29 September 2010

Guess this means it’s columnar jointing week

Posted by Jessica Ball

Well, since everyone else is getting in on the columnar jointing meme/festival/whatever, I suppose I could throw a few photos out there…

Columns in an ignimbrite on the east side of Santa Maria volcano in Guatemala. (A drive-by, unfortunately. I would have loved a close-up look at these!)

Some of the oldest volcanics in the Vulsini volcanic district, Italy: a jointed trachyte from the Bolsena caldera complex. (The tree at the top of the cliff is about 3 m tall.) This is one of my favorite photos, because you can see multiple cooling sets divided by fractures – and it really emphasizes how columns form from inward-directed cooling.

Willow Mountain, Terlingua, Texas (near Big Bend). Beautiful columnar jointing throughout. (No scale on this one, but I’d say it’s about 200 meters from top to base.)

And, finally, an actual example of columnar jointing in basalt, which is probably much more common than jointing in an any of the other stuff I’ve mentioned: a lava flow near St. George, Utah. (Wish I could figure out where, but Google Earth isn’t helping today. Has anyone seen this abandoned water tank?)

Perhaps we should submit a request for a Columnar Jointing Week to someone in Washington. As far as I can tell, we’re sadly lacking in geological observances anyway…