Waterfall and Columnar Basalt © Joe Decker. Used with permission.
I finally found a piece from my nature photographer friend Joe Decker that would make a suitable subject for rock blogging. Y'see, the problem with fine art photographers is that they often forget to do things like add a rock hammer for scale. Also, they can have entire portfolio sections devoted to the Carrizo Plain without once showing an offset stream channel! It must be a very strange way to see the world.
Anyway, this image is of part of a lava flow in Iceland. As a new blanket of rock cools, it contracts. If it cools evenly and slowly enough, this contraction will create regular hexagonal columns. The phenomenon is famous at places like Devil's Postpile and the Giant's Causeway.
Yellowstone National Park is another great place to see cliffs of columnar basalt and also, in a natural dichotomy, cliffs of glassy obsidian. The juxtaposition of tall, regular, hexagonal columns and blubbly, irregular, sometimes jagged obsidian shows the full range of nature and the two extremes it can produce. Of course, given a choice between the two, I have to choose the obsidian. Anyway, that photograph above is so nice I can forgive the lack of a rock hammer or meter stick.
I've been there!
Trivia note: pentagons, heptagons, and octagons can also be found as not-too-terribly-uncommon components of columnar basalts.
Enjoyed the photo - I visited that exact spot ten years ago.
I'm gonna have to owe you that offset stream, I've made two attempts but haven't gotten a result that conveys the offset stream idea well .. yet. :) (Both off the standard viewpoint off of Elkhorn Road.) Just sayin'. :)
Another really good place is one I saw at a forest fire up in Oregon. All around the town of Warm Springs, on the Warm Springs Agency (an Indian Reservation) there are rather thin (10 to 20 metres) bands of columnar basalt interspersed with tufa and ejecta bands. Neat area. And there are very few plants to get in the way of the rocks!