Stratigraphic Columns and How to Keep Your Cool

 

Before going into the field my fellow classmates of 10 and myself received a lesson on stratigraphic columns and how to create one out of an outcrop. Needless to say I was an expert after this one lesson and fairly confident in my capability to rock this. We strut out into the field with our day packs, field notebook and ipads ready to show this massive collection of rocks who’s boss. Well, little did I know that was not me.

It had been only but an hour or two before I began to feel the stress coming on. The beds were becoming harder to define. The sun was in prime position to beat down on hour heads. Some had already taken defeat and were beginning to grow red burnt splotches on their skin. Dripping sweat turned into dried salt stinging our faces. I was slowly but surely beginning to lose confidence in the assignment as I stared at the rocks beds above me. Then the TA approached me asking to take a look at my notes. He then explained to me that my drawings were incorrect. It was then I could feel the real frustration brewing. I wanted to give up but instead I took a breather and got back to work.

To avoid this kind of frustration for later generations I have outlined a wiki-how on “Keeping your cool while designing stratigraphic columns”.

Step 1:
Before you go out into the field you will need to collect these necessary items.
A grid paper notebook
a grain size card
a meter ruler
a hand lens.
Things highly recommended would be:
a full bottle of sunscreen (unless that is you want to burn)
full water bottle

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(Figure 1) All supplies needed to prepare stratigraphic columns in the field

Step 2:
Draw out the stratigraphic columns. It is recommended you draw a couple of these before you get out into the field, this will help reduce stress levels later. The graph consists of 5 columns. The first one marking which bed you’re focusing on; the second is the height of the bed; the third is going to be a sketch of the bed; the fourth column is dedicated to detailed descriptions in macro, meso and micro scale and the fifth is your interpretation how the rocks came to settle that way. The vertical axis will be the height in meters of each bed and the horizontal axis will be the length in millimeters of the actual clasts themselves.

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(Figure 2) An example of how a stratigraphic column should be drawn

Step 3:
This is where the real fun begins. Settle your things on any piece of land. Try not to spend time trying to find a less ashy place, there is no such thing, trust me. Now that you’re in front of your outcrop step back about 30 feet and begin to look at it from a macro scale.

Take down description of color, for example, is it tan, maybe greyish? Perhaps there is staining in the pumice due to oxidation. Then take down the different contact or lenses you find. Contacts separate the different beds from each other and each bed represents a different volcanic event. A sharp contact is one of the easiest to see. It is a clear change in the event that caused that layer. And erosive contact happens when a stream starts to cut down on rocks and can cause a truncation or dip in the contact. A gradational contact comes from slow changes of processes of the eruption switching back at forth.

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(Figure 3) Sam looking at an outcrop in a nearby quarry

Step 4:
Good job you’ve made it to the fourth step and you aren’t confused yet! Or maybe you are, but that’s all part of the fun! This is just the beginning so get strapped in. What we are looking at now is the mesoscale. Now we must examine what types of rock compose the bed we are looking at.

Get close enough to touch the bed. Pull out the meter ruler and take down a measure of the height of the bed you are examining. Then determine what kind of rocks make up the bed. Then take out the handy dandy grain size card and begin to find an average size for each type of rock. From here we are going to determine the amount of lithic fragments. This will help us to tell whether the bed is well or poorly sorted. This can be found on your grain size chart as well. If it is well sorted everything will be close to the same size, if poorly the grain sizes will be varying highly along with a large amount of lithic chunks.

From here we can also see how the clasts are supported. If we describe them as clast supported then all of the rocks of the same clasts are touching. If they aren’t, then they will be matrix supported. You will then determine the type of grade the bed has. Normal grading is when the larger rocks are at the bottom and the smaller are on top. Reverse grading is when the smaller rocks are at the bottom and the larger are toward the top. This can happen through water saturation or how efficiently the gas is released during an eruption.

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(Figure 4) Hanako preparing the ruler to measure a bed

Step 5:
Don’t worry the stuff dripping down your back is not collective ocean spray but the water you drank evaporating out of your skin. But its okay, take a sip of water and reapply sunscreen to your crispy skin and let’s continue.

This next step consists of looking at the clasts through the microscale. Take out your hand lens and begin looking at sample rocks in order to describe them in finer detail. Are there any vesicles in the sample? What about phenocrysts? Both of these descriptions tell us in what conditions these rocks were formed. For example, if there are many vesicles present it means that the magma was gas rich and pressure was rapidly decreased when the volcano erupted. If there are phenocryts it means that the rock cooled slower allowing it time to form crystals.

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(Figure 5) Sam uses a hand lens to take a look at the vesicles of a piece of pumice.

Step 6:
Now that you have analyzed the bed you need to begin to draw it. First determine a set key to let someone know the significance of a squiggle (pumice) or a group of dots (ash). In your drawing you will mark the height of the bed and the variation in rock length set by the vertical and horizontal axis respectivley. Remember in order to show the size of the rocks you must draw the bed to size with the millimeters on the bottom. If you find that the bed also had lithic fragments make sure to add those in too.

