ColumnarJoints

Giant’sCauseway, Ireland

http://www.ivanweb.net/images/Mondo/Irlanda/Giants%20Causeway,06.jpg

Giant’s Causeway, Ireland

http://www.gaschurman.com/world/europe/northern%20ireland/giant%20causeway/slides/giant%20causeway%2013.jpg

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

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

In a side view, you don’t see the polygonal pattern as well, but you dosee the columnar nature of the columns

~1m

Koloa (rejuvenation series) basalt flow, Kaua‘i-

Here, on the S. flank of E. Maui volcano, the tops of the columns areexposed in a gully. The surface is not horizontal because the flow itselfwas emplaced on a slope; the columns are almost always perpendicularto the cooling surface.

Kepuni Gulch, East Maui

Wai‘opai gulch, E. Maui-

Wai‘opai gulch, E. Maui-

Onomea,Hawai‘i

Here are the tops of some ~2 million year-old columnsabove Makapu‘u, O‘ahu.Notice that weathering haspreferentially attacked the fractures - eventually thesewill become individual boulders.

The next geologic event was cutting ofthe gully.

Then, a later flow flowed down the gully,and cooled in it. Because this flowplastered against the walls of the gully,its cooling surface was at an angle,therefore its cooling joints are alsoat an angle (perpendicular to thewalls of the gully).

The older flows are ~horizontal sotheir joints, although poorly-developed,are ~vertical.

Here, in spectacular Wai‘opae gulch,E. Maui, are two generations of lavaflows, both with columnar joints.

-

Dikes also show columnarjointing, but because dikesare essentially vertical(and therefore have ~verticalcooling surfaces), theircolumnar joints are ~horizontal.

It is these pre-broken fragments of dense dike rockthat were often made intoadzes by Polynesians.

Dikes in Waihanau valley, E. Moloka‘i-

A dike cutting diagonally across lava flows on Kaua‘i

Notice that even though the dike margins are at a strange angle, thecolumnar joints are still perpendicular to the margins

Devil’s tower (Wyoming), a remnant of athick lava flow or dome, is probably the mostfamous example of columnar jointed rockin the USA.

Photos by Steve Mattox, from: http://volcano.und.edu/vwdocs/volc_images/north_america/devils_tower.html