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.
Making My Own Worlds 