Why are deep-sea mid-ocean ridges, Earth's longest mountain system, so volcanic even though they lie underwater?

Answer: Tectonic plates pull apart there, and hot mantle rises and melts as pressure drops

Tectonic plates pull apart there, and hot mantle rises and melts as pressure drops ✓ — Right. As the plates separate, mantle rock rises to fill the gap. The drop in pressure lowers its melting temperature, so it melts and feeds basaltic eruptions that build new seafloor. This decompression-melting engine continues all along the ridge, which is why the chain is so volcanic.

Tectonic plates grind together there, and frictional heat melts mantle rock into magma — Not quite. Friction can melt rock in some tectonic settings, but a mid-ocean ridge is where plates move apart; there is no deep grinding along a strike-slip or collisional fault. The melting comes from pressure release in the rising mantle, not from rubbing. Earthquakes along the ridge are caused by extension, not by plates scraping past each other.

A deep mantle plume punches upward through the plate, melting a narrow channel — That describes a mantle plume, the kind of upwelling that powers hotspot volcanoes. A plume is narrow and localized, whereas spreading at a mid-ocean ridge makes mantle rise broadly along a long boundary. That broad upwelling produces a nearly continuous volcanic chain, not one narrow melting channel.