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The Earth's mantle, away from the occasional magma chamber, is solid. But it flows over time and so is described as "pliant", "hot chocolate", "toothpaste", etc.

Glaciers flow much faster. But ice is rigid, solid and brittle, reflected in words such as "hailstone" and "on the rocks" (although above cryogenic temperatures it is much softer than most minerals).

Would most of Earth's mantle material, in situ, feel rigid like crustal rock? All the way from the upper mantle to the edge of core (extreme pressures counteracting the searing heat)? My (indestructible) hands are too impatient to sense deformations over geological time...

Edit: To clarify what (I think) the case is, based on the answer below:

  1. Indeed, ice and most silicates (if in crystal form and not glass form) are brittle rocks all the way to their melting point at low pressure. No "taffy", "chocolate" etc. They get weaker as they get hotter, but always shatter under a hammer blow.

  2. Both can flow slowly when near their melting point (glaciers). Flow happens in between crystal grains, as atoms re-arrange themselves to relieve the applied stress.

  3. At extremely high pressure we can still talk about "compressive strength" (and brittleness, etc) in terms of how much difference in pressure the rock can take. We could in theory set up a hammer to strike the rock while the whole system is immersed in hot high-pressure helium. No voids could form in the material because of the pressure; it can only break from the surface by letting helium in, and even that isn't an instant process. This means the only choice under a heavy a hammer blow is to deform without breaking. Not really "taffy" more like "blacksmith metal". A Bingham plastic from miliseconds to years and a non-newtonain fluid on thousand or million year time-scales.

Kevin Kostlan
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  • feel is a difficult thing to assess on those pressure scales you couldn't feel anything. Any force you can exert is microscopic compared to the forces on the rock. – John Oct 04 '23 at 22:17
  • @John: The hydrostatic component of the stress is, but the shear stresses (how much the stress is direction-dependent) that drive mantle flow are only ~10-100MPa which can be reached with a simple hammer blow. So if a mantle rock was placed in helium at the correct temperature/pressure and you somehow could play with it. – Kevin Kostlan Oct 04 '23 at 23:27

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Though the mantle is not homogeneous, it is overwhelmingly solid and convects as a rigid body. But partial melting can happen, under mid ocean ridges, in subduction zones or over hot spots. Just very little partial melt already change the viscosity by much by "greasing" the contacts between minerals.

One can say it is "plastic", and there may even happen toothpaste like effects (thixotropy = melt under stress), but the correct term is ductile = "smithable".

I can't say what it "feels like", but maybe give some visual impressions of what ductile means:

In this video, at 11:06 they show 4 cylinders of a rock that was exposed to a longitudinal compressing stress. It shows brittle deformation when the pressure of the environment is low, and ductile deformation when the environmental pressure is that of a depth of 30km.

Now 30km is not mantle everywhere on earth, and the material is a sample from continental crust (marble, a metamorphic rock), but I think one gets the impression that pressure alone is enough to make rocks start to move.

Given enough time, some rocks even deform under surface conditions.

Further reading: https://opentextbc.ca/physicalgeologyh5p/chapter/stress-and-strain/

  • A blacksmith deforms metal in a fraction of a second. Glacial ice deforms over years but still shatters under a hammer blow. Mantle rock deforms over millions of years. This is why I think the mantle would also be brittle on human time-scales, at the temperatures and pressures along the geotherm and away from localized melt-zones. – Kevin Kostlan Oct 04 '23 at 20:18
  • plastic means it permanently deforms, which is an accurate description, things brittle on a small scale can undergo a lot pf plastic deformation on a large scale due to grain interaction. – John Oct 04 '23 at 22:15
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    @KevinKostlan: What you think is wrong. If you read the links I posted you'll see that ductile rock deformation also happens - depending on process - in a very short time, seconds to years. I have seen bedrocks close lodes in deep mines in a matter of years. John: yes, but I wanted to be more precise than that. –  Oct 05 '23 at 02:48
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    Oh wow that's much faster than I expected! – Kevin Kostlan Oct 05 '23 at 18:55
  • That's a really good video. The presenter is a great science communicator. – Wayne Conrad Oct 13 '23 at 20:37