Columnar Joints

1. Columnar Joints Giant’s Causeway, Ireland http://www.ivanweb.net/images/Mondo/Irlanda/Giants%20Causeway,06.jpg

2. Giant’s Causeway, Ireland http://www.gaschurman.com/world/europe/northern%20ireland/giant%20causeway/slides/giant%20causeway%2013.jpg

3. Mud cracks illustrate, in 2-d, the process of volume contraction. As the mud dries, its clay minerals contract, eventually pulling away from one another when the contraction stress exceeds the mud strength.

4. In a lava flow, extend the mud crack process into the third dimension, and you have columnar joints

5. - Koloa (rejuvenation series) basalt flow, Kaua‘i In a side view, you don’t see the polygonal pattern as well, but you do see the columnar nature of the columns ~1m

6. Here, on the S. flank of E. Maui volcano, the tops of the columns are exposed in a gully. The surface is not horizontal because the flow itself was emplaced on a slope; the columns are almost always perpendicular to the cooling surface.

7. Kepuni Gulch, East Maui

8. - Wai‘opai gulch, E. Maui

9. - Wai‘opai gulch, E. Maui

10. Onomea, Hawai‘i

11. Here are the tops of some ~2 million year-old columns above Makapu‘u, O‘ahu. Notice that weathering has preferentially attacked the fractures - eventually these will become individual boulders.

12. - Here, in spectacular Wai‘opae gulch, E. Maui, are two generations of lava flows, both with columnar joints. The older flows are ~horizontal so their joints, although poorly-developed, are ~vertical. Then, a later flow flowed down the gully, and cooled in it. Because this flow plastered against the walls of the gully, its cooling surface was at an angle, therefore its cooling joints are also at an angle (perpendicular to the walls of the gully). The next geologic event was cutting of the gully.

13. - Dikes in Waihanau valley, E. Moloka‘i Dikes also show columnar jointing, but because dikes are essentially vertical (and therefore have ~vertical cooling surfaces), their columnar joints are ~horizontal. It is these pre-broken fragments of dense dike rock that were often made into adzes by Polynesians.

14. A dike cutting diagonally across lava flows on Kaua‘i Notice that even though the dike margins are at a strange angle, the columnar joints are still perpendicular to the margins

15. Devil’s tower (Wyoming), a remnant of a thick lava flow or dome, is probably the most famous example of columnar jointed rock in the USA. Photos by Steve Mattox, from: http://volcano.und.edu/vwdocs/volc_images/north_america/devils_tower.html