...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.