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backyard geology igneous stuff pet rock

A girl and her pet rock (4)

So I just realized that I owe a post for today. I spent my entire day working on my geology term paper, so I have nothing all that clever or interesting to say. So I guess instead of falling down on the job completely, I shall share the rough draft of my paper. Yay?

Unfortunately I can’t really include the pictures, so you will miss the treat of my extremely awful, juvenile-looking hand-drawn cross section. It’s one step up from MS Paint, but a small step at that.

I’ve also now submitted my two grad school applications. Keep your fingers crossed for me!

Also as a note, the pictures of what I’m referring to in the petrochemistry section can be found right here.

Genesis of the Green Mountain Kimberlite

Introduction
The Green Mountain Kimberlite is located in a mountain park/open space near the city of Boulder, Colorado, at approximately latitude and longitude 39º59.431’N, 105º18.09’W.

The Green Mountain Kimberlite intrudes in to the Boulder Creek Batholith, which is primarily composed of Precambrian granodiorite. There are no rocks other than the granodiorite and kimberlite exposed in the immediate area, and no evidence of other intrusions at the surface.

The exposed kimberlite contains no identifiable rock fragments that are younger than Precambrian in age. Larson and Amini (1981) attempted to track the age of the kimberlite using fission track ages on apatite and sphene within the rock. The apatite fission tracks yielded the highly suspect age of 77.1 ±5 million years, while the sphene fission tracks yielded a more reliable age of 367 ±15 million year. This number agreed with other kimberlite emplacements near the Colorado-Wyoming border and was considered reasonable at the time, under the assumption that all kimberlites in the region were emplaced at approximately the same time during the Devonian. However, a later study used 40Ar/39Ar of phlogopite from the kimberlite to determine a maximum emplacement age of ~865 million years, though that age was considered suspect within the study due to problems with Ar degassing and anomalously low initial 40Ar/39Ar ratios. Using 147Sm/144Nd ratios taken from megacryst samples from the kimberlite, the same study found an age of 572 ±49 million years for emplacement (Lester et al, 2001). This dating of the Green Mountain Kimberlite agrees with that of the Chicken Park Dike in the same study; the two kimberlite intrusions are compositionally similar to each other, while being significantly dissimilar to other kimberlites in the area, making the difference in age seem both reasonable and logical. At this time, the evidence points to the Green Mountain Kimberlite being emplaced in the Paleozoic, at 572 ±49 million years ago.

Lester et al (2001) have suggested that their dating of the Green Mountain Kimberlite as Neopaleozoic in age puts the emplacement in line with the break up on the Rodinia supercontinent and suggests a tentative link between the two events. If this is the case, the Kimberlite resulted from an extensional tectonic setting, in which the kimberlitic magma flowed up through deep fissures and zones of crustal weakness related to the extension. The formation of the kimberlite came from the melting of mantle peridotite mixed with volatiles, most importantly CO2, though the source of these volatiles is not immediately apparent in the scenario of the Rodinia breakup. Another possible scenario for the generation of the kimberlitic magma is hot spot activity, though the evidence for such activity in North America is so thin as to be nonexistent (McCandless 1999).

Petrochemistry
The Green Mountain Kimberlite is a porphyritic, with a fine-grained ground mass surrounding large phenocrysts. The phenocrysts in the thin section examined were serpentanized olivine sometimes with apparent remnant olivine, phlogopite, biotite, and large calcite crystals. There was also a 1-2mm in diameter opaque of unknown type in the sample, and infrequent but identifiable orthopyroxene. The ground mass is fine grained and rich in calcite, as well as opaques. Boctor and Meyer (1979) identify the major mineral components of the kimberlite as diopside, ilmenite, Cr-rich and Cr-poor almandine, olivine (serpentanized and not), orthopyroxene, biotite, phlogopite, and calcite. No large garnets were identified in the thin section, but it is very possible that some small garnets exist in the ground mass, which remains mostly dark at all angles under crossed polars. Ilmenite is an opaque mineral and as such cannot be identified with true certainty in the thin section, but considering its abundance within the kimberlite, it is likely that a significant percentage of the opaques in the ground mass are ilmenite. The ground mass is also rich in calcite.

Boctor and Meyer also note the presence of Perovskite within the Green Mountain Kimberlite, though it is a mineral not easily identified within the thin section. However, the presence of the perovskite does suggest that the mantle peridotite source of the kimberlite interacted with CO2-rich fluid, which allowed the chemical interactions to create the abundance of Nb and REE in that mineral.

