How can a mantle plume, which is hot solid rock rather than a lava pipe, make magma?
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Answer: Hot rock rises into lower pressure and partially melts.
Hot rock rises into lower pressure and partially melts. ✓ — As hot mantle rock rises, the surrounding pressure drops, allowing some of the rock to melt if it stays hot enough. This is decompression melting: melting begins because pressure falls, not because extra heat is added. The remaining solid rock can keep flowing slowly while magma separates and rises.
Seawater sinks into the plume and melts the rising rock. — Seawater is not required for plume melting. Water released by a sinking plate can lower the mantle's melting temperature at subduction zones; a hot plume can instead melt as it rises into lower pressure. These are distinct ways to make magma.
The plume burns through the crust to turn it into magma. — A mantle plume does not need to burn a molten tunnel through the crust. Its hot rock can rise while mostly solid and begin melting as pressure falls. Magma may later heat or melt some crust, but that is not how the plume first produces magma.
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