A mantle plume is hot solid rock, not a lava pipe. How can it make magma?

Answer: Pressure drops upward

Pressure drops upward ✓ — Right. Hot mantle can remain solid deep down because pressure is enormous; as it rises, pressure falls faster than heat is lost, so partial melting can begin. The counterintuitive part is that melting often starts by unloading rock, not by adding a blowtorch. That is why ridges and hotspots can both make basalt from rising mantle.

It burns through crust — This is the movie version, not the physics. Plumes do not have to be molten tunnels that burn their way up. NOAA describes hot plumes rising and partially melting only in the shallow mantle, while the Geodynamics source frames plumes as hot solid mantle rock. The magma appears late in the trip.

Seawater melts mantle — Water can matter in subduction zones, where fluids help lower melting temperatures above a sinking slab. Hotspots are different: the key move is buoyant rise and pressure release. Mixing the two mechanisms hides the payoff, because Earth makes magma by several recipes, not one universal furnace.