November 9, 2010

Ternary Personalities

Posted by Evelyn Mervine

Ternary personality diagram of Dartmouth 2005 field camp (“The Stretch”) members.

Ternary diagrams are frequently used by geologists to classify rocks. A ternary diagram is a triangle that has three end-member compositions A, B, and C at the points. Rocks identified using such a diagram are plotted in terms of their proportions of A, B, and C. Depending on where the rock falls on the diagram, it will have a certain name. Ternary diagrams are a little confusing, at first, if you’ve never seen one before. The best way to learn is through examples. After awhile, you’ll develop an eye for reading ternary diagrams and will be able to easily estimate where a particular rock should plot.

An excellent description of how to read a ternary diagram is located here:

Andrew Alden discusses ternary diagrams for sedimentary rocks here:

For my thesis research I am studying alteration of the Samail Ophiolite, a fragment of uplifted ocean crust and mantle located in Oman. I work primarily in the mantle section, so I mostly use this particular ternary diagram for classifying mantle (or ultramafic) rocks:

IUGS classification diagram for ultramafic (mantle) rocks. From Wikipedia Commons.

To use this diagram, I estimate the percentages of the minerals orthopyroxene, clinopyroxene, and olivine in my peridotite rocks and then figure out where my rocks plot on the diagram. Most of my samples are lherzolites or harzburgites, though I have the rare dunite. Since I work on altered peridotites, the hard part is actually estimating what percentage of minerals there used to be in my rocks when they originally formed. Most of the olivine is long gone and has been replaced by red clays. Note that the green field in the above diagram represents the range of values typically seen in mantle peridotite rocks. Wherlites are relatively rare. I remember this by thinking, “Where (or wher) is the wherlite?”

Of course, you not only have to develop an eye for how to read ternary diagrams, you also have to develop an eye for estimating percentages of minerals or grain types that are present in the outcrop or rock that you are trying to identify. Again, after awhile you become fairly good at this. I actually find that if I work on sample descriptions in lab for a long time, my eye becomes well-calibrated. I also start trying to estimate the percentage compositions of EVERYTHING. For instance, a few years ago I was on a research cruise and spent much of my time estimating the percentages of plagioclase and olivine in basalts during routine sample descriptions. After one really long day describing rocks I, finally, went back to my cabin. As I was brushing my teeth before bed, I found myself absently estimating the percentage of blue flecks in the white bathroom tiles.

However, if I haven’t had to estimate mineral percentages for awhile I find it’s best to use a cheat sheet. My go-to cheat sheet is found in the back of the trusty “Rite-in-the-Rain” All-Weather (well, perhaps not wintry mix) Geological Field Notebook. This field notebook has a great plastic scale and pages of useful information for the field geologist. Included are sedimentary ternary diagrams and- gasp- a quaternary diagram for classifying igneous rocks. There is also a size chart for identifying sedimentary grain sizes. Importantly, there are two pages (below) that can be used for estimating percentage composition of grains and/or minerals. Very useful!

Rite-in-the-Rain All-Weather Geological Field Notebook, turned to the percentage composition diagram, with its own plastic ruler for scale. The perfect gift for any geologist! Photo taken November 2010.

So, now that I’ve talked a little about the conventional way that geologists use ternary diagrams, let me discuss a less-common way that geologists use ternary diagrams: to classify personalities! Next time you in the field with a group of geologists (or anyone, I suppose, but if they’re not geologists they might think you’re weird/nerdy) or taking/teaching a geology class, you can make your very own ternary personality diagram!

Making such a diagram is fairly simple, but generally goes more smoothly if delegated to two or three enthusiastic and perceptive members of your group. I suppose that you should also make sure that your everyone in your group is okay with having their personality classified. The first picture in this blog post is a ternary personality diagram made by Dartmouth geology students during the Fall 2005 field program. You may notice that I am roughly 55% Iona, 40% Lisa/Jon, and 5% Hannes. Dartmouth students, especially geology students, are pretty chill, so no one in our group seemed to mind their personalities being classified.

Here’s how to make your very own ternary personality diagram:
1. Find a group of geologists who are related in some way. They could be on a field trip together, taking the same geology class, or perhaps students/professors in the same department. Generally, it’s best to make the diagram after the group members have spent some time together and have become familiar with each other’s personalities. Field camp is the perfect environment for this!
2. Figure out the three members of the group with the most extreme/different personalities. Draw a ternary diagram triangle with these three people as end-members.
3. Plot everyone else on the diagram, based on what percentage of each end-member personality each person has.
4. Drink beers/soda and eat pizza and laugh about how similar/different in personality you are to the other members of your group.

I hope you have enjoyed my geological musings so far! Late Wednesday night I leave for a geology-themed wedding in Costa Rica. Two dear friends of mine are going to be married near Arenal volcano, so I’ll be out of town for a few days. However, I promise to post volcano pictures upon my return!