re: "plastic" is more like it.
The way I learned it, the "plastic" behaviour of the mantle is approximately that of mild steel. You need huge forces to make it move, but move it does...
I happen to have a Ph.D. in Mantle dynamics, so even though I've forgotten a lot in the past 25 years, I do speak from a position of knowledge.
The Mantle is clearly solid and elastic, since it transmits seismic waves efficiently and at high speed. It is not mushy, slushy, or in any way "wet" or liquid.
Even the softest bits of the mantle (the low velocity/partial melt zones near the top and especially under mid ocean ridges) are only a few % melt, at best. Mostly it's <0.1% melt even in a low velocity zone.