地幔，另一方面，是固体。热羽与较冷环境的混合极其缓慢，仅通过传导发生。岩石是糟糕的热导体，所以它们只能绝热冷却 ，这根本就不多。如果我们看一下我画得很糟的相图:

你可以看到一个上升的岩石(红色和蓝色线)没有迅速冷却到足以错过固相体(曲线表示岩石熔化的 p - t 条件)。因此，它会融化。< / p >

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.

1)保持上升的地幔物质固体所需的压力被释放，物质变成液体(就像迈克尔粗略画的那样，上图)。记住，只要有足够的压力，任何东西都可以加热并保持固态……即使是氢，也被认为存在于地核中。< / p >

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.