In the mantle, minerals contained in buoyant plumes of rock undergo phase changes from solid to liquid (decompression melting). In the atmosphere, waters contained in buoyant plumes of air undergo phase changes from gas to liquid (clouds and precipitation).

Plumes in both systems experience decreasing pressure and decreasing temperature as they rise. Why do the phase changes go in opposite directions, i.e. why do rising mantle minerals melt while rising atmospheric waters condense?


2 Answers 2


There is no comparing the chemical composition of "atmosphere" with that of mantle rocks and the consequences of rising in each have almost nothing to do with one another. The cooling of rising atmosphere is a fairly simple thermodynamic process requiring neither molecular change of ingredients, chemical release of water, nor thermally impervious boundaries.

By contrast, rising mantle materials will undergo different melting processes depending on their water content and chemistry. In the simplest case, like a mid-ocean hot-spot, unpolluted by melt fluxes like water and carbonates, rising mantle rock undergoes "decompression melting" in which 2 things happen:

1) The pressure needed to keep rising mantle material solid is released and the material becomes liquid (like that crudely drawn by Michael, above). Remember, given enough pressure, you can heat up ANYTHING and keep it solid...even hydrogen, like that believed to exist in the earth's core.

2) Constituent minerals undergo phase (mineralogical) changes on their way up to the base of the upper crust. Remember that mantle materials come to the earth's surface as basalts which are mineralogically different from the olivene-rich peridotites of the mantle. Despite being chemically similar, deep mantle rocks contain minerals that are crystallographically denser than those found basalts. As they are brought up, these minerals undergo a phase change that alters their crystalline structure; they become different minerals, lighter and more buoyant, further reducing the density of the rising melt.

So, there it is (in a nutshell): reducing pressure on hot solid material allows it to become liquid and also invokes phase changes in minerals that make the overall material less dense.

  • $\begingroup$ I don't think solid hydrogen exists in the Earth's core; you may be thinking of Jupiter. The melting point of hydrogen at 65 GPa is only about 1000 K and decreases above that, per this paper: arxiv.org/pdf/0803.2321.pdf The Earth's core seems far, far too hot. $\endgroup$
    – user25972
    Mar 1, 2017 at 17:26
  • $\begingroup$ Metallic hydrogen within the earth's core has been theorized since at least the 1980s and especially with the discovery of stable metallic hydrogen at room temperature, at less than 150GPa, in 1989 by a group at Carnegie Mellon. $\endgroup$ Mar 3, 2017 at 0:51
  • $\begingroup$ The disputed discovery of solid metallic hydrogen only occurred this year. Your claim of solid metallic hydrogen being discovered in 1989 requires proof. Please also do not conflate metallic with solid. Hydrogen could be a liquid or solid metal, or a liquid or solid nonmetal, under different conditions. Theories of solid hydrogen in the core are nonetheless worth considering if you would please provide citations. $\endgroup$
    – user25972
    Mar 3, 2017 at 14:07

When humid air rises, it reaches higher altitudes where it meets colder air. Because air is a fluid, it readily mixes with the colder air around it, lowering the vapour pressure of H2O and condensing as liquid or solid (i.e. water or ice, respectively). This is very similar to heat transfer by advection and convection. (also note comment by casey below)

The mantle, on the other hand, is a solid. Mixing of the hot plume with the colder surroundings is extremely slow and happens only by conduction. Rocks are terrible conductors of heat, so they only cool down adiabatically, which is not much at all. If we look at my badly drawn phase diagram:

decompression melting in the mantle

You can see that a rising rock (the red and blue lines) does not cool rapidly enough to miss the solidus (the curve denoting the P-T conditions where rock melts). Therefore, it melts.

I would also like to add that most rocks in mantle plumes don't actually melt, or they do so to a limited degree. Mantle plumes cause widespread melting because they rise and pool below other rocks that are less refractory (easier to melt). This can be because of their composition which is less mafic, or because they have fluxes such as water in them. The heating of these crustal rocks by the mantle plume is what usually causes widespread melting.

  • 2
    $\begingroup$ Note that rising air cools (dry or moist) adiabatically as it rises due to expansion (the parcel does work on the environment to expand and the parcel cools). This reduces the saturation vapor pressure and works without invoking entrainment of environmental ambient air into the rising parcel. Entrainment tends to surpress clouds as it can be quite dry aloft. $\endgroup$
    – casey
    Jan 28, 2017 at 1:44
  • $\begingroup$ @casey point taken $\endgroup$
    – Gimelist
    Jan 28, 2017 at 2:44

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