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CrustalTrudger t1_j6mt8do wrote

> How viscous is the magma in the mantle?

The mantle is solid. That being said, even though it is demonstrably solid, the mantle flows like a fluid on geologic time scales and generally tends to behave like a Non-Newtonian fluid (i.e., stress and/or viscosity are a non-linear function of the strain rate). If you want a deep dive on mantle rheology (i.e., how it deforms), this slide-deck has a thorough treatment. Given that it's ideally a Non-Newtonian fluid, it's hard to ascribe a single viscosity to mantle materials, and it will vary as a function of background conditions (i.e., temperature and lithostatic pressure) and the rate of strain the material is experiencing.

> Are there points of greater viscosity and lesser?

As described above, in detail the viscosity will vary as a function of strain rate, i.e., as strain rate increases, viscosity will decrease. Those complications aside, we do have a variety of estimates for what we could sort of think of as the "background" viscosity (or the viscosity that, when assuming the mantle instead behaves like a Newtonian fluid with a single viscosity, best explains observations) for the mantle and these do vary as a function of location. For these, we typically consider the upper mantle (nominally the mantle above 660 km) and lower mantle (mantle between 660 km and the outer core) separately.

For the upper mantle, this has a pretty wide range of viscosities anywhere from 10^(18) Pa s to 10^(21) Pa s (e.g., Dixon et al., 2004) with significant lateral variability. As discussed in that paper (and other sources), this high degree of variability is largely ascribed to differences in water content or temperature that reflect both modern and past tectonic histories and where higher water content and/or higher temperatures lead to lower viscosities (i.e., materials that flow easier / are effectively weaker).

For the lower mantle, depending on the source, similar order of magnitude ranges have been suggested but tend to be uniformly higher and the variations are more in terms of depth as opposed to laterally. For example, both Lau et al., 2016 and Mitrovica & Forte, 2004 suggest viscosities ranging from 10^(21) to 10^(23) Pa s with lower values at the top of the lower mantle and a peak in viscosity near the bottom of the lower mantle (though all estimates suggest that viscosity decreases significantly to <10^(20) Pa s approaching the core-mantle boundary). In contrast, other estimates like Cízková et al., 2012 and van der Meer et al., 2018 suggest less variable lower mantle viscosity mostly between 10^(22) and 10^(23) Pa s but still with a peak near the bottom of the lower mantle.

It's worth briefly discussing where these numbers come from. For the upper mantle, it's primarily from using the response to glacial isostatic adjustment, i.e., the lithosphere flexed down under the weight of large ice sheets and is still flexing back up after they melted and the rate and spatial patterns of that flexing back up (which we can measure) is in part controlled by the viscosity of the mantle (we also use similar response to responses to large loads like lakes, etc to work out viscosity estimates for areas far away from those that were glaciated). Similar data is used for some of the lower mantle estimates (e.g., Lau et al., 2016 and Mitrovica & Forte, 2004), but alternatively, papers like Cízková et al., 2012 and van der Meer et al., 2018 both use the sinking rate of detached subducted slabs imaged by seismic tomography to estimate viscosity.

> If it weren't for the heat, could you swim in it? What's an everyday substance that might have comparable viscosity?

For reference, the viscosity of water is around 0.001 Pa s, higher end viscosities of honey will be around 10 Pa s, average peanut butter is around 100 Pa s, and the viscosity of pitch (e.g., pitch drop experiments) is ~10^(8) Pa s, so still at minimum 10 orders of magnitude less viscous than the least viscous part of the mantle.

In short, the viscosity of the mantle is high enough that on human timescales, a material with a similar viscosity at room temperature would be for all intents and purposes a solid, so no, you could not swim in it. This is why we talk about the mantle as a solid. It is in the way we experience solids, and the fact that it has a viscosity and flows is only relevant if you're considering timescales of several thousands or tens of thousands of years at the minimum.


agate_ t1_j6nidr1 wrote

One way to think about the fact that “the mantle is a solid but it flows” is to think about ice. Ice is a solid, just look at an ice cube, and yet a kilometer-thick glacier flows.

Like ice, the mantle isn’t perfectly solid. But if you compare it’s viscosity to that of a glacier, you find that the mantle is roughly a million times more solid than ice.

(Ice is also a non-Newtonian fluid, so a precise comparison isn’t possible.)


[deleted] t1_j6muy1z wrote



dukesdj t1_j6nk0kf wrote

This is not really the definition of fluid as there actually is no strict boundary between what is and is not a solid. Indeed as others have noted but incorrectly commented on, things like the mantle, pitch, and jelly are examples of substances that have a dual nature in both being solid AND liquid.


To quote George Batchelor (taken from An Introduction to fluid dynamics), "The distinction between solids and fluids is not a sharp one, since there are many materials which in some respects behave like a solid and in other respects like a fluid." ... "But, even supposing that these two definitions could be made quite precise, it is known that some materials do genuinely have a dual character.".


What this really means, and what fluid dynamicists recognise, is trying to constrain a substance/object into being a solid or a fluid has more to do with humans desire to define things in discrete buckets and less about the actual physical world.


CrustalTrudger t1_j6nmo19 wrote

Thanks for clearly stating what I was trying to express somewhat sloppily. This is largely why in discussions of rheology (for rocks at least), talking about them as either "solid" or "fluid" is uncommon and instead you tend to see them described just as "materials", i.e., when texts introduce useful analogues for thinking about the stress-strain or stress-strain rate relationship (i.e., the various combinations of a sliding frictional block, spring, or dashpot that would produce some sort of equivalent stress-strain or stress-strain rate response) they tend to do so in terms of just materials, e.g., "Maxwell materials" or "Voigt materials" etc. Not all geology texts are good about this though.


phdoofus t1_j6npg6i wrote

A good (reasonably not bad) example of the rheology of the mantle is Silly Putty.

If you put a ball of it on a desk and hit it with a hammer, it shatters. A good analogy for earthquakes (brittle response)

If you just leave it on the desk it will become nice flat putty pancake (ductile response)

Two different responses to two different stress/strain regimes. Particularly effective in class


CrustalTrudger t1_j6mx7hi wrote

This is actually kind of a misleading "clarification" though. Pitch is a useful example, i.e., a viscoelastic solid that will deform on long time-scales under its own weight. At room temperature and timescales sufficiently short (i.e., less than a few years), pitch would meet the simple definitions of a "solid", but observed on long enough time scales, it can be observed to flow.


OlympusMons94 t1_j6ng2lg wrote

Liquids (and solids) are much less compressible than gases, but they are still compressible. Constant volume (incompressibikity) is just (sometimes) a useful simplifying assumption. (In other contexts like sound/seismic wave speed, it would be, well, complicating to say the least, given that would result in an infinite wave speed.)

If you have a tall enough, a column of metal, or even rock, it will deform under the pressure from its own weight. A penny is just very small and light, and deformation is negligible. Like a solid, a liquid will also not deform without without some force being applied, but the type of deformation is different.


muskytortoise t1_j6o1zlo wrote

That is a common definition, yes, but scientifically liquid is a state of matter. Fluid =/= liquid. Liquid is a phase, fluid is behaviour. You said liquid when you described fluid and while that is correct in common meaning it's unnecessarily confusing people when the difference is described.