Geological travels in Northern Ireland, Part VI:

The word "joint" in geology refers to any fracture in a rock unit along which movement has NOT occurred. (If movement DOES occur along a fracture, that makes it a fault.)

The Giant's Causeway in County Antrim, Northern Ireland, shows jointing of a particular pattern: the intersection of the joints divide the rock into column-shaped pieces, shaped roughly like an un-sharpened pencil.

This is an image of two of the "Causeway basalt" layers exposed in a gorge east of the Giant's Causeway itself. Note their difference in size: slower cooling produces larger columns. Faster cooling produces smaller columns. Therefore the lower flow cooled off more rapidly than the upper flow.



Lava, when hot, takes up more volume than cold igneous rock. As it cools, the solidifying lava contracts. Because the whole volume of rock is contracting, evenly-spaced centers of contraction develop. Cracks open up to accomodate that contraction. This makes a honeycomb-style pattern, because 3 crack orientations is the minimum number necessary to allow contraction in every direction. These three orientations meet at an average angle of 120º.

The same phenomenon can be seen at Devils Tower, Wyoming.

The weird columnar jointing patterns at the Giants Causeway were used on the cover of Led Zeppelin's album Houses of the Holy (1973). While I was there, I thought about re-creating the album cover with geologists (clothed!) in the same positions as these kids, but I forgot to bring along the album as a reference. Tragic, isn't it?

The overall loss of volume of the (hot versus cold) rock can be estimated with a photograph like this. Divvie the photo into equal units of area, and then count up how many are solid rock and how many are empty air. About 1% shrinkage is seen here -- more than in other places I've seen columnar jointing.

Once formed, these joints allow water to penetrate into the lava flow. Water encourages both physical and chemical weathering of the basalt, enlarging the size of the fractures. Water, being the universal solvent, helps catalyze many chemical reactions. Basalt is a rock that is stable under certain conditions in the Earth's interior, but it is not stable at the Earth's surface, where conditions of temperature, pressure, and humidity encourage it to break down. These break-down chemical reactions start on the surface of the column and work their way inward, like a thousand mice nibbling on the exterior of a large block of cheese. Physical weathering takes place when the water freezes. When water becomes ice, it expands in volume by about 9%. This "wedges" open the cracks even more. Once widened, they can accomodate more liquid water, which can then freeze again, widening the cracks further.
The end result of these physical and chemical weathering processes is to break down the rock, from the outside in. Rotten rock sloughs off in sheets, exposing fresh rock from the interior for weathering to attack. This produces an overall "onion skin" effect. An original polygonal chunk of rock become spheroidal over time, as weathering reduces it in size and volume. Pound coin for scale.

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