Giant’s Causeway

Giant’s Causeway is the most popular geological tourist attraction in the island of Ireland and it was one of the places I definitely wanted to see. I have to say that it really is worth it.

Columnar basalt in itself is not an uncommon phenomenon but here it is especially well exposed and there is actually much more geologically interesting stuff to see than just the small cape named the Giant’s Causeway.

The weather was just awful — it was cold, windy, and rained all the time. I guess it was bad even according to Irish standards because there were surprisingly small number of visitors which was good.

http://picasaweb.google.com/107509377372007544953/Ireland#5755679505310466834
Reddish layer of laterite (Port na Spaniagh laterite) underlying the Causeway basalts. Laterite is weathered basalt. In this case it indicates that there is a time gap between the formations of basalt layers.

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Closeup of laterite. Laterite is red because of oxidized iron. Iron is plentiful in mafic rocks like basalt.
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Spheroidally weathered basalt.
http://picasaweb.google.com/107509377372007544953/Ireland#5755679641366670258
This seastack is known as Camel’s back. I guess it is a Bactrian camel — it has two humps. Geologically it is a dike cutting through the basalts underlying the Causeway.
http://picasaweb.google.com/107509377372007544953/Ireland#5755679712907545954
There is even littlebit of sand. Giant’s Causeway is a small cape in the background.
http://picasaweb.google.com/107509377372007544953/Ireland#5755679723704864450
It seems to consist mollusk shells and basalt fragments.
http://picasaweb.google.com/107509377372007544953/Ireland#5755680149443967634
The columns are the real Giant’s Causeway, but the boulders resting on top of them probably represent the upper curvi-columnar (known also as entablature) part of the Causeway basalt formation.
http://picasaweb.google.com/107509377372007544953/Ireland#5755679858067423794
A boulder of curvi- or pseudo-columnar basalt. It is not as regularly jointed as the main sequence because of faster cooling. This part of the sequence was closer to the surface.

Giant’s Causeway. It really is amazing even if you are prepared to see something like this.

http://picasaweb.google.com/107509377372007544953/Ireland#5755680201708222018
The basalt forming the Giants’s Causeway was originally a lava lake filling a topographical depression.
http://picasaweb.google.com/107509377372007544953/Ireland#5755680303316870658
Columns are the result of contraction — solid rock is denser and takes less space than liquid lava.
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Lava layer loses heat through their bottom and top surfaces where the shinkage joints start to develop.
http://picasaweb.google.com/107509377372007544953/Ireland#5755680064970623618
The columns have a variable number (3-7) of sides.
http://picasaweb.google.com/107509377372007544953/Ireland#5755679835071347426
A sideview of the Giant’s Causeway. It can be seen that because of contraction there are also horizontal joints (ball and socket joints) separating each column into many shorter colums.
http://picasaweb.google.com/107509377372007544953/Ireland#5755679931225861122
This outcrop known as the Giant’s Organ is located few hundred meters away from the Causeway. I guess it is not reached by many visitors of the Causeway, but geologically it is very interesting outcrop because here one can see the whole sequence of regular columns and curvi-columnar section directly above it. The term “entablature”, which is borrowed from architecture, was first used to describe the upper section of this outcrop by Sergei Tomkeieff who is perhaps well-known to petrologists because of his Dictionary of Petrology. He called the regular columns below “a colonnade”.
http://picasaweb.google.com/107509377372007544953/Ireland#5755679987053228162
Unfortunately it rained so heavily that I couldn’t take a clear picture of the Organ but hopefully you can get an idea how it looks. Here you can see that the sequence can be separated into three parts The upper layer above the entablature was named “pseudo-columnar” by Tomkeieff.