My feelings about rock names

A sedimentologist friend of mine just graduated, and last week she was organizing her high school’s rock collection. We got to talking about how rocks are named, and concluded that sedimentary rocks (clastic at least) have the best naming system of all the rock groups (sedimentary, igneous, and metamorphic). This week in the geology 101 lab that I teach we start talking about rocks, so this seemed like an appropriate time to blog about rock names.

Let me take this moment to say that there is nothing serious about this post. I am a sedimentologist. I am extremely biased. There will undoubtedly be many flaws in my reasoning. Just don’t get your knickers in a twist if you’re a geologist and have different opinions. I’m also going to make a ton of simplifications for the non-geologists reading this.

If you’re not a geologist, which I assume most of the people reading this are, I’m gonna try to keep this simple. There are 3 main groups of rocks: sedimentary (sandstone, mudstone, etc.), igneous (volcanic rocks, granite, etc.), and metamorphic (marble, schist, slate, etc.). (I’m hoping you’ve heard of at least a couple of those rock names.) They are all united in the rock cycle. This will be relevant later.

Below are a few sedimentary rock name charts. Rock names often have 2 parts: one for the grain/crystal size, and one for the composition. Sedimentary rock names are the best because they have clearly defined grain size cutoffs. The grains also don’t grow into each other like they do in metamorphic and igneous rocks, so it’s easier to determine size in most cases (this is somewhat of a simplification, but now is not the time to explain cements). When you’re looking at a rock sample, it’s pretty easy to figure out grain size with your eyes, maybe a little hand lens (magnifying glass) and one of many small charts you can take into the field. It’s even easier in a microscope, but generally that’s not necessary to determine the grain size part of the name. If the grains are bigger than 2mm, it’s a breccia or a conglomerate. If you can see sand grains with the naked eye, it’s a sandstone. If it all looks like one uniform thing, and you can’t see grains, even with a hand lens, it’s in the siltstone/mudstone range. That’s it.

Composition for sedimentary rocks (mostly sandstones) is based on the relative amounts of quartz, feldspars, and rock fragments (lithics). A rock’s composition is plotted on a ternary (triangular) diagram, and wherever it falls determines the rock’s compositional name. The label at each corner of the ternary diagram indicates that 100% the rock is made of that grain type. The less the rock has of that grain type, the further away it plots from that corner. Most rocks fall somewhere between the 3 corners. The ternary diagram has nice, easy, straight lines dividing the different names.

The only group of sedimentary rocks that I find a bit confusing are the carbonates. Those seem a bit subjective to me too, but the Dunham classification (chart below) is about as straightforward as it gets.

 

Metamorphic and igneous rocks, on the other hand, exist on more of a spectrum of names. I’ll save igneous rocks for last, because I think they are the biggest nightmare. When I teach the Geo 101 igneous rock lab, my students always struggle. The following week, they have to ID both metamorphic and sedimentary rocks in one lab, and they always find that process significantly easier.

Metamorphic rocks are mostly problematic when you’re talking about slate, phyllite, schist, and gneiss, which are all on a spectrum. Shale (a sedimentary rock made of silt-sized grains) turns into slate when it get metamorphosed by extreme changes in temperature and pressure. Further metamorphism turns the slate into phyllite, and then schist, and finally gneiss. More metamorphism generally means more shinyness, and larger crystals. I kid you not.

So those rocks I just discussed are foliated, which means the minerals align themselves perpendicular to pressure (that’s how you get the shinyness). In the chart above, there are also non-foliated metamorphic rocks. Individually, these are pretty easy to distinguish and name. So metamorphic rock names get points for that. But the foliated rocks are still on an annoying spectrum. And if you want to know more metamorphic rock names, here are a few more:

Finally, we have igneous rocks, which are the most obnoxious to deal with of all. Igneous rocks form when magma cools and crystallizes. Technically, they are divided by crystal size. Big crystals are intrusive, which means they cooled slowly in the crust. Tiny, microscopic crystals are extrusive, which means they cooled really fast on the surface. Seems straightforward, no? But then you throw in the rocks that have both big AND tiny crystals, and sometimes this makes them “porphyritic,” but it’s kind of subjective (in my experience).

