Geological travels in Northern Ireland, part IV:


"The Giants Causeway" is the name of this peninsula of land sticking out into the North Sea. Note the people on it for a sense of scale. Admittedly, it doesn't look too impressive from a distance. But when you get closer, an interesting pattern emerges...



The Causeway is made of thousands of columns of basalt. Oriented a few degrees shy of vertical, these columns formed when an ancient lava flow cooled down and contracted. Cracks developed on the top of the flow (the coolest part) and propagated downward, dividing the rock into these uniformly-shaped chunks.





Viewed from above, each column's shape becomes apparent: they are polygonal: mostly 6-sided, but there are also 5-sided, 7-sided, 8-sided, and 9-sided columns. There is a one-pound coin placed on the middle column in this photo to provide a sense of scale.

Casey sits in a natural "throne" made by the columns as they have been weathered by the pounding waves. You can see here that they are not quite vertical on the west side of the Causeway -- but instead are plunging steeply to the west.







On the east side of the Causeway, a tall outcrop of columns shows them plunging steeply in the opposite direction -- to the east. In between the two sides (down the middle) of the Causeway, the columns are approximately vertical. Note also the ~horizontal joints which divided each column into a series of cake-like stacks. You can tell that these joints came later, because they do not continue uniformly across columns (look at the lack of alignment at the bottom of these columns, for instance).












The overall sequence in the events of the formation of the Causeway would look something like this diagram, shown in cross-sectional view.

First, the "Lower Basalts" were eroded, and a valley was carved out.

Second, the "Causeway Basalts" were erupted, filling the valley. Columnar jointing began at the top of the flow and propagated downward.

Third, the "Causeway Basalt" lava had completely solidified, with columnar jointing dividing up the igneous rock into subterranean columns. Note the radial "splay" of columns in the paleo-valley. On the eastern side, they plunge to the east. On the western side, they plunge to the west.

Fourth, erosion attacks the landscape, removing some material. The Causeway itself pokes up above sea level.


Tourists clustered on the tip of the Causeway.

Labels: antrim coast, basalt, geology, giants causeway, northern ireland, structure