What drives the mid-ocean ridges that erupt more lava every year than all land volcanoes combined?

Answer: Plates pulling apart underneath

Plates pulling apart underneath ✓ — Correct — plate spreading drives it. Two oceanic plates slowly pull apart; the mantle below rises into the gap, the pressure on it drops, and it partially melts. The new molten rock builds fresh ocean crust along the seam. That seam stretches about 65,000 km worldwide and quietly produces most of Earth's igneous rock — a continent-spanning fountain that never stops, almost entirely underwater.

Sea pressure heating deep magma — Sea pressure heating magma has cause and effect backward. Pressure doesn't add heat — it raises the temperature at which rock would melt. The mantle under a ridge melts because the pressure on it DROPS as it rises, not because the ocean weighs it down.

Saltwater dissolving the crust — Saltwater dissolving the crust is chemistry in the wrong place. Where seawater meets fresh basalt you get hydrothermal vents and black smokers — a real and important system, but it doesn't create vast new crust. New crust comes from melting, not from being dissolved.