Why do deep mid-ocean ridges usually ooze lava instead of blasting ash?

Answer: Seawater pressure cages gases

Cold water stops magma — Cold water does chill the outside of lava fast, making glassy crusts and pillow shapes. But cooling alone would not make an eruption quiet; sudden water heating can also help explosions. The deep-ocean difference is the heavy water column, which keeps bubbles and steam from expanding freely.

Seawater pressure cages gases ✓ — Deep water adds a huge confining load, so gas bubbles in rising magma cannot swell the way they do near air. Around most mid-ocean ridges, that pressure favors effusive lava and pillow flows rather than ash columns. At 2,500 m, the water adds roughly 250 atmospheres, a quieting lid most land volcanoes never feel.

The lava has no gas — Basalt is often less gas-rich and less sticky than rhyolite, but it is not bubble-free. The Surtsey system was basaltic and still produced phreatomagmatic blasts when it reached shallow water. Composition matters, yet the surprising switch in the Reykjanes case is mostly the pressure drop with depth.