Conclusions
The formation mechanism for kimberlite magmas in particular is still a topic of great discussion among geologists (Heaman et al, 2004), and unfortunately the genesis of the Green Mountain Kimberlite remains murky. In general, the kimberlitic magma that produced the Green Mountain Kimberlite must have formed due to the interaction of mantle peridotite with volatiles, particularly CO2 and water. This volatile interaction is further supported by the abundance of calcite phenocrysts and in the ground mass of the kimberlite, as well as the Nb and REE-rich Perovskite found within the kimberlite by Boctor and Meyer (1979). Probably prior to the partial melting, the peridotite had undergone at least one episode of metasomatism. The source of the volatiles for this metasomatism and melting is unclear; there is little evidence for a mantle plume in the area, and the existence of a nearby subduction zone is likewise unclear (Heaman et al, 2003). After the formation, the magma was forced upward under high pressure, most likely following deep crustal fissures or zones of weakness related to the break up of the Rodinia supercontinent. This rapid, pressurized intrusion (and ultimately extrusion) of the kimberlitic magma explains the existence of granodioritic xenoliths within the kimberlite, taken from the surrounding Boulder Creek Batholith during the kimberlite’s intrusion. With even the age of the Green Mountain kimberlite still a matter for debate, little more can be said about the rock’s formation with any degree of certainty.

References
Boctor, N. Z., Meyer H. O. A. Oxide and sulfide minerals in kimberlite from Green Mountain, Colorado. In: The mantle sample – inclusions in kimberlites and other volcanics (F. R. Boyd and H. O. A. Meyer, editors), Proceedings of the Second International Kimberlite Conference, AGU, Washington DC, v. 1 (1979), pages 217-229.

Heaman, L. M., Bruce A. Kjarsgaard, Robert A. Creaser, The temporal evolution of North American kimberlites, Lithos, Volume 76, Issues 1-4, Selected Papers from the Eighth International Kimberlite Conference. Volume 1: The C. Roger Clement Volume, September 2004, Pages 377-397, ISSN 0024-4937, DOI: 10.1016/j.lithos.2004.03.047.

Heaman, L. M., B. A. Kjarsgaard, R. A. Creaser, The timing of kimberlite magmatism in North America: implications for global kimberlite genesis and diamond exploration. Lithos, Volume 71, Issues 2-4, A Tale of Two Cratons: The Slave-Kaapvaal Workshop, December 2003, Pages 153-184, ISSN 0024-4937, DOI: 10.1016/j.lithos.2003.07.005.

Larson, E. E., M. H. Amini. Fission-track dating of the Green Mountain Kimberlite diatreme, near Boulder, Colorado. The Mountain Geologist, v. 18 (1981), pages 19-22.

Lester, A. P., E. E. Larson, G. L. Farmer, C. R. Stern, and J. A. Funk. Neoproterozoic kimberlite emplacement in the Front Range, Colorado. Rocky Mountain Geology, v. 36, no. 1 (2001), pages 1-12.

McCandless, T.E. Kimberlites: mantle expressions of deep-seated subduction. In: J.J. Gurney, J.L. Gurney, M.D. Pacsoe and S.H. Richardson, Editors, Proceedings of the Seventh International Kimberlite Conference vol. 2 (1999), pp. 545–549.

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backyard geology igneous stuff pictures

Finally, pictures!

I finally got off my butt and uploaded the photos from my two field trips. I was intending to get a flickr account, but didn’t feel like resurrecting my old Yahoo e-mail address. So let’s try Picasa and see how it works!

Field trip 1
Field trip 2

These are all the photos I took of the field trips. I’ve put captions on most of them, so hopefully they’ll be clear. And all of this stuff is less than a day’s drive from Denver!I do have all the photos from the Moab field trips I did a year ago, so I’ll try to get those together soon.

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backyard geology volcano

Backyard Geology: The Valles Caldera

There’s still some geology left for me this semester – this coming Monday I get to start cutting my thin section from the kimberlite I picked up at Green Mountain. Eventually the thin section will be made in to a slide and I’ll be doing a petrographic analysis, taking a photo micrograph of it, and writing a paper. Which is fine. Kimberlite is super cool.

But the field component is definitely done. This last trip was another jaunt down to New Mexico, this time over by Los Alamos. We spent most of our time between the Bandelier National Monument and the Valles Caldera. It’s a very cool area. Our reason for being at Bandelier was to look at the Bandelier Tuff, as well as some other volcanic rocks in the area. The tuff was produced by the Valles Caldera blowing out about 1.2 million years ago.

