Thursday, October 9, 2008


There's a discussion going on in the geoblogosphere right now on the use of analogies to describe geologic phenomena. This is a topic near and dear to my heart; here are a few of my favorites.
  • The development of a film of fat and scum on a stew is analogous to the development of the earth's crust. Due to chemical and physical differences of the original components, with cooking, the less dense components separate and rise to the surface. The remaining components can be thought of as more concentrated since there is less of the (now) floating fraction present in the mix. This analogy can be extended to magmatic segregation- which is really what we're talking about.

  • Silica concentration vs. viscosity in lava or magma can be related to varying sugar concentrations in syrups and candies. So corn syrup is pretty thick, "fake" maple syrup (not real- too expensive to play with!) is less thick, molasses is really thick. Fudge will deform under pressure, but is basically solid (like a rhyolite or dacite dome/plug).

  • Likewise, hot (or cold) fudge topping can help illustrate the relationship between temperature and viscosity in silica melts.

  • There are a huge number of analogies to help get at the idea of deep time. One that I have developed and used with a large number of classes is the idea of one earth year passing for each human second. In that framework, the earth is about 146 years old, human civilization a bit less than three hours old, and the the KT transition was a bit more than 2 years ago. A passing reference is here, and a more extended development in the comments here.

There are scads of these, and the examples above are just a few favorites. I have to point out that my least favorite, by a long shot, is the vinegar and baking soda volcano. I know, I know, this "experiment" (actually, silly activity) is loved by every kid. But hear me out:

  • Acid plus carbonate yields carbon dioxide through a chemical reaction. Volcanic eruptions' degassing results from exsolution of dissolved gas- mainly water. Chemical reactions per se are largely irrelevant.

  • The source of the materials is totally ignored.

  • Most volcanoes, even less violent ones, should not be characterized as "gently effervescing."

  • Putting the whole thing on top of a paper mache or modeling clay cone obscures the fact that volcanic cones are built by previous eruptions.

  • The very aspect of this activity that appeals to kids, its unfamiliarity, defeats its utility as an analogy- using the known to make sense of the unknown.

  • Worst of all, this activity is always the endpoint. If it was used as an introductory activity to start to get into the actual physical reality and behaviors of volcanoes, it would be much less cringe-inducing to me. Which brings me to my final point:

Analogies, by their nature, are incomplete and inaccurate. They can be very, very useful in connecting the familiar to the unfamiliar, and can lead to important insight- not only into the unfamiliar, but back to the familiar. A component of using analogies, whether in one's own thinking, or in education, should always include the question, "How do the analogs differ?" It is in understanding how real volcanoes are different from an acid-carbonate chemical reaction that these exciting and beautiful phenomena start to take on their own reality. So that, for example, I can use Newberry Volcano in Central Oregon as an analogy to try to understand the sulfur volcanoes on Io.

(From here; north is to the bottom in this image. See here for a nice source list)

(From here; photo from Paulina Peak looking NE over toe of the Big Obsidian Flow. Caldera walls background, resurgent volcanism has separated Paulina [left] and East [right] lakes)

(From here- highly recommended photo essay on Jupiter and moons from The Big Picture)

A better volcano analogy, for me? A shaken two-liter bottle of pop, opened suddenly. Observers/participants get the idea of an explosive, short-lived event (while still addressing safety constraints), and of the whole thing being driven by the power of exsolving gas. Further, nearly everyone is familiar with a soda-pop Plinian eruption. But they've never thought about it that way. You can extend this analogy by comparing the "eruption" of a refrigerated bottle of pop to a bottle of pop warmed in a water bath.

1 comment:

Silver Fox said...

Kids have fun building a volcano - and it often interests them in volcanoes - but you're right, it might be better to build a volcano layer by layer. Though the time constraints and wandering interest might get the better of that project!