Activity discovered at Yellowstone Supervolcano - I talked about supervolcanos a while back. Don't worry, it's still in no danger of blowing up any time soon. (And here, we're talking geologically soon, which is a much, much longer span of time than a human soon.) The two cool things in this article are the discovery that the Tetons are getting shorter, and that there's a "bulge" that's expanded and deflated at Yellowstone.
The Tetons getting short is interesting because, in the normal course of things, mountains do get shorter. That's just the way things work. Mountains are built, erosion wears them down. However, the Tetons are getting shorter much, much faster than they ought to be. Now why is that? Mountain ranges are normally criss-crossed with faults, some of which may be very, very large. The faults are from where the rock broke, unable to handle the strain, when the mountain range was thrust up. There's a very large, active fault at the feet of the Tetons. The way such faults normally work when active is that the valley at the foot of the mountains drops, while the mountains move higher up. Except that the fault between the Tetons and their valley is going in the exact opposite direction as normal - the valley is rising, the mountains are sinking. It'll be interesting to find out what the exact mechanism is. The current hypothesis is that this abnormal movement is due to the expansion and contraction of the Yellowstone volcano; the volcano puffs up, it pushes on the valley. The valley creeps up the side of the mountains, which forces them down.
Now, the "bulge" is actually a pretty normal thing, volcano-wise. Contrary to what you might think, rock is actually very elastic. If put under pressure (pressure that isn't overwhelming, that is) for a long period of time, rocks will deform. When rocks are put under too much pressure too fast, they will break, which is what causes faults. Volcanoes tend to bulge as magma builds up, putting pressure on them from the inside. (One of the heralds of the Mt. Saint Helens eruption was the enormous bulge on the side of the mountain.) In many volcanoes, this bulge builds up and builds up until the volcano erupts. In this case, the bulge deflated a bit before rising again, which indicates a temporary relief of pressure; it also happened pretty rapidly - at seven inches in three years, that thing is sprinting when you think about things geologically. The bulge is probably caused by the movement of magma from the mantel plume that feeds Yellowstone.
* * *
Massive gypsum crystals in a cave in Mexico
These are SO COOL. Look at the first picture carefully - that's a person in there for scale. These crystals are in a limestone cave, which was probably created by water from a hydrothermal vent coming in through a fault and dissolving the rock. (Limestone is very prone to dissolve when in contact with water.) The water deposited the minerals that formed these crystals (and the precious metal veins exploited in a nearby mine) and the crystals formed over time. The area is still very active as a hydrothermal vent; the temperature of the cave is around 125-150F and the air's at a constant 100% humidity. Brutal!
If you didn't know, hydrothermal activity is associated with volcanic activity. When rock is subducted at a plate boundary, it normally carries a lot of water with it. The water is superheated and seperates from the rock; it escapes rapidly through whatever avenues are available to it, normally through faults that form vents. Due to the nature of how the rock melts, the superheated water often carries rare elements with it (such as precious metals) that it deposits along the vents as it cools, moving to the surface.
The giant crystals in this picture are gypsum. Gypsum is a pretty cool mineral. It's a 2 on Moh's hardness scale, which means that you can scratch it with your fingernail. When you get a nice crystal that hasn't been banged up (and it's hard to find those, sometimes, because just about anything will mark gypsum because it's so soft) they're usually transparent. When you touch gypsum, it's smooth and feels faintly soapy or waxy.
Monday, March 26, 2007
Wednesday, March 21, 2007
Arkose & Alluvial Fans
Today was my second field trip with my Sed Strat class. We went up to Settlers' Park to look at some exposed facies there. (A Facies is a group of sedimentary structures you see in a rock that points to a particular environment that the sediment was deposited in.) If you're ever in the Boulder area and up for a little bit of an uphill hike, I recommend it. The facies we looked at belonged to the Fountain Formation and the Lyons Formation.
The Fountain Formation is pretty famous, at least locally. A formation is a unit of rock that is geographically contiguous (it's all connected) and clearly seperate from the formations above and below. Sometimes this seperation is due to a change in rock type, since formations are usually of a single lithology - which is to say composed of just one type of rock. However, sometimes formations are seperated by unconformities, which are boundaries caused by erosion and other events.
So calling it the Fountain Formation means that it's a big unit of a single kind of rock that covers a definite geographical area. (In this case, a broad swath at the feet of the east face of the Rocky Mountains.) Fountain is the name of the formation. It is composed of sandstones and Arkose; the Arkose is the most famous and gives it its beautiful color. Arkose is a particular kind of sedimentary rock. Arkose is normally primarily quartz, but it has at least 25% Feldspar in it. This will give the rock a definite pink cast, or if its been exposed to any weathering, the Feldspar will cause iron oxide (remember: rust is iron oxide) that stains the rock anywhere from orange to a beautiful, deep red.
