Geology of Devils Tower

Devils Tower National Monument


Here at the Tower, we have two main types of rock, we have the sedimentary rocks that are all around the tower and the rock of the tower itself. This is our beautiful rock from the tower it’s called Phonolite, Phon-o-lite. A lot of people when they look it they think it looks a lot like granite, and it is kind of a cousin of granite but it doesn’t have any quartz crystals. And to be granite you have to have quartz crystals, all of these shiny white crystals are actually feldspar. In between some you see some tiny black crystals and also a whole bunch of gray area both the black crystal and the gray area are rich in iron and as they rust they give the tower its color There have been three accepted theories around for quite a while one is called a stock and that would be just a big blob of rock that kind of stopped underground was about the shape the tower is now but somewhat bigger and that would be called as I said a stock The next theory is that it came up and found a weak layer still well underground and moved out into that layer and kind of blew up until it became a mushroom shaped big hunk of molten rock that geologists call a laccolith. The third theory is that it came up and then above the tower there would have been a pipe that goes all the way to the surface, and remember the surface at this time could have been as much as a mile and a half above. Up there you would have had eruptions and the big cone and all that kind of stuff, but that is not where the tower was, the tower was down in the plumming, and that would be called a volcanic plug. A new theory has come out and that involves ground water. The idea is that when the hot molten magma came up it would find water underground, which happens a lot. When this hot molten rock hit that water it formed a huge amount of steam, which exploded a big pipe and a crater on the surface. And then after things calmed down a bit the rest of that molten rock came up from below, up that pipe that was made weak by the explosion, and cooled in the bottom of the crater, so in this theory the rock would be on the surface right from the beginning, however it would be at the bottom of a big crater, so not at the land surface, You may have noticed in pictures of the tower that it has columns, how did those columns get created? Well that is part of the cooling process. When you have a big mass of molten rock it starts to cool from the surface and on that surface it will start creating crack pattern, molten rock occupies more space then solid rock does so it has to shrink, and you can imagine that if you have a huge piece of molten rock and it you start to cool it everything starts to shrink from everything else and you will develop a crack pattern. You may notice that we have a boulder field, and that is from pieces of the rock that have fallen off through the millennia, we do lose a couple of boulder each year they tell us, they bring them down when there is no one around to get hurt by them, but the last time a full column has fallen they say is maybe 10,000 years ago. A lot of folks want to know what’s on top? The top is fairly flat, a little bit domed from the center and through the years it has developed some soil and actually has plant growing up there, grass is the most common thing but you also have things like cactus, sage brush, and goose berries not only do you have plants you also have animals, there is a whole bunch of small rodents like chipmunks, mice, and wood rats. And whenever you have a banquet like that you have something that likes to eat it so we have snakes up there as well, and of course the birds and the bugs.


A more in depth look at the Geology of Devils Tower National Monument.


5 minutes, 6 seconds



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