Then there’s composition. This is where igneous rocks get their awful spectrum just like the foliated metamorphic rocks. Composition boils down to the relative amounts of dark and light colored minerals. Compositions are curvy and highly variable.

 

Igneous rocks are a nightmare. I don’t understand how people can comprehend how to name them. And I haven’t even mentioned the volcanic igneous rocks, like obsidian and pumice. Admittedly, those are easier to identify, but they can still be confused with things like rhyolite.

These are the primary tools we use to name rocks. I think the sedimentary rocks make the most sense, and perhaps that’s part of why I’m a sedimentologist. You can take a look at these charts and diagrams and draw your own conclusions about which group of rocks are easier to name. If you have any questions about rock naming and identification, leave a comment and I’ll do my best to use my teaching assistant skills to answer them.

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What is sequence stratigraphy?

I went fishing for the first time today, and I’d love to write about it because it was REALLY fun – but I only managed to get one photo, and I technically was in between casts, so I wasn’t really fishing. Just sitting on a boat looking cool with a dog. Oops. So stay tuned for a post about fishing later, after I’ve had a few goes at it (and taken some more photos!).

Instead, I’m going to attempt to explain sequence stratigraphy and my graduate thesis project to non-sedimentary geologists!

First, some Geology 101:

Sedimentary rocks are rocks that are made up of sediment (gravel, sand, clay, mud, etc. derived from other rocks that were weathered and eroded). Water (and other things, like wind, but mostly water) moved this sediment around via rivers and waves and tides. Eventually, this sediment settles down for the long haul and slowly gets buried by more and more sediment. This burial causes the sediment to squish together and compact. At a certain point, the sediment “lithifies” and becomes a sedimentary rock. Welcome to my favorite part of the rock cycle.

Sedimentary rocks are conglomerates (gravel-sized sediment held together by smaller sediment), sandstones (basically sand that has become a rock via the process described above), shales (really fine grained stuff, generally too small to see without some kind of magnifier), and mudstones (the finest grained sediment).

The type of sedimentary rock you’re looking at, the size of the sediment grains, and any sedimentary structures that were preserved (like ripples, crossbeds, planar beds, etc,) can tell you what kind of environment the sediment was deposited in. Composition of the sediment grains (minerals and rock fragments) can also help, but sometimes the grains are too small to determine that without a microscope. You might imagine that a sedimentary rock created by a lake (mostly very fine-grained sediments like mud and clay) would be very different from one created in a beach environment (mostly sand), which would in turn be very different from a rock from a river environment (gravel and pebbles). Of course, all of these environments can be variable, but I’ll get to that in a minute.

 

Stratigraphy is the study of these sedimentary deposits/rocks, and how they are layered.

One more thing before we start putting things together into sequence stratigraphy. Over time, sea level around the world rises and falls for a variety of reasons. Sometimes it’s climate – either global (eustatic) or local. Sometimes it’s caused by plate tectonics – the movement and interaction between crustal plates.

Now, let’s put these concepts together:

I’ve talked about depositional environments, and I’ve talked about sea level change. At the most basic level, putting these two concepts together is sequence stratigraphy. Burial is what ties them together.

I’m going to put this into context with a coastline, because that’s what I work with, and that’s what makes the most sense to me. On a coastline you might have the river meeting the ocean, a beach, a tidal flat, and the deeper, offshore environments.

Imagine you’re on a beach, and sea level begins to rise. Pretend you can breathe under water/sediment, and you’re immortal, so you can totally watch things change on a geological time scale.