A large portion of the Fountain Arkose was deposited by alluvial fans. Alluvial fans are a phenomena found at the base of mountains. What happens is that there are canyons through the mountains - formed by rivers. During the spring melt (or intense storms), massive amounts of water will flood through these canyons, picking up lots and lots of sediment along the way. These canyons let out at the base of the mountains, and the water suddenly spills out in a characteristic fan-shape. (To visualize this, turn on a hose that's laying on the ground. Notice how the water spreads out in a fan at the end of the hose.)
While the water is shooting through the canyon, it's going very, very fast. This translates to the water having a lot of energy - and the more energy water has, the bigger rocks it can carry. As the water spills out of the canyon, it loses a lot of that velocity because it's no longer directed in a channel formed by the canyon walls. So it drops everything that it was carrying.
Alluvial fan deposits are very interesting to look at. They're composed of layer after layer of different kinds of mudstones, sandstones, and conglomerates. When the water first comes out of the canyon, it drops all of the big rocks that it picked up - anything from coarse sand to even boulders! The rocks formed from that are conflomerates - there's a wide range of how big the clasts (the bits of rock that the river dropped) are, and some of them are very large. At other times, the water wasn't moving fast enough to carry large rocks, and it will just drop sand, or even mud. So you will layers with all different clast sizes in them. Mudstones are often far darker than the layers above and below them, so you will see stripes running through the formation.
The Fountain Arkose formed from the erosion of the Ancestral Rocky Mountains - the mounains that existed in the past before today's Rockies. They were worn completely down, and then a new session of mountain building brought today's Rockies up. The Ancestral Rockies were made of granite as well - that's where the Feldspar in the Arkose comes from. Feldspar is an "unstable" mineral. It is subject to chemical weathering, and because of its physical properties, it breaks into tiny pieces easily. So large deposits of Feldspar are normally found close to their source. If they're buried quickly, they can't be weathered away!
If you want to see the Fountain Formation, there are many good places to see it in Colorado. In Boulder, you can go to Settlers' Park, where its been uplifted into a hogback - the originally flat layers of rock are standing vertically. Also in Boulder, the Flatirons are part of that formation. Red Rocks Amphitheatre is built in another exposure of the Fountain Formation. It can also be seen in Garden of the Gods. If you ever have a chance to go to any of these places, I recommend it. They're beautiful, and there's some good hikes in those areas along with great geology!
The Fountain Formation is pretty famous, at least locally. A formation is a unit of rock that is geographically contiguous (it's all connected) and clearly seperate from the formations above and below. Sometimes this seperation is due to a change in rock type, since formations are usually of a single lithology - which is to say composed of just one type of rock. However, sometimes formations are seperated by unconformities, which are boundaries caused by erosion and other events.
So calling it the Fountain Formation means that it's a big unit of a single kind of rock that covers a definite geographical area. (In this case, a broad swath at the feet of the east face of the Rocky Mountains.) Fountain is the name of the formation. It is composed of sandstones and Arkose; the Arkose is the most famous and gives it its beautiful color. Arkose is a particular kind of sedimentary rock. Arkose is normally primarily quartz, but it has at least 25% Feldspar in it. This will give the rock a definite pink cast, or if its been exposed to any weathering, the Feldspar will cause iron oxide (remember: rust is iron oxide) that stains the rock anywhere from orange to a beautiful, deep red.
A large portion of the Fountain Arkose was deposited by alluvial fans. Alluvial fans are a phenomena found at the base of mountains. What happens is that there are canyons through the mountains - formed by rivers. During the spring melt (or intense storms), massive amounts of water will flood through these canyons, picking up lots and lots of sediment along the way. These canyons let out at the base of the mountains, and the water suddenly spills out in a characteristic fan-shape. (To visualize this, turn on a hose that's laying on the ground. Notice how the water spreads out in a fan at the end of the hose.)
While the water is shooting through the canyon, it's going very, very fast. This translates to the water having a lot of energy - and the more energy water has, the bigger rocks it can carry. As the water spills out of the canyon, it loses a lot of that velocity because it's no longer directed in a channel formed by the canyon walls. So it drops everything that it was carrying.
Alluvial fan deposits are very interesting to look at. They're composed of layer after layer of different kinds of mudstones, sandstones, and conglomerates. When the water first comes out of the canyon, it drops all of the big rocks that it picked up - anything from coarse sand to even boulders! The rocks formed from that are conflomerates - there's a wide range of how big the clasts (the bits of rock that the river dropped) are, and some of them are very large. At other times, the water wasn't moving fast enough to carry large rocks, and it will just drop sand, or even mud. So you will layers with all different clast sizes in them. Mudstones are often far darker than the layers above and below them, so you will see stripes running through the formation.