The tuff starts pretty far away from the Caldera, as you might expect from the sort of massive volcanic explosion that would come from a caldera-forming eruption. At the first place where we examined it, I think we were at least 20 km away from the Bandelier National Monument, and the tuff and pumice layers were about 50 feet thick. The layering of the rocks in the area moving toward the caldera are pretty interesting. There are alternating layers of fairly unconsolidated pumice, tuff, ignimbrite. The tuff is basically pumice that has been partially welded back together by heat, and contains some phenocrysts. Sometimes the extremely well-welded ash units look eerily like basalt flows from a distance, which is very cool. By the time we got in to Bandelier National Monument to see the cliff dwellings, the tuff was about 500 feet thick.

The tuff and pumice makes for some pretty bizarre rocks. You normally expect rocks to be heavy, but the pumice feels almost as if it’s made of styrofoam. The cliff dwellings were actually cut in to the tuff layer, which is only slightly heavier and more solid than the pumice itself.

We also drove in to the caldera, which is a stunning area. It’s basically a massive, rolling plain covered with grass, which is surrounded entirely by a ring of large hills. The plain itself is dotted with smaller hills, which are actually obsidian domes that have formed at one time or another since the caldera collapsed. The biggest of the hills within the caldera is the resurgent dome. I do have some pictures (still need to pull them and the ones from the previous field trip off my camera) but for now, here’s a couple nice shots from Wikimedia Commons:
One of the domes in the Caldera
A couple more domes, during the winter

The pictures really can’t give you an idea of the scale of the place. You’ll just have to go there yourself, some day. Also, if you want a piece of Bandelier Tuff for yourself, you obviously cannot collect in the national park. However, there are several road cuts outside of the national monument where you can pull over and pick up large pieces of pumice and tuff, as well as some where you can find obsidian-like extrusions. It’s some very cool stuff.

Not far outside the caldera itself, there’s a picnic area where you can catch a trail up on to Battleship rock, which is made of ash deposits. The trail up to the rock is pretty tough. It gave my knees hell going back down particularly. But you do get a fantastic view from the top.

Also at that picnic area, you can catch a trail to the McCauley Warm Springs. It’s about a five mile round trip, and if you have knee problems like I do, I’d really recommend some walking sticks for this one. They make progress faster and much less painful. It’s a tough enough hike that there weren’t too many people in and out of the area, even on a beautiful and warm Sunday. The Springs themselves aren’t what you would expect. They’re meteoric hot springs, which basically means that rain water gets down in to the magmatically active zone via fissures and then is expelled to the surface. This means that they’re not too mineralogically strange – and don’t smell like sulfur, for example. (There are other sulfur-rich springs in the area which are hydrothermal in nature.) They’re also not as hot as you’d think – they’re more “warm” springs than hot springs. The temperature was like being in a very pleasant swimming pool, which is more remarkable than it seemed at the time considering that temperatures were getting down below 40 degrees F at night in the area. There’s a lot of algae growing in them, but the water’s warm enough that they certainly don’t smell like an active breeding ground for cyanobacteria. So it was a nice little excursion and a nice soak. There are also a lot of little fish that live in the springs. My feet got gently nibbled at a lot, which felt very ticklish and was quite amusing. I recommend having a beer (if you’re old enough) while relaxing in the springs.

Overall, an amazing experience courtesy of Giant Geological Features That Could Kill Us All.

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backyard geology volcano

Backyard Geology: Capulin Volcano

Four days in a row of hiking (since even though we came back on Sunday, we did another hike on Monday) have just about destroyed me. I’m limping around like an old lady today. Lots of very, very cool stuff was seen on the field trip. Yes, I took many pictures. No, I haven’t uploaded them yet. I’m working on it, though. And there’s lots of very amazing geology stuff to write about. I may never catch up, considering that school has apparently slowed me to a one-post-a-week crawl.

For the three day trip, we spent most of our time in the Raton Volcanic Field down in New Mexico, though on Sunday we did head back in to Colorado for the Spanish Peaks. (Which are a whole other cool thing to write about.) The Raton Volcanic Field (RVF from here on out) was and still is caused by the rifting near the Rio Grande River, where there’s hot, plastic mantle (asthenosphere) welling up to within 30 kilometers of the surface, which on a continent is a Very Big Deal. Normally, the asthenosphere minds its own business and stays at a depth of 100-200 km. At the Rio Grande Rift, it’s poking its steaming head above the Moho, which means there’s a lot of very hot rock where it really has no business being, and that makes for a lot of volcanic activity.

