Tuesday, January 15, 2008

What geological concepts are "thresholds"?

While cleaning out my office for the beginning of the semester, I ran across an article in the November 2007 Journal of Geoscience Education on "threshold concepts," and I've been wanting to talk about it for a couple weeks. The article itself isn't going to be available online until next year, but there are lots and lots of related articles here.

According to the authors, a "threshold concept" is something that is difficult to grasp, but which transforms one's understanding once understood. It makes other things make sense, but only if the student really gets it. If the student doesn't get it - well, maybe he/she will get by with parroting answers, but it will be difficult to use the concepts.

A meeting in the UK came up with a list of some possible threshold concepts in the earth sciences (including geography). It's a really long list, and I'm not sure I buy all (or even most) of it.

But I'm curious what you think. Professors, post-docs, grad students, undergrads, interested non-geologists: what concepts have made parts of geology suddenly make sense to you? And do you think that there are any universally confusing concepts, or would the list depend on where a person grew up, and what a person learned as a child, and what kinds of cultural stories the person believes?

For instance, I always tell geology majors that there are two classes that, in my opinion, really transform their understanding of geology. Mineralogy is the class in which rocks are transformed from dull grey things into storytellers. Structural geology is the class in which geology acquires a third dimension (or at least, a third dimension underground). But I may be biased - after all, I do structural geology on metamorphic minerals, and clearly I'm the one interested in the coolest and most fundamental stuff in the discipline!

And my big ah-ha moment in my introductory class had nothing to do with minerals or structures. It had to do with correlation of flat-lying sedimentary rocks. I simply couldn't believe that it was possible to know the relationship between rocks under two different hilltops - at least, until I went on a field trip and made a map and saw it for myself. But then I grew up in Maine, on top of glacial till and multiply deformed metamorphic rocks, and flat-lying stratigraphy was not part of my experience. Somehow I expect that my students from the Colorado Plateau don't have that same problem.

So I'm curious: what were your big ah-ha moments? What concepts made it all suddenly make sense?

Because if we could really figure out what the fundamentally important stuff is, I could spend the time on it, and leave out things that fall into place more easily.

Reference: Stokes, A., King, H., and Libarkin, J.C., 2007, Research in science education: threshold concepts: Journal of Geoscience Education, v. 55, n. 5, p. 434-438.


MJC Rocks said...

Great post, it really got me thinking, way back. I still have a vivid memory of being 12 years old, and visiting the north rim of the Grand Canyon, and collecting fossil crinoids in the Kaibab Limestone. That wasn't the threshold moment, but the question that nagged at me, unanswered at the time: why were these ocean fossils at 8,000 feet on the top of a dry plateau? Years later, in an intro geology class, we did one of those simple geologic time exercises with cash register tape, with the dates of significant geological events compared to important human events. In that moment, I came to undertand how changes and movements amounting to millimeters per year could amount to mountain ranges and shifting continents. And how those fossils ended up on top of a plateau.

It's a pretty simple example, but I also realized just now how the lack of outdoor education damages the learning process. Kids who have never seen rocks in their original setting, who've never picked up a fossil, or never explored a cave, have a hard time even coming up with those nagging questions that lead to threshold moments years later.

Callan Bentley said...

For me it was the concept of deep time. As Herodotus said, "if one is sufficiently lavish with time, everything possible happens." At some point during my undergraduate career, I remember thinking, "Damn: the Earth is really, really, really, really old." I recall sitting there, my mind reeling. "Whoa."

As John Playfair noted during his visit to the unconformity at Siccar Point, I felt a yawning sense of vertigo. Time is DEEP. It gave me a perspective on life that has never left: not just geology, but also a reality check on the ups & downs of everyday life.

Great post, Kim. You're good at this.

ScienceWoman said...

It's a big concept but the one that I struggled with in my intro geology class years ago was plate tectonics. It's one I'm struggling again with now in my teaching. I teach an intro physical geography course and the powers that be have declared that my course shall not have plate tectonics (because that's geology, not geography). But how can I talk about things river and coastal terraces without directly invoking tectonics. And it even comes up when we do the carbon cycle. It's very frustrating. So that's the first threshold concept that came to my mind.

Thermochronic said...

This sounds simple, but I remember doing block diagrams and figuring the relative timing of events based on cross-cutting relationships, unconformities, etc. For some reason, this lab we had in my first geology class totally clicked with me. I still imagine block diagrams in my head when I am trying to put things in order. Semi-related was my first experience palinspastically restoring a cross section. Perhaps this points to the way I learn things visually, but doing that for the first time all of a sudden made geologic maps, field work, everything really, just make sense.

Tuff Cookie said...

For me, I think at least one of those moments might have come during a field trip to the Colorado Plateau my freshman year (yes,that one again; turns out it was pretty life-altering).

