Monday, January 14, 2008

Hot, gooey fillings

My least favorite geological misconception would have to be one that I come across a lot as an aspiring volcanologist: The Earth's mantle is a molten sea of liquid, and the crust "floats" on it.

Now, I suppose this is a perfectly good way to provide a simple explanation to young children who are too young to understand the rheology of rock in the mantle, or the fact that it can flow and not be a liquid. (I certainly believed it when I was little. Mind you, this was pre-elementary school and all I had to go on was "The Magic School Bus Inside The Earth.")

But things are not so simple! (Insert totally fake tone of wonder and awe here.) While it would be a whole lot easier to call the crust solid and the mantle molten and leave it at that, it isn't accurate. Better to divide the Earth up by the way things actually are (and behave) - using the lithosphere and asthenosphere (or aesthenosphere, which I prefer but my spell-checker doesn't) and their associated subdivisions. The asthenosphere being, of course, the fun part, where rock actually deforms plastically - in short, flows - and where only a few percent of the material is actually molten.


Gasp! You mean what my kindergarten teacher taught me (and what a lot of people I come across seem to believe) is a lie? I was being deliberately decieved? All those layered models of the earth that are so diligently trotted out and explained in three (or occasionally, if we separate the core into an inner and outer core, four) layers are totally wrong?


Well, yes and no. I wasn't being taught the correct structure of the Earth (at least right off). But when I think about how difficult it is to explain the concept of rock (or a solid) that flows to your average elementary-age child, I suppose I can't get too upset. And since few people seem to get much instruction in Earth science beyond their elementary or middle school years, it's easy to see why they cling to the "simple" explanation of the Earth's structure.
Unfortunately, unless they take an upper-level course in Earth science in high school or college, there's not much opportunity to correct this. TV shows, especially on the Discovery and Science channels and the like, seem to be taking a good stab at it, but again, it's a limited audience - and those shows are totally overwhelmed by all the other stuff that's on the air nowadays.

Anyway, maybe it's easier to just say that the mantle is molten rock and leave it at that, but I would appreciate it if the science teachers of the world (those that don't already do this, that is) would take a stab at explaining that while the mantle is actually solid, the heat and pressure involved at that depth make it behave like a liquid. Much as I like having the opportunity to impart a little geologic knowledge to the people I meet, this particular misconception isn't particularly difficult to head off before it becomes permanently implanted in someone's understanding of the planet they live on.

Of course, this means the teachers had better be prepared with some material to help answer the inevitable "Why?" and "What do you mean, a solid can flow?" questions, but there's nothing wrong with a healthy spirit of inquiry and they might even start some bright young minds on the path to a career in the geosciences.

As for pie and the Earth Sciences...well, I have the perfect example.

As you might have read in an earlier post, my first real experience with geology was on a 3 1/2 week-long field course to the Colorado Plateau. We started out in the Guadalupe Mountains of Texas and made our way up through New Mexico. On our way into Arizona, about 80 miles West of Socorro, NM on Highway 60, we came across a wonderful place:

Pie Town, New Mexico!

Yes, by golly, there's actually a Pie Town. And yes, they sell pie - at The Daily Pie Cafe. And they tell you what kind of pie they have that day with...you guessed it, a "Pie Chart".

I had Spiced Apple, and it was delicious. An experience not to be missed.

6 comments:

Julian said...

That misconception was so pervasive on labs, midterms, and daily multiple choice questions in my Geo 001 class that I think the professor's head would have exploded in rage if one more person said "the mantle is molten" without a "not" between "is" and "mantle" on anything graded. Considering I've seen him since then and his head was still attached to his neck, I'm guessing most people finally got the point by the time of the final.

Maybe elementary school teachers could use silly putty as a better comparison than straight-up liquid. While it still certainly isn't an exact analogue for ductile rock, it's "solid" by little kid definitions (ie. it doesn't get you wet if you touch it), but it still flows. It might be easier to adjust someone's thinking to the idea of ductile solids if they're already thinking in terms of unusually-behaving substances than to go "EVERYTHING YOU KNOW IS WRONG haha it's not liquid!"

Yami McMoots said...

Silly putty is solid by grown-up definitions, too - if you smash is just right with a hammer, it shatters. Or maybe that's not so grown-up.

I've heard horror stories about graduate students spouting this misconception during quals.

Tuff Cookie said...

Ooh, I hadn't thought of the Silly Putty. Not a bad analogue - I'll have to remember it if I end up teaching Intro Geo labs in grad school.

Yami - Grad students? *Shudder*

Any geo department that graduates someone who believes that is in BIG trouble.

MJC Rocks said...

Good point, and good post. The related misconception along this line is that students may even understand that the mantle is essentially solid, but because the outer core is liquid, that must be where all the lavas come from.

caboolture said...

A really nice example of a solid flowing can be found at http://www.physics.uq.edu.au/pitchdrop/pitchdrop.shtml . I've found this a useful analogy in the past!

kathleen said...

Geol grad from MTSU but interested in the new concepts. My own quirk is that the continents in motion idea is that they WERE, from an extra terrestrial impact or whatever. They aren't now; they are almost antipodal so don't need to move. EXCEPT that the three previous megacontinents somehow got together. SO, maybe we are headed back? And what would make them do that?

Carey said that all the surface that was is. I agree only I see it as a higher land mass from two low ones to make this as simple as possible. Imagine one pancake then two, one on top of another.

Land that was barely above the sea becomes miles high. Works for me. Land mass DOES NOT GET SUBDUCTED, ONLY OVERLAIN.

I see now that geologists are finding land masses that were (they claim) subducted, and they point to the chalky white material to prove it only that now this same material is up on top of the mountainous land mass that overrode the original land mass.

Looking for papers that explore this idea that all the land that was is. Thank you Dr. Carey.