Posted by: Alan Pitts | September 21, 2011

Giant’s Causeway

So of course I wasn’t going to write about primary igneous structures and Ireland with out covering the Giant’s Causeway.

This is probably Ireland’s most famous geologic feature and a place I’ve wanted to visit ever since I was just a wee intro student.

These columns, located in Northern Ireland, are part of a cenozoic aged tholeittic basalt unit which erupted on the surface about 60 million years ago.

Photo by Aaron B.

Here are some of my fellow field students, probably discussing something like how they were going to pull off recreating a man-sized version of the famous Led Zeppelin shot from the album cover to Houses of the Holy. ( no, really)

These columns form as the basalt cools and contracts creating the hexagonal shape. I wrote about some columnar basalt in the Blue Ridge Province of Virginia last spring, which are much older. But of course they really cant compare to the Giant’s Causeway.

Most of these have 5 or 6 sides, although I have been told there are 7 or even 8-siders to be found.  I spotted only a few 7-siders, but nothing greater.

Like this one.

And this one.

Here is a view of the columns from the side along with myself proudly displaying my shirt from Citizen Chain, my good friends and the very best bike shop in San Francisco, maybe even the world.

Another student, making his way across the columns.

Here is another view of a column that is pretty wavy.

Maybe this is something that can be explained by weathering, or maybe is the result of a varied cooling front.

For more on cooling fronts and such, here is a link to my former structural geology professor Callan Bentley’s great post on cooling fronts and column formation.

Here is one more shot of Ireland’s iconic igneous structures.

Photo by Aaron B.



  1. wow-nice photos! i am interested to know what causes the horizontal fractures that make such an interesting staircase to walk on (i.e. 3rd photo) – they are almost perfectly parallel to each other. Also, many of those have sagging middles taht pool water- is this weathering (perhaps from wave action?) or a by product of the cooling as well?

    • Hi ! And thanks for the comment !

      Great questions. Im actually not totally on sure about both of the features you asked about. First the horizontal fractures. When I saw these in person as a field student my first inclination was arrest lines. But I dont think that is correct, these are fractures, and different from the vertical fractures because they dont all line up. I’ve read about these described as “ladder fractures” and are possibly a result of thermal expansion/contraction on the vertical scale instead of horizontal. The little pools of water you mention are also interesting because there are also convex up surfaces all over the place. Also after a little reading I think these little domes and basins are also related to the ladder fractures.
      Unfortunately we didn’t receive a lot of instruction at this location, we were basically turned loose like a bunch of kids at the McDonalds play house. So Im not all together sure. Im thinking that I will write a blog post in the future to tackle these problems and hope to explain them graphically. Or Perhaps someone else with a little more experience in this area will comment and provide a better answer.

  2. Gorgeous pics, thanks for sharing! Someday I’ll get there… someday!

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