Well, school has started; expect to see a lot of backyard geology posts for the next month, since I’m in an igneous and metamorphic field geology class. We’ll be going to some very cool places!
Sites first, then a bit of background:
Valmont Dike is a very striking feature in Boulder. The best place to see it is at Valmont Road, where it splits into Butte Mill Road and Valmont Drive east of the city. Unfortunately, you can’t actually get up near the dike; it’s completely fenced off. This is due to two current issues: there’s an old mining mill there and the soil may be contaminated, and there’s a Native American tribe claiming the dike as a holy site. There is a little gravel shoulder where you can pull off and at least get a good look at the dike. Be careful since there is a waste management facility in that area as well so there may be big trucks coming on to Valmont Drive.
The dike is very impressive looking, though. The igneous intrusion itself is about 30-40 feet wide and sticks up about 200 feet from ground level. It’s basically a vertical block of alkali basalt. (Though apparently it’s not as homogeneous as it appears; we just can’t get up to look at it and see how the mineral composition changes.) One either side and to its front, there’s a “wrapping” of Pierre shale. You can see from the shoulder that the bedding in the shale is almost entirely undisturbed; it’s got the normal 10 degrees east dip that all of the sedimentary rocks in the area have. So basically the dike cut through the shale without really disturbing it. You can also see that the shale against the dike is much paler than the rest. This is most likely some contact metamorphism where the shale was “baked” a bit by the hot magma that made the dike.
If you drive along Valmont Road, you can see the dike at various points for about the next mile and a half. There’s also another bit of the dike farther west, but it’s fenced off as well and not nearly as cool looking.
There’s a sill that’s very easy to find on Flagstaff Mountain. To get there, take Baseline up past Chautauqua Park and continue along up the mountain. Past the Flagstaff House restaurant, up a couple more switchbacks there’s a little gravel parking lot. As you park, the sill will be pretty much straight ahead of you, to the south.
The Flagstaff Sill is very easy to pick out, though not quite as awe-inspiring as the Valmont Dike. Most of the rocks in the area are from the Fountain Formation, various red-colored sandstones, conglomerates, and mudstones. The sill stands out as a salt-and-pepper granodiorite. You can follow it fairly easily along the ground. You’ll also notice that the sill seems to be broken up in a particular way, in vertical chunks. If you look from a distance, you can actually tell that this is the remnants of columnar jointing in the igneous rock.
Now, the background:
On its face, volcanism in Colorado seems to be a strange prospect. We’re not at an active plate boundary like the dramatically named “Ring of Fire.” We haven’t been even in geological history. Now, sometimes volcanoes are caused by “hot spots” in the mantle, but there’s not any evidence that we’ve been sitting on one. However, if you look at a geological map of Colorado, you’ll see a lot of ancient lava flows. These all date from the late Cretaceous to early Paleocene, which is when the Laramide Orogeny was occurring.
In general, an orogeny is an uplift event, where mountains are built. The Laramide Orogeny was actually caused by subduction on the western coast, where present-day California is. Normally oceanic crust subducts at a fairly steep angle and the zone of volcanic activity associated with it stays pretty close to the coast. (Think about where volcanoes are currently located in Washington state and Alaska.) In this particular case for some reason the angle of the subducted crust was incredibly shallow. So the oceanic crust that was getting pushed under the North American continental plate extended as far east as Colorado.
The way this caused volcanism isn’t intuitive. If you’re like me, you imagined that it’s from all the rocks rubbing together and melting or something like that. Actually, it’s far more interesting. The oceanic crust moves through the mantle, and since that crust is actually relatively cold at that point, it locally cools the mantle. However, the surrounding heat and pressure causes water and other volatile compounds to be effectively “steamed out” of the crust. That water lowers the melting point of the mantle above the subducted crust, and so the mantle partially melts in to magma.
This magma had to go somewhere. In some cases, it came out as lava flows. The two things I’m going to mention here, though, are dikes and sills.
Dikes and sills are both small, sub-surface intrusions of magma into already existing rock. The magma didn’t break to the surface in these cases; it’s just an underground blob or line. Often dikes or sills will feed into a larger body of magma, like the magma chamber of a volcano. They don’t have to, however.
The big difference between a dike and a sill is how they intrude into the rock. A dike cuts through beds of rock. So if you imagine a horizontal shale unit with lots of thin little beds in it, the dike cuts vertically through that. A sill cuts between beds or layers of of rock. You can basically imagine it like sticking a sheet of paper between the pages of a book.
Interestingly enough, the Valmont Dike and Flagstaff Sill actually have fairly different compositions. The dike has a lot of iron, a relatively large amount of sodium and potassium, and is less than 50% silicate. That makes it an alkali basalt, which is the sort of basalt you tend to get in mid-continental volcanism. The sill has about 20% more silicate and a lot less iron. It’s not quite a granite, but it’s an intermediate between diorite and granite.
Both sill and dike have been dated using potassium/argon radiometric dating to be about 64 million years old. They definitely come from the same event – the Laramide Orogeny – and from melts that occurred at about the same time. One cause for the different composition may be that the dike came from a basaltic magma that went directly to the surface, so its composition is basically that of the melt. The sill may have been caused by a more basaltic magma pooling and melting the silica-rich continental crust above it. This mixing and dilution of the basaltic magma with what would basically be rhyolitic magma would account for the change in composition. And for all we know, sub-surface there might be a gradual change in composition to the sill. We don’t know how far down it extends or really anything about it other than what we can see at the surface. To find out more, we’d probably have to do seismic and magnetic studies.
Expect more about volcanic activity in Colorado soon!