The Fountain Arkose formed from the erosion of the Ancestral Rocky Mountains - the mounains that existed in the past before today's Rockies. They were worn completely down, and then a new session of mountain building brought today's Rockies up. The Ancestral Rockies were made of granite as well - that's where the Feldspar in the Arkose comes from. Feldspar is an "unstable" mineral. It is subject to chemical weathering, and because of its physical properties, it breaks into tiny pieces easily. So large deposits of Feldspar are normally found close to their source. If they're buried quickly, they can't be weathered away!
If you want to see the Fountain Formation, there are many good places to see it in Colorado. In Boulder, you can go to Settlers' Park, where its been uplifted into a hogback - the originally flat layers of rock are standing vertically. Also in Boulder, the Flatirons are part of that formation. Red Rocks Amphitheatre is built in another exposure of the Fountain Formation. It can also be seen in Garden of the Gods. If you ever have a chance to go to any of these places, I recommend it. They're beautiful, and there's some good hikes in those areas along with great geology!
Friday, March 02, 2007
Why you should love sedimentary rocks.
New year, new semester, new tax return, new FAFSA. Where does the time go?
Mineralogy last semester ended well, though I can't say I'm sorry to see it done. It takes a special kind of person to want to spend all your time staring into a petrographic microscope and thinking about 3-D crystalline forms. I got an A, and I wrote a rather boring paper about Enstatite, an igneous mineral that comes in rather pretty olive colored crystals. (Maybe I should post that paper here so you can look at it and... marvel, if that's the word I want.)
This semester, my geology course is sedimentary stratigraphy.
Sedimentary rocks are pretty much the unsung heroes of the modern age. Well, to be precise, rocks in general are unsung heroes. They just sort of lay there, as far as most people are concerned, and they don't actually do anything...
Except that they do. Off the top of my head, here's what rocks have done for you lately:
1) Kept you from plummeting into an ocean of subsurface magma.
2) Supported your house, roads, office building, etc.
3) Provided some pretty scenary, if you live near mountains.
4) Acted as building materials (or storage for building materials) for at least half the objects you interact with on a daily basis.
5) And so on and so on.
But among the rocks, sedimentary rocks are the work horses for human concern. Now, all rocks are linked together, by something called the rock cycle. Sedimentary rocks are formed by the weathering and erosion of igenous, metamorphic, and even other sedimentary rocks. Weathering is the process by which rock is broken down into little pieces, and erosion is how those little pieces are carried away, most commonly by water, followed by wind and gravity. These little rock pieces are called clasts; they're carried along by the wind or water and eventually dropped somewhere. This is called deposition. When enough clasts have been dropped in the same location, they build up, compact under their own weight, get covered with more clasts, and eventually get squished and cemented into a sedimentary rock.
That's the really simple, basic view of it.
Unlike igenous or metamorphic rocks, sedimentary rocks don't have to be melted or cooked or squished and twisted out of all recognizeable shape. This means that you find an absolute multitude of things in sedimentary rocks that you can't possibly find in metamorphic or igneous rocks. Things like: Fossils (bones and footprints and things like that), oil, and drinking water.
Sedimentary rocks often also preserve ingenious little clues that tell us a great deal about where they were formed and what the Earth was like at that time, and in that place. You can find ripples preserved in rocks, mud cracks, even the impression of rain drops falling on a desert plain in the distant past. These rocks are our window into a time so far back that human beings didn't exist to write down what was happening. Remember, in the lifetime of the earth, we are barely the blink of an eye.
So, every time you go to the museum and look at the dinosaur bones, you're looking at something that was preserved in a sedimentary rock. Every time you put gas in your car, you're using a product made from oil, which forms in shale (a sedimentary rock formed in deep water conditions), and then hides in subsurface reservoirs, most of them found in either sandstone or limestone (also sedimentary rocks). If you drink water from an aquifer, that water often has filtered a long distance through a formation of sandstone, which has acted as a natural filter so it's clean to drink.
Isn't that a weird though, water or oil flowing through rocks? In some of these reservoirs, it's just finding its way through cracks in the rock. But in the case of sandstone, it is literally travelling through the rock. This is because of the way sandstone is made.
Sandstone is made of sand-sized clasts. Now, these clasts can really be any sort of rock or mineral, but most commonly you'll find them made of quartz. This is because quartz is pretty hard, and has a property called conchoidal fracture. That means that when a little piece of quartz gets rolled or bumped along by the wind or water, it breaks in a special way. It doesn't get sharp corners - it breaks in a very round, smooth fashion. So quartz sand, once its old enough and has been moved around enough, tends to be the roundest, smoothest sand you'll ever find. Then when you pack this quartz sand together, there's space between the little sand grains. Think about what it looks like when you put a bunch of marbles in a bowl. There's still plenty of space in between the marbles for liquid to fit in, even if they're packed as tightly as possible.
