Monday, October 13, 2008

Analogies, analog modeling, and squashed chocolate

While I was away, Callan asked about geobloggers' favorite analogies. Several other people responded. I'm a week late to the party, but I want to play, anyway.

A few weeks ago, I bought a chocolate cookie from one of my former intro students at the local farmers' market.

"You like chocolate, don't you?" she asked.

"Ummm. Yeah." (I didn't have the cookie in my mouth quite yet, but I may have been drooling.)

"I remember you talking about it all the time in our class. Especially about leaving chocolate in a hot car or putting it in the freezer."

So when I came back from GSA, it was time to give my (one, tragically brief) lecture about rock deformation. (That's the sad thing about teaching Earth Systems Science. I can't spend weeks talking about my favorite things.) It's maybe a bad idea to try to explain one's entire specialty in a single lecture after spending week listening to experts talk about the newest, coolest stuff in one's field. In any case, I was having a harder time than usual distilling the subject into its most basic essences.

When I've only got one lecture to do all of structural geology, I spend most of my time talking about faults. Faults can have earthquakes; earthquakes can kill (or at least make life very unpleasant). But I try to explain that ductile deformation exists, and the whole bottom half of the continental crust (not to mention the mantle) deforms that way.

I don't try to explain ductile deformation with rocks. It's hard to imagine something as hard as a rock squishing, even when the rock looks like this:

Photo: disharmonic folds in marble below the Snake Range decollement

So I resort to describing other materials. Silly Putty is great for describing ductile behavior, but it doesn't explain how cold materials behave differently from hot materials. (Well, I don't think it does. I've never put my Silly Putty in the freezer.)

Chocolate, on the other hand, is perfect. Put a chocolate bar in the freezer overnight. When you take it out, you need a rock hammer (or some other implement of destruction) to break off pieces. Cold chocolate is brittle.

Leave the same chocolate bar in your car, though - not tonight, because it's cold out tonight, but maybe during the day tomorrow. Take it out. If it's still solid, it will probably bend in your hands without breaking. (It might have melted, in which case it's now igneous chocolate, which has its own appeal, but which messes up the analogy.) Warm chocolate bars are ductile.

I've used this explanation for years, and normally I get exactly the responses I want. And this year, the students described the cold chocolate just fine, but when we got to the warm chocolate bar...

"What kind of chocolate?" they asked.

I shrugged. "What kind of chocolate do you want?" I generally prefer dark chocolate, but milk chocolate will do, as well.

"How about a Snickers bar?"

I frowned. "You know, a Snickers bar is great in the field, especially if the bears don't eat it, but I think its behavior is a bit too complicated for this analogy."

"Ok, then, what about a Milky Way?"

The point was not getting across. "No, a Milky Way is still made of too many different things, with different behavior. I think we should make our model as simple as possible to begin with. Because, umm, we want to avoid edge effects or something." I am not a modeler myself, but I had just gotten back from GSA.

"Oh. You mean just a Hershey's bar?"

"Yes. Yes, a Hershey's bar will do."

"How about if it has almonds in it?"

"NO! Just a Hershey's bar."

I have a horrible feeling that the other students in the class will write very confusing essays about caramel, nougat, and almonds. Or at least that they will threaten to unless I feed them.

Edit: I almost forgot that I've seen an example of class experiments deforming candy: the deformation of Charleston Chews, used in Structural Geology classes by Arlo Weil from Bryn Mawr. It's a great experiment/demo/in-class exercise... but I'm told that the smell of the candy makes people feel sick. And I'm trying to get students to make positive associations with rock deformation. So maybe pure chocolate is better for the thought experiment.

1 comment:

GeologyJoe said...

When I was in college I had a CCTV show "the half-hour knowleged hour". One episode I did expirements with silly putty, one of which was freezing it then smashing it with my rock hammer.

It shatters like the homogeneous solid that it is.

ah the good ol' days.