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!

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