. As the newer lava slowly cooled, highly regular cracks appeared, forming the hexagonal shapes we see today. Similar results can be seen at locations in Scotland (such as Fingal’s Cave in Staffa) and elsewhere in Ireland.
A third lava event covered these shapes up, but after millions of years, erosion caused by wind and water laid bare the unusual structures. They’re about 18 inches wide and almost all are hexagonal, or six-sided. It’s estimated that we can see about 40,000 of these hexagons.
Today, the Giant’s Causeway is not only a major tourist attraction and UNESCO World Heritage site, it helps geologists better understand our planet’s geological history.