We were working on a mapping project in Sunglow Park in Utah. I had been having a tough time of it, since I knew nothing about field mapping and not much about identifying rocks. We'd been told to memorize the stratigraphic column for the areas we were visiting, and I'd been working on it for some time, without a whole lot of success. At one point, we sat down on a fault contact between some volcanic and sedimentary units, and were discussing the map patterns, and something definitely clicked. I put the stratigraphy into context, made connections between the different sedimentary units and realized that the fault was related to a system that we'd been exploring for the past few days. I finally saw the area as a whole. That was the big moment: everything on the ground coming together to tell a story about the area.

Of course, this was after a great deal of frustration on my part, mostly because I was inexperienced, and I didn't appreciate it much at the time (being hot and tired and wishing my professor would fall off a cliff). But in retrospect I realize that that particular mapping exercise was an excellent one, and helped me develop a sense of geology not just as a series of individual events but a flow of continuous changes, causes and effects.

BrianR said...

Kim...this is a fascinating topic and thread...I have no time to add my $0.02 right now...soon, very soon

Anonymous said...

Definitely did not have a 'sputnik' moment all at once (at least not yet), but understanding plate tectonics in general terms, in concert with geochemical cycles really brought my understanding together of the entire Earth system and how it differs from other planets.

Actually, Bowen's reaction series is another good example, both in rock formation and weathering processes.

Phase diagrams as well - the (lack of) relationship between quartz and nepheline, for example.

Hah, if I thought about it some more, I'm sure I could keep going!

Kim said...

Thanks, everyone! This may be a reason to figure out if blogger's poll capabilities allow people to choose more that one option - I'm curious how many people agree on these.

mjc and Callan - the concept of geologic time was one that the UK conference had on their list, and one that I agreed with. I don't remember when geologic time first made sense to me - I don't remember an AH-HA moment. And I've done work with geochronology and rates of tectonic/structural/metamorphic processes, so I must have had to think about it. Hmmm.

Sciencewoman and anonymous - I wondered whether plate tectonics would be, or not. I didn't know about plate tectonics until I got to college in 1985, so it was definitely a big deal for me. But I wondered if students who are more than 20 years younger than me take it for granted - if it's a threshold concept, do a lot of kids pass that threshold in grade school? Or do the kids learn misconceptions?

(Sciencewoman, you should ask the Powers That Be if you can discuss gravity in physical geography, because that's physics, after all. Actually, I understand why there's a problem - the lines between geology and physical geography are blurry, and therefore people overemphasize the difference to justify teaching both disciplines. And geography has a rough time making non-geographers understand that they teach things beyond state capitals. But economic geographers must talk about supply and demand... don't they?)

Map patterns and block diagrams are something I'm dealing with (in some ways) in structure labs right now. And, yes, definitely a threshold concept.

Phase diagrams, too. A lot of geologists never quite get them (which may be a sign of a more advanced threshold?).

Tuff Cookie said...

Oh, phase diagrams...bane of my existence. I suspect my difficulty in grasping that particular concept is going to come back to haunt me.

Kim said...

Tuff - yeah, if you're going into volcanology, understanding phase diagrams probably will come back to haunt you. But if you're interested in going into academia, I've found that I do a better job in teaching concepts that I struggled with. Ideas that made too much sense to me... well, I have trouble figuring out how to explain them to people who don't find them intuitive. (Perhaps that's one of the problems with math classes, particularly for students who struggle with math?)

yami said...

It's possible to skate by as a physical volcanologist without looking at very many phase diagrams, if you happen to like fluid mechanics.

Euler poles were both tricky and useful for me. Stereonets were tricky, but not so useful. I think the biggest deal for me was basic elastics - the relationship between stress and strain (hell, even getting the concept of strain as a dimensionless number), and particularly figuring out what shear really means. It wasn't explicitly taught in my undergrad curriculum - the geo department assumed the physics requirements would do it, I guess, but the physicists were more interested in teaching us to wank around with Hamiltonians - and I had to bang my head against it in several contexts until it finally made sense.

The Lost Geologist said...

My best a-ha moment was when I realised the connection between plate tectonics, bowens series, magma differentiation and the formation of mineral deposits. Of course that didn't all happen in one day.

I never quite understood it (plate tectonic, differantion, etc.) until I began getting into mineral deposits in detail. Before that everything appeared like totally unrelated issues but I saw that they are all inter-connected. So I would call my threshhold concept the realisation that everything is connected. Everything has a reason.

saxifraga said...

I'm late here, but still wanted to add my opinion. I think the 3D perspective the single concept I have struggled the most to grasp. In undergrad it took a while before I really learned to see the axes on the wooden models of mineral structures and I also had difficulties with structural geology. As someone who is now a specialist in sedimentary architecture/ geometry, which is highly three dimensional, it turned out I was able to learn it, but for me it was a threshold.

I don't know if it's an universal threshold, but I do think that the main concepts that many struggle with are perceptions of time, shape, continuity that are rarely used to this extent outside of geology. I guess if I had been artistic as a child and painted a lot I might have had a better grasp of the three-dimensionality.

GeologyJoe said...

StereoNet Diagrams were really the first thing to make me say " whoa, so all this data CAN tell and show me something."

Mohr Circles i still fumble with.