Tuesday, February 17, 2009

Another Followup: Does Glass Flow as Fast as the Mantle?

Mike said:
...if water has a viscosity of 1 and honey has a viscosity of a few thousand, rheids in the mantle are on the order of a few billion.
Roland asked:
How does glass compare?
Oh boy, the whole "glass is a liquid" thing. Yes, I was taught this in a grammar school science class, and it is wrong, wrong, wrong! Sorry, don't mean to get excited here, but oft-repeated scientific fallacies are a big pet peeve of mine..."glass is a liquid", "there's no such thing as centrifugal force", "the moon is bigger on the horizon", "toilets flush backwards in the southern hemisphere", etc. They all drive me nuts. For a while I even flirted with the idea of registering scientificfallacies.com as a domain name for a site specifically designed to debunk these.

Anyway, yes, glass is an amorphous solid...I think the Corning site (a manufacturer of glass) has an excellent write-up about this.

I particularly like the bit about lead flowing 1 billion times faster than glass. For perspective, the whole justifying argument of "glass is thicker at the bottom than the top of old church windows" would mean that astronomical telescopes with large glass mirrors would go out of focus in a matter of weeks.

Now, how does glass flow compare to rheids in the mantle? Well, it's a little hard to do so, particularly since the mantle covers such a wide range of temperatures. At depth, the magma viscosity is lower (i.e. it flows more easily) than near the crust, simply because the temperature increases as you go down.

That said, though, the upper mantle is usually pegged with a viscosity in the neighborhood of 10^20 poise, shockingly almost the same as that given for glass. The reason why the rheids in the mantle flow better than glass in the church window is simply a matter of pressure. In the church window, the only force compelling the glass to flow is gravity. This is extremely weak in comparison to the mantle being forced by the pressure of gigatons of material above it.

Put in another way, remember that viscosity is just a measure of resistance to flow, not flow itself. A pool of honey will move much more slowly if you poke it with your finger than if you hit it with a hammer, even though the viscosity doesn't change. Similarly, mantle rheids and glass both have about the same resistance to flow...just that in the mantle case the applied force is much, much greater such that it flows on the order of centimeters per year.



  1. Good article. We're Astronomers and Space Artists, now working in hot glass, so I was particularly interested in this comparison; finding it odd to see something in the recent posts list about glass flow amongst astronomy topics. Very well written. Thanks. We'll be following you.

    Found you through a Blogger Twitter tweet while following a trail on the new Followers Gadget that they had just produced.

  2. I "heard" many years ago that the reason why church glass is thicker at the bottom than the top is that it was made by pouring liquid glass onto a flat surface or spinning a "wheel" of glass, hence there were often irregularities. When placing the glass into the window lead the fitters put the thicker, heavier pieces at the bottom of the frame. They were fitting small pieces of glass of course, not sheets of plate glass like we have today.
    If you imagine a wheel of glass, spun from a rod, then the part near the centre would be thicker than the edge. The piece from the middle where the rod was would be discarded and hence used by people in domestic windows. This is why houses have those "bulls eye" glass window lights and churches don't.

  3. Watching the sky is an integral part of being a gardener. I'm glad I found your blog.

  4. Hi Paul,

    The reason you give for irregularities in church windows is absolutely correct.

    In fact, some people have even gone back to this old method of spinning a wheel of glass for windows to produce that "old-timey" nostalgic effect.

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  6. I am surprised to see that glass does not flow! Thank you for this info. Very well written article.