First, your beach sand would get buried by finer grained sand from the tidal flat, and as the water continued to get deeper, the beach sediment deposit and the tidal flat deposit would get buried by the deep offshore deposits (really fine muds with maybe a little really fine sand).

That, my friend, is a sequence. If you cut a slice into the sediment right where you were standing when sea level began to rise, you would see this stratigraphic sequence, and the sediment would be getting finer closer to the top. We call this a “transgressive” cycle, because the shoreline is “transgressing” across the land – it is moving landward. Also, the furthest the shoreline extends at the end of transgression creates a surface called the “maximum flooding surface” – hopefully this seems pretty obvious: as sea level rises, you are flooding the environments that were there before sea level began to rise.

Now, pretend you are still standing in the same place on that beach (now buried under quite a lot of sediment), and sea level begins to drop. You might see the return of tidal flat deposits, and eventually you’d see the beach again, and if sea level drops far enough, you might even see the river environments at the very top. This is what we call a “regressive” cycle – the shoreline is regressing away from the land and moving seaward.

If you were to step back and take a slice out of this whole sequence I have described – from the first beach deposit to the fluvial deposit, and then studied how these depositional environments changed laterally and vertically, you would be studying sequence stratigraphy.

Of course, it is a TON more complicated than this, especially since these processes are often erosive, so you don’t always get a perfect sequence that records an entire cycle of sea level rise and fall, but hopefully you get the idea. There are also these things called “significant surfaces” (the maximum flooding surface is one of them), which help us define sequences. They are usually created by some form of erosion – either transgressive or regressive, but involve some kind of shift in either the direction or speed of sea level rise or fall.

One important aspect of sequence stratigraphy is the source of sediment, and this is where the focus of my thesis project lies. You can hopefully imagine that river sediments come from somewhere upstream, while beach or tidal flat deposits might be sourced from somewhere else on the coastline, or they might get sediment from the ocean. Sediment comes from all over the place.

In my field area, the previous graduate student identified three different sources of sediment. I’m going to be looking at the composition of all the sandstones (sorry, sand can be found in many different environments, try not to thing about it too hard) in my rock formation and comparing them to see if there are significant differences between these different sources – and if the sands from the same source have similar compositions. Again, it’s a bit more complicated than that, but that’s the general idea. Also there are GREEN minerals in my sandstones (not a common sedimentary mineral color). I get to identify them – I’m pretty stoked.

Sequence stratigraphy is like studying history, but it’s history of the earth rather than of people, and that’s what I love about it. Sequences of depositional environments is very intuitive to me. Plus, looking at things in a powerful microscope (a few different kinds actually), is really fun.

That thing up in the right corner that looks like plaid? That’s called “tartan twinning.” It’s a potassium feldspar grain. It GREW like that. Plaid is found in nature, guys. Chew on that.

If you’re curious, or you need me to explain something differently, please feel free to leave questions in the comments, and I’ll do my best to help you understand! This stuff comes as second nature to me (and I already find it fascinating), so it’s difficult (as any specialty can be) to break it down and keep it interesting. I hope you at least learned something about sedimentology by reading this post.

 

Fun fact: “Sedimentology” is not recognized by computer dictionaries. My entire area of study does not exist to technological devices.

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Cutting rocks with a diamond saw

Yes, I mean actual diamonds. I cut rocks with diamonds. Be jealous.

Ok, so they’re REALLY tiny (microscopic, even), and they’re synthetic, so it sounds a lot cooler than it actually is.

Oh, who am I kidding, cutting rocks is one of the best things about being a geologist. Especially when you’re cutting sedimentary rocks with a diamond rock saw – it’s like slicing butter with a hot knife. If someone ever asks you if you want to try cutting a rock, just say yes.

Last Friday I got to cut some rocks as part of my thesis work. I’m going to attempt to explain my thesis project in non-geologist terms in a later post, but right now I just want to brag about cutting rocks and feeling a little bit like a god while doing so. This particular batch of rocks were all from a fresh rock core (someone drilled a tube into the subsurface and pulled out a cylinder of rock – essentially). The core was also thankfully already sliced in half. You might imagine that flat edges would make rock cutting much easier, and you’d be right.

