Geology & landscape
Imagine yourself standing on the causeway and picture a gently rolling landscape with clumps of trees above a base of chalk rock.
About 60 million years ago, the first in a series of volcanic activity had a big impact on what we now call the Giants Causeway. Lava at 1100 degrees Celsius obliterated the vegetation and hardened on top of the limestone base to form the first in a series of layers of basalt. If you look east from of the Causeway, towards the chimneys, you can see these lower basalts as five dark bands (starting from the sea and working upwards).
As the eruptions which formed the Lower Basalts got less and less frequent, there was a longer dormant period (of at least a hundred thousand years). A warm and wet climate wore down the top of the basalt; plants grew and helped form a deep red soil. You can see this thick red layer, called laterite in the cliffs below the chimneys and elsewhere around the causeway site - like a jam layer in a huge cake!
After that, a river flowed through the area digging out a valley. Then, more lave poured over the landscape - this time in a thicker layer, filling the bowl of the river valley. These Causeway basalts cooled in a giant pool in the valley hollow. The chemical properties of the lava and the way in which it cooled formed the special columns you can stand on today.
In our causeway lava pool, the hardening surface cooled quickly. It cracked and fizzled with each wash of fresh water. Meanwhile, at the slowly cooling bottom of the pool a much more even pattern was forming, starting with 3 pronged cracks. The cracks inched out in lines to form patterns of shapes - most with five or six sides. As the lava cooled slowly, the cracks moved upwards through the solidifying rock - forming the columns of the Giants Causeway.
The basalts containing the causeway columns were then covered by further layers of lava, which like the lower basalts formed much less regular shapes. Lastly another layer of laterite was formed with further basalts on top (these last two layers - the upper-basaltic layer and the upper basalts have disappeared at the coast but can be seen inland).