The RVF actually isn’t in the Rift Valley itself; it stands on the margins. The area is very topographically interesting; generally you have a lot of rolling plains there, but there are also stair-step like mesas and very prominent hills poking up from the landscape. Each of these prominent, conical hills is an extinct volcano. The mesas are caused by basalt lava flows that came from the volcano. So the basic process of the RVF is that a volcano pops up in a valley (where the crust has thinned a bit due to the rifting to the west), puts out a lava flow or two, there’s more rifting and a new valley created, and then the process repeats.

Now, most of the volcanoes in the RVF (with such prominent exceptions as Sierra Grande, which is a shield volcano) are cinder cones. Many of them are now covered with vegetation of some type, but I did see some prominent and presumably younger (since they still had their very distinctive shape) cinder cones that were completely naked. Naked cinder cones tend to erode down very quickly, since they’re basically made of layers of ash and other pyroclastic debris that aren’t well consolidated. As far as volcanoes go, cinder cones are fairly well understood. There are a lot of active cinder cones today, and one in Mexico even started its formation a little more than 60 years ago: Paricutin.

Capulin Volcano is one of the RVF cinder cones. It’s relatively young, between 58 and 62 thousand years old, and it is rather well vegetated. The vegetation layer has helped preserve the volcano’s shape, so it’s very distinct and pretty. The volcano itself is a national monument, and there are several extremely nice trails. One goes around the volcano’s rim, another goes down in to it, so you can look at the blocked-off vent that spewed all the ash and debris, and a third goes out on to the lava flow at the volcano’s feet. As is common, Capulin did put out a basaltic lava flow, but not from its central vent as we’ve come to expect from the normal images of composite and shield volcanoes. Since cinder cones are structurally weak due to their composition, most develop a vent at their base and that’s where the lava comes out.

Other than the simple OH MY GOSH COOL of begin able to walk on and down in to a volcano, there’s some very nifty geological stuff to be seen. At several of the road cuts on the volcano, you can see the layers of ash that make up the cone. They come in a lot of different colors and are fairly distinct. You can also see volcanic “bombs” all over the place. These are chunks of magma that got spewed into the air and solidified in distinct chunks. As you look over the lava flow at Capulin’s feet, there are several visible tumuli, which are dome-like features where hot lava welled up through the cooler, thin crust on the lava flow surface. Also, in the fields that cover most of the lava flow now, you can still see the ghost of pressure ridges, which are ripples preserved in the flow. These are also caused by the movement of hot lava under the cooler surface, causing deformation.

All this cool volcano stuff, and it’s only a four hour drive or so from Denver! I do have some pictures of Capulin that will hopefully be posted soon, but they’re not going to do the volcano much justice. Soon after we left, it started raining and then rained extremely hard for the next eight hours. So as you can imagine, while we were at Capulin it was extremely overcast. (And also shockingly cold.)

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backyard geology igneous stuff pet rock

Backyard Geology: The Green Mountain Kimberlite

Unfortunately, I can’t provide very good directions to this one, and there’s a good reason for it. We drove up to Green Mountain (near Boulder, Colorado), got on one of the trails, and then at a random time just sort of bombed off into the underbrush. It involved going down and back up an extremely steep stream valley where there wasn’t even the hint of a track. Steep, like I’m clinging to trees to keep myself from tumbling down the slope steep. It was a very, very, very rough hike for someone with bad knees and an often embarrassing lack of balance. It’s only about a mile and a half, but it feels much, much longer.

About the best I can do right now is give you the lat/lon of the outcrop: 39º59.431’N, 105º18.09’W. These coordinates should have about a 20 foot accuracy if you believe the claims of the GPS unit’s manufacturers.

That said, the hike is very, very worth it. Big important note, though: the kimberlite is in park land. I honestly have no idea what trouble if any there could have been for us going off trail the way we did, but I know for certain that you’re not supposed to bring in a rock hammer and whack samples off the outcrop.

The kimberlite itself is very interesting. It intrudes through the Boulder Creek granodiorite, which is a holocrystalline intrusive rock with large crystals of quartz, feldspar, and mafic minerals. If you run across Boulder Creek outcrops, they have a distinctive “salt and pepper” appearance. In comparison, the kimberlite is a porphyritic extrusive rock where the ground mass is extremely dark. The samples we found contained large garnets, ilminites, and olivines. The weathered surface of the kimberlite is gray rather than black, with the chemically altered phenocrysts much more obvious by color difference.

The outcrop is mid to upper slope and stands out fairly well from the landscape. There are no trees growing in it. The outcrop itself is about 100 feet in diameter, though on the down slope it elongates into a teardrop-like shape due to the erosion of the slope.

So, a tough hike, but very cool rocks.