So, when you get a whole load of these quartz sand grains together and pack them in tightly, then squish them some more and cement them together to make a piece of sandstone, even if the rock looks solid, there's actually a lot of empty space in it, hiding between the quartz grains!
This space is what oil and water move through. So when someone drills a well down to contact the sandstone the oil or water is in, it happily moves into the well - because the pressue in a well that goes all the way to the surface is a lot less than the pressure all that oil or water is under when it's in a rock, under the ground.
There's a lot more to talk about with sedimentary rocks. Hopefully I'll be able to ramble about them some more, soon!
Mineralogy last semester ended well, though I can't say I'm sorry to see it done. It takes a special kind of person to want to spend all your time staring into a petrographic microscope and thinking about 3-D crystalline forms. I got an A, and I wrote a rather boring paper about Enstatite, an igneous mineral that comes in rather pretty olive colored crystals. (Maybe I should post that paper here so you can look at it and... marvel, if that's the word I want.)
This semester, my geology course is sedimentary stratigraphy.
Sedimentary rocks are pretty much the unsung heroes of the modern age. Well, to be precise, rocks in general are unsung heroes. They just sort of lay there, as far as most people are concerned, and they don't actually do anything...
Except that they do. Off the top of my head, here's what rocks have done for you lately:
1) Kept you from plummeting into an ocean of subsurface magma.
2) Supported your house, roads, office building, etc.
3) Provided some pretty scenary, if you live near mountains.
4) Acted as building materials (or storage for building materials) for at least half the objects you interact with on a daily basis.
5) And so on and so on.
But among the rocks, sedimentary rocks are the work horses for human concern. Now, all rocks are linked together, by something called the rock cycle. Sedimentary rocks are formed by the weathering and erosion of igenous, metamorphic, and even other sedimentary rocks. Weathering is the process by which rock is broken down into little pieces, and erosion is how those little pieces are carried away, most commonly by water, followed by wind and gravity. These little rock pieces are called clasts; they're carried along by the wind or water and eventually dropped somewhere. This is called deposition. When enough clasts have been dropped in the same location, they build up, compact under their own weight, get covered with more clasts, and eventually get squished and cemented into a sedimentary rock.
That's the really simple, basic view of it.
Unlike igenous or metamorphic rocks, sedimentary rocks don't have to be melted or cooked or squished and twisted out of all recognizeable shape. This means that you find an absolute multitude of things in sedimentary rocks that you can't possibly find in metamorphic or igneous rocks. Things like: Fossils (bones and footprints and things like that), oil, and drinking water.
Sedimentary rocks often also preserve ingenious little clues that tell us a great deal about where they were formed and what the Earth was like at that time, and in that place. You can find ripples preserved in rocks, mud cracks, even the impression of rain drops falling on a desert plain in the distant past. These rocks are our window into a time so far back that human beings didn't exist to write down what was happening. Remember, in the lifetime of the earth, we are barely the blink of an eye.
So, every time you go to the museum and look at the dinosaur bones, you're looking at something that was preserved in a sedimentary rock. Every time you put gas in your car, you're using a product made from oil, which forms in shale (a sedimentary rock formed in deep water conditions), and then hides in subsurface reservoirs, most of them found in either sandstone or limestone (also sedimentary rocks). If you drink water from an aquifer, that water often has filtered a long distance through a formation of sandstone, which has acted as a natural filter so it's clean to drink.
Isn't that a weird though, water or oil flowing through rocks? In some of these reservoirs, it's just finding its way through cracks in the rock. But in the case of sandstone, it is literally travelling through the rock. This is because of the way sandstone is made.
Sandstone is made of sand-sized clasts. Now, these clasts can really be any sort of rock or mineral, but most commonly you'll find them made of quartz. This is because quartz is pretty hard, and has a property called conchoidal fracture. That means that when a little piece of quartz gets rolled or bumped along by the wind or water, it breaks in a special way. It doesn't get sharp corners - it breaks in a very round, smooth fashion. So quartz sand, once its old enough and has been moved around enough, tends to be the roundest, smoothest sand you'll ever find. Then when you pack this quartz sand together, there's space between the little sand grains. Think about what it looks like when you put a bunch of marbles in a bowl. There's still plenty of space in between the marbles for liquid to fit in, even if they're packed as tightly as possible.
So, when you get a whole load of these quartz sand grains together and pack them in tightly, then squish them some more and cement them together to make a piece of sandstone, even if the rock looks solid, there's actually a lot of empty space in it, hiding between the quartz grains!
This space is what oil and water move through. So when someone drills a well down to contact the sandstone the oil or water is in, it happily moves into the well - because the pressue in a well that goes all the way to the surface is a lot less than the pressure all that oil or water is under when it's in a rock, under the ground.
There's a lot more to talk about with sedimentary rocks. Hopefully I'll be able to ramble about them some more, soon!
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