This is what the rocks looked like before I started slicing them, except for the 2 at the top. The goal is to cut a thin section “blank.” They go by many names (billet, chip…), but the piece you cut before it gets shaved down enough that light can pass through it under a microscope. They’re roughly 1″ x 1 & 7/8″ and about half an inch thick. Then we send them off to a lab where everything is standardized and we get a bunch of perfect thin sections returned like magic. And then I have to count 70,000 individual grains, among many other things. But I’m getting ahead of myself.

The rock saw! It’s basically stationary, and you put your rock on the rack, and move the rack under the blade. That chip sitting on it is a typical thin section blank. Fun fact: it’s pretty difficult to cut yourself on this blade, even though it’s designed to cut rocks. It’s actually pretty blunt – about 1/16″ thick. I meant, don’t get your finger trapped between the rock and the blade, but you could probably hold your finger right on the blade as it spins and it wouldn’t cut you.

Look! I’m doing science! Literally just sliding the rock into the blade, and it just cuts. You gotta go slow, so you don’t fracture the rock or damage the blade. But not THAT slow. At least, not with sedimentary rocks. We were able to cut about 15 samples in about 3 hours – and that includes refilling water buckets and labeling everything. I have to cut about 85 more though… going to be a busy few weeks.

In the end, this is what we had left. I failed to take a photo of any of the actually blanks, because… I have no excuse, it just didn’t happen.

Honestly, I am just really excited to get this part done. Microscopes are fun. Probably I’ll change my mind about this after I spend many hours staring down into them, but rocks look really cool in thin section. I’ll hopefully post some photos of that when I get around to that process. My project is mostly a sedimentary petrology deal (petrology = looking at rocks under a microscope and identifying minerals and figuring out where the sediment came from), and I am just really anxious to get to the data collection part. Collecting and preparing samples is only fun for the first few days, in my opinion.

Then there are the big cabinets in the lab that just say “ACID” on them in giant red letters…

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Thesis Update

The internship is over, and I’m back in Montana. A mere 24 hours after I got home, I went out to my field area, and I’m still there. I have had so much to worry/think about over the last week I hardly knew it was Sunday. I actually kept forgetting it was Sunday until I realized, at 10:30pm, that I needed to write a blog post. I don’t have time to write a whole bunch of words like I normally do, I don’t have time to put together some spam poetry, and the mosquitos are eating me alive (seriously, I look like I have chicken pox right now). Also someone just walked up to me and asked me for the wifi password at 11pm. It is time to go to bed.

Instead, I’ll give you some photos of field work this week!

This was the view from my lunch spot in a tiny patch of shade that only got smaller as we sat there. We then proceeded to climb to the top of the cliff, which is higher than what you see here.

 

This is a trace fossil! I think it’s paleophycus, but I’m terrible at trace fossils. So. Um. Worms did this. In the Cretaceous.

 

 

This is my fancy artistic attempt at a panorama view of my field area. That little bit of civilization out there beyond the cliffs? That’s where we left our car. This is the top of the ridge that you can see in the first photo. There was more of it behind us.

 

This is a bug. I don’t know what kind of bug, because I am not a bug scientist. But possibly cicada? It’s about an inch long plus wings. Much prettier than those giant cockroaches I saw in Houston. If you are a bug person and can identify this, please leave a comment! It died outside our apartment, and I think it was stuck there for a couple of days, making loud noises in the middle of the night. That, or there’s some giant cricket living in the cabin walls trying to torture us in our sleep.

Welp, that’s my thesis update! More to come later when I am not surrounded by flying insects. I am so ready to be home, but I’ve got 4 more days out here. Be glad you are not me right now. I have slept in my own bed exactly once in the last two and a half months.

 

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