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Astronomy Picture of the Day with Devil’s Tower!

Astronomy picture of the day for July 29.

Very, very beautiful. I love Devil’s Tower as a geological formation. I’ve only been there once, when I was just a kid, but I desperately want to go back some day soon. Beautiful picture!

For a full look at the geological history of the Devil’s Tower area, the National Parks Service has a very good description. It covers all the major sedimentary units in the area, as well as talking about the igneous rock that forms Devil’s tower.

Actually, reading over the site was a bit of a learning experience for me. I’d only ever heard Devil’s Tower referred to as a volcanic plug, but there is apparently not a lot of evidence to support ancient volcanic activity from that area. (Though since this evidence could have long since eroded away, that’s not really definitive.) Devil’s Tower certainly has a shape that makes people think “Volcano!” but the rather sheer sides of it have more to do with the columnar jointing that the igneous rock experiences. This means that the sections of rock tend to break into six (or more, or less) sided columns and then fall away when stressed by the contraction experienced during cooling.

Also, for some reason I kept thinking that Devil’s Tower was basalt. Part of this is because columnar jointing is very common in basalt. (The basalt of the Columbia Plateau springs instantly to mind.) But whether Devil’s Tower was formed by an igneous intrusion (making it a laccolith or maybe a stock) or actually is a volcanic plug, basalt would be the wrong, wrong answer.

Basalt is the name for extrusive (read: a volcano barfed it on to the surface of the Earth) igneous rock that is very rich in iron and magnesium. Gabbro is the name for rock of a similar composition that’s cooled under the surface of the Earth – or as the case may be, inside a volcano without ever making it to the surface.

Actually, the rock that form’s Devil’s Tower isn’t even gabbro – it’s technically “phonolite porphyry.” If you’ve never heard of that, it’s okay, I haven’t either. We’re getting in to very persnickity naming of igneous rocks, and unless you’re a geologist who specializes in that kind of rock, it’s not something you’d run across. Basically, it’s an intermediate intrusive rock, which is a bit like granite but lacks the quarts crystals. So I’d guess it’s closer to a diorite. Since it’s in the middle of a continental plate, it’s got too much silica to be mafic like a gabbro, but there was still enough hot mantle material in the mix to keep it from bumping over into the category of granite.

Either way, beautiful, beautiful picture!

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