29 September 2010
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?)