While drawing the bed you want to make sure it accurately represents exactly what you are seeing. So if there are altering layers you would want to put altering layers of dots and squiggles in the drawing.

This part of the stratigraphic column might take a while but don’t worry no one gets it right the first time.

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(Figure 6) A fully drawn stratigraphic column. The two squiggle represent pumice, small dots are ash, and dark odd shape pieces are lithic fragments

Step 7:
You have made it to the last step. Wipe off that sweat and wear your gnarly tan lines with pride. In this step you will write an interpretation of the bed and how you think it came to be. For example, “The bed is reverse graded because the pumice fell out of suspension from water, causing small pumice to saturate faster and bigger pumice slower”. Everyone’s interpretation will be different since this part is more opinion based.

Congratulations, you have now successfully created a stratigraphic column! To make another just repeat steps 1 through and remember to take deep breaths in between each step. Good luck and happy stratigraphing!

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6 thoughts on “Stratigraphic Columns and How to Keep Your Cool”

  1. Becca – I thoroughly enjoyed reading this. It has so much personality and really brings home the challenges (both mental and physical) of making strat columns. Great job.

  2. Hi Rebecca,

    I enjoyed a lot of the language of this post, and the overall tone was humorous and engaging. You established the informal tone well in the first paragraph, specifically the last line. One thing I noted that seemed a little out of place was this line “Needless to say I was an expert after this one lesson and fairly confident in my capability to rock this” because it seemed a little sarcastic when what is around it is self-deprecating but informative.

    The format of this post was unique, and I think it was really effective. The wikihow format allowed you to really clearly state your main ideas and insert a lot of personality into the writing. When you write with so much enthusiasm reading becomes much more interesting for your audience, so this was an excellent post.

    Something to keep in mind for these posts is that your audience may not know as much as you–I definitely don’t–so it helps to define unfamiliar terms like macro, meso, micro scale, clasts, vesicles, and phenocrysts. Defining terms like these helps the audience understand the terms themselves and the context they are in, and it can help you to understand them more by defining them in your own words. Something else that you could more clearly define would just be the actual assignment you were working on: what are stratigraphic columns and what do they show. It’s really interesting stuff, so your audience will want to know what you’re up to.

    For future posts, something you can do that really helps with editing is to read each paragraph out loud. I do it constantly, and though it does come at the cost of appearing only a little insane, when you read out loud you can find all the little stylistic hang-ups that you may have accidentally glossed over. One instance in which it could have helped to revise out loud was “From here we can also see how the clasts are supported. If we describe them as clast supported then all of the rocks of the same clasts are touching.” What I found in this section was that not knowing the definition of clast meant it was difficult to know the meaning of the sentence, and the second sentence seems awkwardly worded. Revision is an essential, if painful, part of writing and it helps immensely. Really fun post this week. I look forward to your next ones.

    Justin

    1. Hi Justin,

      Thank you for your input! I really appreciate the the insight. I guess when you work with the terms everyday you forget others might not know. I will make sure to define certain terms in my next blog. Thank you for taking the time to read and comment my blog, again I appreciate the advice!

  3. Hi Rebecca,

    Making a wiki-post entry: genius. While I’m not a geologist by any means, your subtle humor kept me interested the entire time, and your layout definitely made this post easy to digest! What I mean by this is the different sections accompanied by a picture is simple but efficient.

    As I stated above, the visuals definitely helped, and it gives viewers a chance to rest their eye from a chunk of text. However, another helpful visual could be pictures or diagrams of specific terms or locations. For example, I didn’t fully understand what you meant by “contact” and a picture might clear things up.

    Along those same lines, you defined only certain terms throughout the post. By adding more definitions, you can more easily reach out to readers who are trying to create a stratigraphic column for the first time. It’s important to keep in mind who your audience will be. Another geologist? A student taking a geology course? A retired graphic designer looking to become a geologist as a hobby? While a blog post might be less formal than a research paper, you are still explaining research-paper concepts. By keeping formal and informal qualities, you can reach out to all sorts of readers, or use language and style to pinpoint a specific audience.

    All in all I enjoyed reading your post and I can’t wait for the next one!

    1. Hi Nicole,

      Thank you for taking the time to read my blog. For my next blog I’m going to be a bit more thorough with making sure terms are better defined. I will also keep in mind drawing on pictures to show ideas more clearly.

  4. Hey Becca,

    HA! Your first paragraph fully captures a geologist’s first strat column. Welcome to the club.
    This is a great explanation of the process behind creating strat columns. Your steps are clear and well described. One thing to incorporate in steps 3-5 are where you are recording these descriptions at the varying scales. The figures you included were perfect for the visualization of what the macro-, meso-, and micro-scales look like, but additional pictures of how your strat column develops during each step would be invaluable to your portrayal of how to create a strat column.
    As you begin to explore more of the island, what are some advantages to creating strat columns at each stop? Why should geologists make more than one strat column even though they’re looking at what should be the same deposit from the same eruption?

    Well done.

    Alex

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