Mid-ocean ridges hide Earth’s longest mountain chain. Why so volcanic?

Answer: Tectonic plates pull apart

Tectonic plates pull apart ✓ — Right. At a ridge, plates separate, pressure drops in the hot upper mantle, and basaltic melt rises to build new ocean crust. The twist is scale: NOAA describes the ridge system as nearly 65,000 km long, yet more than 90% of it sits underwater. The planet’s biggest volcanic factory is mostly hidden from view.

Sediments catch fire — Almost, but this is the wrong setting. Coal or carbonate sediments can amplify gases when magma intrudes them, as in parts of Siberia, but ridges mainly work by plate spreading and mantle melting. The useful contrast is that the same lava-looking outcome can come from very different underground triggers.

A hot needle punches up — Not the usual ridge mechanism. A narrow upwelling can feed hotspots and some flood-basalt events, but mid-ocean ridges are plate-boundary factories. A ridge is more like a long zipper opening than a hot needle, which is why it can wrap around the globe rather than make one island chain.