<p>The Earth's mantle, away from the occasional magma chamber, is solid. But it flows over time and so is described as &quot;pliant&quot;, &quot;hot chocolate&quot;, &quot;toothpaste&quot;, etc.</p>
<p>Glaciers flow much faster. But ice is rigid, solid and brittle, reflected in words such as &quot;hail<em>stone</em>&quot; and &quot;on the rocks&quot; (although above cryogenic temperatures it is much softer than most minerals).</p>
<p>Would most of Earth's mantle material, <em>in situ</em>, 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...</p>
<p><strong>Edit:</strong>
To clarify what (I think) the case is, based on the answer below:</p>
<ol>
<li><p>Indeed, ice and most silicates (if in crystal form and not glass form) are brittle rocks all the way to their melting point <em>at low pressure</em>. No &quot;taffy&quot;, &quot;chocolate&quot; etc. They get weaker as they get hotter, but always shatter under a hammer blow.</p>
</li>
<li><p>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.</p>
</li>
<li><p>At extremely high pressure we can still talk about &quot;compressive strength&quot; (and brittleness, etc) in terms of how much <em>difference</em> 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. <em>Not really &quot;taffy&quot; more like &quot;blacksmith metal&quot;.</em> A Bingham plastic from miliseconds to years and a non-newtonain fluid on thousand or million year time-scales.</p>
</li>
</ol>

CC BY-SA 4.0
2023-10-04T22:17:05.030Z
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
CC BY-SA 4.0
2023-10-04T23:27:30.350Z
@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
<plate-tectonics>
Is it inaccurate to describe Earth's mantle as "soft and plastic"?
Kevin Kostlan
# Is it inaccurate to describe Earth's mantle as "soft and plastic"?

(Asked by: username_0 on 2023-10-04)

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 "hail*stone*" 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.

**Tags:** plate-tectonics

> **Comments:**
> **username_1**: 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.  
> **username_0**: @username_1: 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.  

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## 0 answers

