Giant's Causeway

Photograph by Jim Richardson

Stepping stones into the sea, these basaltic columns of the Giant's Causeway in Northern Ireland formed from volcanic activity some 50 million to 60 million years ago. Rock formed from molten matter is known as igneous, and geologists classify igneous rock by its texture. You would need a microscope to see the grains in the 40,000-some basalt columns that form the Giant's Causeway.