My favorite analogies
Tuesday, I asked for my fellow geo-bloggers' favorite analogies, with a promise that I would share mine in 48 hours. The time of revelation is nigh... Here are a few of my favorite "geo-nalogies":
The continental crust is high-proof liquor
I see partial melting as a kind of distillation. Just as "sour mash" can be distilled to concentrate the alcohol it contains (separating it from the water it's dispersed in), so too can partial melting act as a "distillation" of the silicate earth. The minerals with the lowest melting temperatures will melt, leaving behind a solid residue enriched in Fe, Mg, Mn, and Ca, and yielding a magma that is enriched in Si, K, Na, and O. With its~granitic composition, the continental crust is 80-proof Jack Daniels. Where did it come from? It's distilled from the sour mash we call "the mantle":
I love a good chunky cookie. Save your Oreos and Lorna Doones for yourself. What I really like is one of those cookies with chocolate chips, oatmeal flakes, raisins, macadamia nuts, and those sinfully good butterscotch chips. What I like about these cookies is not so much how they taste, but how I can tell the difference between the individual ingredients and the cookie they comprise. I use this analogy early on in Physical Geology to illuminate the difference between minerals and the rocks that the minerals comprise:
Like many of us, I had an old sofa in college. The sofa was ripped, had been scratched by a cat, and had coffee spilled on it. It was draped in several layers of blanket in an attempt to cover up the lousy state of the upholstery. Someone added a pillow to the sofa at some point. When I was working for the C&O Canal National Historical Park (translating their geologic history into non-geology-speak), it struck me that the North American continent* was kind of like that old sofa. It had been scratched by glaciers instead of cats, and lava had been spilled on it kind of like that errant French Roast. It had rift valleys, but unlike the sofa's, North America's rifts didn't have springs poking out. New material had been added in the form of exotic terranes, kind of like that pillow got added to the sofa. And the blankets draping parts of the continent were made of sediment instead of fabric... but essentially the two were alike:
Tectonic plates are UFOs
In cross-section, a tectonic plate could be seen to have a profile kind of like a flying saucer. The thick part in the middle is the continental crust, but then it has a thin fringe encircling it (the oceanic crust). You can hardly blame a visiting Martian for feeling kind of attracted to it:
I didn't come up with this one... But read it somewhere (McPhee, maybe?) that I have since forgotten. Anyhow, the basic idea is that the Washington Monument's obelisk here in Washington, DC can show the difference between the Precambrian portion of geologic time (most of the monument, 88% of Earth history) and the Phanerozoic eon (post-Cambrian, 12% of Earth history). The little pyramid-shaped bit on top is the Phanerozoic. The thickness of a single sheet of paper draped on top of the tippy-top would represent the entire span of human history:
The continental crust is high-proof liquor
I see partial melting as a kind of distillation. Just as "sour mash" can be distilled to concentrate the alcohol it contains (separating it from the water it's dispersed in), so too can partial melting act as a "distillation" of the silicate earth. The minerals with the lowest melting temperatures will melt, leaving behind a solid residue enriched in Fe, Mg, Mn, and Ca, and yielding a magma that is enriched in Si, K, Na, and O. With its~granitic composition, the continental crust is 80-proof Jack Daniels. Where did it come from? It's distilled from the sour mash we call "the mantle":
I love a good chunky cookie. Save your Oreos and Lorna Doones for yourself. What I really like is one of those cookies with chocolate chips, oatmeal flakes, raisins, macadamia nuts, and those sinfully good butterscotch chips. What I like about these cookies is not so much how they taste, but how I can tell the difference between the individual ingredients and the cookie they comprise. I use this analogy early on in Physical Geology to illuminate the difference between minerals and the rocks that the minerals comprise:
Like many of us, I had an old sofa in college. The sofa was ripped, had been scratched by a cat, and had coffee spilled on it. It was draped in several layers of blanket in an attempt to cover up the lousy state of the upholstery. Someone added a pillow to the sofa at some point. When I was working for the C&O Canal National Historical Park (translating their geologic history into non-geology-speak), it struck me that the North American continent* was kind of like that old sofa. It had been scratched by glaciers instead of cats, and lava had been spilled on it kind of like that errant French Roast. It had rift valleys, but unlike the sofa's, North America's rifts didn't have springs poking out. New material had been added in the form of exotic terranes, kind of like that pillow got added to the sofa. And the blankets draping parts of the continent were made of sediment instead of fabric... but essentially the two were alike:
Tectonic plates are UFOs
In cross-section, a tectonic plate could be seen to have a profile kind of like a flying saucer. The thick part in the middle is the continental crust, but then it has a thin fringe encircling it (the oceanic crust). You can hardly blame a visiting Martian for feeling kind of attracted to it:
I didn't come up with this one... But read it somewhere (McPhee, maybe?) that I have since forgotten. Anyhow, the basic idea is that the Washington Monument's obelisk here in Washington, DC can show the difference between the Precambrian portion of geologic time (most of the monument, 88% of Earth history) and the Phanerozoic eon (post-Cambrian, 12% of Earth history). The little pyramid-shaped bit on top is the Phanerozoic. The thickness of a single sheet of paper draped on top of the tippy-top would represent the entire span of human history:
Okay, that's all I've got for today. What have YOU got?
Labels: geologic time, geology, granite, minerals, teaching
4 Comments:
These are wonderful, and I love your cartoons, diagrams, artwork. And the colors.
I think I've heard the Washington Monument one before - it's too bad you can't walk up the steps anymore (is that still true?) - and you'd stay in the Precambrian until reaching the very top!
100 percent agree with Silver Fox! Great post! Mark Twain once wrote that the layer of paint on top of the Eiffel tower was equivalent to human history compared to the height of the tower taken as geologic history (a surprisingly accurate and prescient take given that radiometric dating was decades in the future). I'll see if I can find the actual wording and the source; he finishes up with a hilarious statement that, in effect, any idiot can see that the whole purpose of building the tower in the first place was to hold up that layer of paint.
I suspect that the ultimate source of your Washington Monument/paper analogy is this passage from Twain.
Earth history as a human arm, with the contact between the neck and the shoulder being Earth's origin:
The shoulder is the base of the Archean.
The elbow is the Archean/Proterozoic boundary.
The wrist is the Proterozoic/Phanerozoic boundary (with the bulge of the lower forearm being about where the Ediacaran begins).
The palm is the Paleozoic.
The first two digits of the middle finger is the Mesozoic.
The last digit of the middle finger is the Cenozoic, and a single pass of nail file eliminates the history of Homo sapiens.
I like Thomas Holtz' analogy. But:
The wrist is the Proterozoic/Phanerozoic boundary
I think this is a bit off: my arm's 85cm, so the Paleozoic begins at 540/4600 * 85cm = 10cm, which is the knuckle. Similarily, the rest of the Phanerozoic is shifted towards the fingertip.
I've also written a post about it (sorry, only in German).
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