Kimberlite is actually one of my favorite igneous rocks, mostly because it’s very cool to look at in thin section. Much of the fine-grained ground mass in kimberlite isn’t actually silicate minerals – it’s calcite. This makes it incredibly colorful when looked at with crossed polars.

The story behind kimberlites is also very cool. They are effectively volcanic dikes, but rather unusual ones. Kimberlitic magma is produced when there’s a critical mass of volatiles in an area of the mantle, normally carbon dioxide and water. (The large amount of carbon dioxide present is the reason kimberlites contain so much calcite.) The volatiles lower the melting point of the surrounding mantle material, and with the sudden pressure on a body of volatile-filled magma, the results are explosive. The magma exits the mantle upward and comes exploding out of the crust, in some cases at the speed of sound. This incredibly explosive, violent eruption of magma under high pressure is what gives kimberlites their characteristic carrot or funnel-like shape.

Also, since the eruption of a kimberlite is so violent, they often carry significant chunks of everything they went through to get to the surface. This includes pieces of mantle peridotite – most of what we know about the mantle composition came from samples brought up in kimberlites. In certain areas, this also means that the kimberlite brings up pieces of old continental crust – most importantly, pieces of the remaining cratons from the Archean. And these craton bits are where diamonds come from. Kimberlites can be small (like the one on Green Mountain) or enormous, like the ones that are mined for diamonds in Africa.

And the best part? Technically speaking, we could get another one erupting out of the ground at any time. There’s no way of knowing. There’s just something cool about that thought, though I wouldn’t want to be standing on top of one when it made its appearance.

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backyard geology

Backyard Geology: Recent Volcanism in Colorado (part I)

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!

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backyard geology igneous stuff

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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backyard geology

Backyard Geology: Garnet/Magnetite sands from the Great Lakes

One of my coworkers (a geologist that I do a lot of work for), who shall for the purposes of this blog be called “Tim,” recently went home to Michigan to visit his family. He brought back a very cool little sand sample that he’d scooped up off the beach at Lake Huron. On the site, the sand looked like very unremarkable brown sand, the kind you’d get off of any beach. Under the microscope, though, it had some real character!

When you look at sand through the lens of its characteristics as sediment, the three factors you’re most interested in are:

1) Sorting: If sand is “well sorted,” it means that most of the grains are about the same sign. The sorting of sediment has implications on how the sand was transported. For example, poorly sorted sediment might have come from something like a landslide, where everything got jumbled together, or from glacial till, which just gets pushed around indiscriminately by the glacier. Well sorted sediment indicates that there was probably longer transportation, and normally by wind or flowing water.

2) Size: Is it big or is it small? A lot (but definitely not all) of sediment begins its life as a larger rock, so this can also be an indication of length of transport, or mechanism that created the sediment.

3) Rounding: Is it round or is it angular? The more round a grain of sediment is, the more punishment its taken over the course of its life, which smooths out the rough edges. (A possible metaphor, here?)

The sand that Tim brought back from Lake Huron was pretty fine in size, very well sorted, and very well rounded. The sorting comes from the fact that it was put on a beach by the lake, and goes hand in hand with the particular grain size for that area of the beach. What the rounding means is that these sand grains have been worked a lot, no doubt by the lake, but possibly by other means.

The normal, average sand that those of us in the continental US are used to seeing is primarily made out of quartz. This is because quartz is very tough and very common as minerals go, so there’s lots of it, and it can take a lot of punishment without breaking down. What tells us the most interesting things about sand and where it came from are the other minerals that you can find in it.

In this case, there were very well rounded grains of deep red garnet, and also quite a bit of magnetite. This is some very cool stuff, and not what we see around in Colorado. Garnet comes pretty exclusively from metamorphic rocks, and magnetite is found in both igneous and metamorphic rocks. So the source of this sand was most likely metamorphic basement rock that’s been crunched up and worn down into sand.

Knowing what we do about the geologic history of the great lakes – they formed from the ice sheets melting at the end of the last ice age – this sand may very well have started its life as glacial till, dropped into the bottom of the newly formed lakes during the melting.

There was actually a very surprising amount of magnetite in the sample. Tim separated it out by dragging a magnet through the sand, and it made an impressive black fuzz. This is not necessarily how much magnetite you’d get if you dredged a similar sample from the bottom of the lake. Apparently streaks of black sand are common on the lake shorelines, because once the sand has had a chance to dry, the wind blows the lighter quartz and other mineral grains farther up the beach, leaving the heavy magnetite behind. So it’s very possible that the surprising amount of magnetite Tim brought home was due to this sorting action.